Eu Incorporation into Sol–Gel Silica for Photonic Applications: Spectroscopic and TEM Evidences of...

Eu Incorporation into Sol−Gel Silica for Photonic Applications:Spectroscopic and TEM Evidences of α‑Quartz and Eu PyrosilicateNanocrystal GrowthAndrea Baraldi,*,† Elisa Buffagni,†,∥ Rosanna Capelletti,† Margherita Mazzera,†,⊥ Mauro Fasoli,‡

Alessandro Lauria,‡,# Federico Moretti,‡,∇ Anna Vedda,‡ and Mauro Gemmi§

†Department of Physics and Earth Sciences, University of Parma, Viale G.P. Usberti 7/A, 43124 Parma, Italy‡Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 55, 20125 Milano, Italy§Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Piazza S. Silvestro 12, 56127 Pisa, Italy

*S Supporting Information

ABSTRACT: The problem of Eu incorporation into silica as dispersed dopants,clusters, separate-phase nanoparticles, or nanocrystals, which is of key importancefor applications in the fields of lasers and scintillators, is faced by applying to sol−gel silica doped with nine different Eu3+ concentrations (0.001−10 mol % range)various spectroscopic techniques, including crystal field and vibrational modeanalysis by means of Fourier transform absorption and microreflectivity (in the200−6000 cm−1 and 9−300 K ranges), radioluminescence, and Raman scatteringstudies at 300 K. The variety of methods revealed the following concordantresults: (1) amorphous Eu clusters grow when the Eu concentration is increasedup to 3 mol % and (2) Si−OH groups are completely removed and ordered phaseseparation occurs at 10 mol % doping, as suggested by the remarkable narrowingof the spectral lines. Comparison with polycrystalline Eu oxide, Eu silicates, and α-quartz spectra allowed the unequivocal identification of Eu2Si2O7 pyrosilicate andα-quartz as the main components of nanocrystals in 10 mol % Eu-doped silica.Such conclusions were brilliantly confirmed by transmission electron microscopy and electron diffraction analysis. Phononcoupling and anharmonicity were analyzed and are discussed for a few vibrational modes of nanocrystals.

1. INTRODUCTION

Optical materials aimed at applications in the fields of lasersand scintillators often exploit the luminescence of foreignelements such as rare-earth (RE) or transition-metal ions thatare introduced as dopants in crystalline hosts and act asluminescent centers. However, in some cases the growth ofbulk single crystals is difficult and expensive as a result, forexample, of a very high melting point of the considered system.Therefore, alternative preparation routes are pursued to obtainbulk samples of specific compounds.An interesting technology that has given satisfactory results

in recent years is optical ceramics, where a powder of micro- ornanometric dimension is pressed (often at high temperature) toobtain a transparent or translucent bulk polycrystalline material.For example, optical ceramics were obtained for severalsystems, including YAG:Nd and Y2O3:Nd,

1,2 YAG:Ce,3,4

Lu2O3:Eu5 and LuAG:Ce,6 LSO.7 This technology still faces

some problems, as the presence of grain boundaries gives riseto surface defects causing afterglow processes, as shown forLu2O3 and for YAG:Ce.8−10 Moreover, transparent materialscan be obtained mostly with cubic structures that do not displayanisotropy of the refractive index.Nanocomposites are also considered as an alternative

morphological form for optical materials. In this case,

luminescent particles are embedded in a transparent supportingmedium, which can be liquid or solid. Very recently, growinginterest has been devoted to liquid nanocomposites forapplications in the photodynamic therapy of cancer.11,12

More extended studies concern solid nanocomposites, such assimple RE oxides,13−18 silicates,19,20 CdTe nanoparticlesembedded in BaFBr:Eu,21 nanoporous silica “soaked” byCdSe/ZnS luminescent nanoparticles or rhodamine,22 andorganic−inorganic hybrids.23 For these materials, the particledimensions should be lower than a few hundreds of nanometersto avoid scattering of the emitted light. The embedding stagecan be performed after powder preparation; a critical point inthis case is that particle aggregation into bigger, light-diffusingclusters can easily occur. Alternatively, the sol−gel process canbe exploited to obtain doped xerogels and glasses in whichluminescent nanoaggregates are spontaneously formed becauseof the low solubility of the dopants in the silica matrix. Themain difficulty is control of the degree of crystallinity and thespecific crystalline form of the aggregates, which are determinedby thermodynamic conditions and cannot be directly tuned.Crystalline but not luminescent CeO2 nanoparticles form in

Received: October 11, 2013Published: November 25, 2013

Article

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© 2013 American Chemical Society 26831 dx.doi.org/10.1021/jp4101174 | J. Phys. Chem. C 2013, 117, 26831−26848

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Ce-doped sol−gel silica sintered in an oxidizing atmosphere,24

while amorphous clusters with a structure close to that ofsilicates are formed in Ce-doped silica sintered in a reducingatmosphere and in Gd-, Tb-, and Yb-doped silica.25,26 In all ofthese cases, the dopant concentrations are equal to or higherthan 1 mol %. The particular behavior of Ce can be explainedby its tendency to be incorporated with 4+ valence in the silicamatrix under oxidizing conditions, at variance with the othermentioned REs. An exception to amorphous clustering isrepresented by silica-based glass ceramics obtained by heavy Gddoping (40 mol %) and codoping with Ce: crystallineaggregates either of Gd2Si2O7 pyrosilicate or with theGd4.67O(SiO4)3 apatite-like structure were revealed.27

In the present work, the analysis is extended to incorporationof Eu into silica prepared by the sol−gel method. A widedopant concentration range (from 0.001 up to 10 mol %) wasconsidered, which allowed the clustering phenomena to befollowed from the beginning up to the phase separation anddevitrification. The Eu clustering in silica was previously facedby molecular dynamics simulation studies.28−30 For Euconcentrations ranging from 1 to 5 mol %, Afify andMountjoy29 envisaged the onset of clustering already atconcentrations as low as 1 mol %, but they complained aboutthe lack of experimental evidence of a cluster structure, adeficiency that the present results try to fulfill. Up to now,indirect evidence of Eu clustering has been argued mainly fromfluorescence spectra in the visible region, without providing anyhint about the cluster and/or separate phase structure. Forexample, inhomogeneous line broadening due to the shortdistance between Eu ions in clusters favors energy transfer andcompromises the site selectivity of an exciting laser,31,32 whileconcentrations higher than 3 mol % induce emissionquenching.33 More recently, our preliminary Fourier transforminfrared (FTIR) measurements34 showed that (1) the areaunder the Eu 7F0 →

7F4 absorption (in the 2500−3200 cm−1

range) exhibits a supralinear trend when plotted versus Euconcentration in the 0.1−3 mol % range, thus suggesting clusterformation, and (2) increasing the Eu doping level from 3 to 10mol % drastically changes the absorption and Raman spectra,with the appearance of narrow lines, at variance with thebehavior displayed by other REs.26,27 The crystal field (CF) andvibrational spectroscopy results available at that time34,35 wereable to make apparent the growth of generic Eu silicate-likeclusters but not to identify their specific structure. The peculiarbehavior displayed by Eu in silica glass was the motivation forthe present work, which aimed to understand the detailednature of the clusters. Europium (1) works as a microscopicprobe of its environment through the splitting of the electroniclevels of the 4f6 configuration by the CF and (2) modifies, evenheavily, the vibrational modes of the silica network. Thus, CF,vibrational absorption, and microreflectance spectra weremeasured, even at low temperature, by means of FTspectroscopy in the 200−6000 cm−1 range and werecomplemented by Raman and radioluminescence (RL) spectra.The spectroscopic approach was further integrated withtransmission electron microscopy and electron diffractionanalysis.

2. EXPERIMENTAL SECTIONSilica samples were obtained by the sol−gel method byhydrolizing tetraethyl orthosilicate (TEOS) in ethanol (Si/H2O1:8 molar ratio; TEOS/ethanol 1:3 volume ratio). Eu dopingwas achieved in the sol by adding under stirring proper volumes

of ethanolic solutions of europium nitrate pentahydrate[EuIII(NO3)3·5H2O, 99.9%, Aldrich], ensuring the homoge-neous dispersion of the dopant before addition of water.Doping levels from 0.001 up to 10 mol % were considered. Thesols were put into polypropylene screw-cap flasks (Kartell) andtightly sealed. After gelation, bulk xerogels were obtained byslow solvent evaporation at 40 °C. A sintering process leadingto fully densified composite materials was performed by heatingat a rate of 4 °C/h up to 1050 °C. All samples were kept at thistemperature for times never longer than 2 h, and then theywere slowly cooled (the time required to reach 150−100 °Cwas roughly 6−7 h). The whole thermal treatment wasperformed in a pure O2 flux. Some samples were submitted to afurther postdensification rapid thermal treatment (RTT) by anoxidizing oxygen−hydrogen flame. Such a treatment typicallyfeatured a very fast (2−4 s) temperature increase up to 1500−1800 °C, where the sample was kept for approximately 10 sbefore rapid cooling in air. The temperature was monitored byan optical pyrometer (Impac IE 120) working at 5140 nmemission. Most of the samples were transparent, but a few ofthem (e.g., the 0.001 mol % sample) showed foggy regions. The10 mol % sample was brittle after densification and almostcompletely milky. This feature is not unusual because brittlesol−gel samples were also obtained as a consequence of heavy(8−10 mol %) Pr doping.36

Reference powdered materials were also prepared by solid-state reaction, as Eu oxyorthosilicate (Eu2SiO5) and Eupyrosilicate (Eu2Si2O7), or purchased, as crystalline europiumoxide (Eu2O3, 99.99% from Aldrich), and natural smoky quartz.The Eu2O3 powders were submitted to a preliminary 1 hthermal treatment at ∼1000 °C to remove some adsorbedmoieties (e.g., water) responsible for spurious lines at ∼850,1087, 1494, 3410, and 3594 cm−1 detected in the absorptionspectra; however, the treatment was unable to remove traces ofcarbonate groups (bands around 1500 cm−1), very likelyintroduced during the oxide synthesis procedure37,38 (seeFigure 3, curve f). Reference slices of natural α-quartz and fusedpure silica were also investigated.CsI-based pellets were prepared to measure the absorption

spectra of either powdered compounds (oxyorthosilicates,pyrosilicates, and oxides) or silica (Eu3+-doped or undoped)and α-quartz in the spectral ranges where the massive samplesexhibit very high absorption coefficients. In the last cases, chipsof the samples were ground and then mixed with CsI powder.The sample/CsI weight ratio ranged between ∼0.6 mg/100 mgand 20.2 mg/100 mg (Table 1).The optical absorption measurements over the wavenumber

range 200−6000 cm−1 were performed by means of a BomemDA8 Fourier transform spectrophotometer capable of aresolution as fine as 0.02 cm−1. The microreflectance spectrawere measured at room temperature (RT) in the 600−6000cm−1 range by coupling the spectrophotometer to an IR PLANmicroscope (Spectra Tech). As a reflectance standard, a goldmirror was used. Microreflectance measurements were choseninstead of the specular reflectance ones because some of thesol−gel samples were brittle, mainly those heavily RE3+-doped,and thus, only small-sized samples were available for measure-ments. The sample area investigated was less than 1 mm2. Inthe case of optically inhomogeneous samples, the analysis wasextended to different areas. Diffuse reflectance spectra ofpowdered compounds were measured at RT by coupling thespectrophotometer to a Praying Mantis diffuse reflectionaccessory (Harrick Scientific Products). As a diffuse reflectance

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp4101174 | J. Phys. Chem. C 2013, 117, 26831−2684826832

standard, powdered CsI was used. Standard RT absorption andreflectance spectra were measured at a resolution of 1 cm−1 byacquiring 500 scans, while for 9 K absorption spectra theresolution was improved to 0.5 cm−1 and the number of scanswas increased to 1000. The temperature at which theabsorption spectra were measured could be varied in the 9−300 K range by assembling the samples in a model 22 CryodineRefrigerator (CTI Cryogenics) equipped with KRS5 windows.Raman spectra were measured at RT using a micro-Raman

spectrometer (Labram, Jobin-Yvon). The excitation source waseither the 488 nm line of an Ar+ laser or the 632.8 nm line of aHe−Ne laser. Unpolarized Raman spectra were collected in thebackscattering configuration through a CCD detector.X-ray-excited RL measurements at RT were performed using

a homemade apparatus featuring a CCD detector (Jobin-YvonSpectrum One 3000) coupled to a monochromator (Jobin-Yvon Triax 180) with 300 grooves/mm grating. The spectralresolution was 2 nm in the 300−700 nm range. X-rayirradiation was realized with a Philips 2274 tube operated at20 kV.Transmission electron microscopy (TEM) was carried out

on a FEI Tecnai F20ST microscope working at 200 kV at theEarth Science Department of the University of Milan and on aZeiss Libra 120 microscope working at 120 kV at the Center forNanotechnology Innovation@NEST in Pisa. The Libra micro-scope was equipped with an in-column omega filter, aNanoMEGAS DigiSTAR P1000 tool for precession electrondiffraction and diffraction tomography, a NanoMEGAS ASTARmodule for phase and orientation mapping, and a scanningTEM (STEM) unit with a high-angle annular dark-field(HAADF) detector. Electron diffraction tomography and theASTAR module were used to study the crystal structure ofnanoparticles. In electron diffraction tomography, a series ofelectron diffraction patterns is collected every 1° while thecrystal is tilted around the goniometric axis39 in a variable rangebetween 90° and 120°. In this way, a large portion of thereciprocal space is sampled and, via software, a 3Dreconstruction of the crystal reciprocal space is obtained,40

allowing the unit cell parameters to be derived easily.41 PETSsoftware was used for the unit cell determination andrefinement,42 and ADT3D software was used for 3D visual-ization of the reconstructed reciprocal space.43 In ASTAR phaseand orientation mapping, a series of diffraction patterns iscollected while the beam is scanning a given area. The patternsare recorded by taking pictures of the fluorescent screen with afast CCD that can collect tens of patterns per second, thus

reducing the data collection time to a few minutes in the case ofan area of 1 μm2 sampled every 5 nm.44 The recorded patternsare then compared via software with simulated patterns of thecrystal phases that should be present in the sample. In this wayit is possible to obtain a phase mapping by recognizing thephase to which each pattern belongs and an orientationmapping by determining the crystal orientation that corre-sponds to each pattern. In the Libra 120 microscope theresolution of the mapping is 10 nm because this is theminimum beam size that can be used.

3. RESULTS3.1. FTIR Spectroscopy. Figure 1 compares the CF

absorption spectra (measured at RT in the 2100−5500 cm−1

range) of sol−gel silica doped with different amounts of Eu3+

(curves b−e) with that of undoped silica (curve a). The spectrashow that when the Eu3+ concentration is increased from 0.1 to3 mol % (curves b−d), the following occur: (1) broad and weakbands due to the presence of Eu3+ grow at ∼2670, 2826, 3020,3900, 4526, and 4760 cm−1; (2) the 3673 cm−1 peak, attributedto the Si−OH stretching mode,34,45 smoothly decreases; (3) abump at ∼3600 cm−1, indicated by a vertical arrow, overlaps theSi−OH peak in the 1 and 3 mol % Eu3+ samples and is moremarked in the latter (curve d, magnified as curve a in the inset).Dramatic changes were monitored in the 10 mol % Eu3+-dopedsamples (curve e): (1) the Eu3+-related absorptions arecharacterized by rather sharp lines; (2) some features relatedto typical SiO2 absorptions are heavily modified, for example,the O−Si−O asymmetric stretching overtone at ∼2260 cm−1 46

is replaced by a more structured peak at 2238 cm−1; (3) the Si−OH peak is no longer detected; (4) the baseline increases withincreasing wavenumber, accounting for the light scatteringcaused by the milky sample (compare curves d and e in Figure1).The structures displayed by Eu3+ peaks (curve e in Figure 1)

represent a rather unusual feature for RE3+ absorptions inglasses. To get a better resolution of them, the spectra of 3 and

Table 1. Sample/CsI Weight Ratios (WRs) for the AnalyzedPelletsa

sample WR (mg/100 mg)

undoped SiO2 0.6smoky α-quartz 2.3; 19.5SiO2:Eu

3+ 3 mol % 0.66SiO2:Eu

3+ 10 mol % (milky) 2SiO2:Eu

3+ 10 mol % (transparent) 3.8Eu2SiO5 2.4; 19.6Eu2O3 2.3; 20.2Eu2Si2O7 3.8; 19.6YAlSi2O7 0.9; 9.7

aFor samples with two WRs, the lower WR was used for thefundamental vibrational mode analysis and the higher WR for overtonemodes and CF studies.

Figure 1. Optical absorption spectra measured at room temperatureon SiO2 samples doped with different amounts of Eu

3+ in the region ofthe 7F0 → 7F4,

7F5,7F6 Eu3+ transitions. Curve a, undoped sample;

curve b, 0.1 mol %; curve c, 1 mol %; curve d, 3 mol %; curve e, 10 mol%. Curves a, d, and e were taken from ref 33. Inset: curve a coincideswith curve d in the main frame; curve b is for a 3 mol % Eu-dopedsample submitted to RTT (see section 2). For clarity, some spectrahave been vertically shifted.



10 mol % Eu3+-doped samples were measured at 9 K (Figure2A, curves b and c) and compared with those of undoped silica

(Figure 2A, curve a) and a 1 mol % Eu3+-doped CdF2 singlecrystal (Figure 2A, curve d). The absorptions in the 3 mol %Eu3+-doped sample remain broad even at 9 K (curve b), whilethe 10 mol % Eu3+-doped sample shows a line-rich spectrum(curve c): with decreasing temperature, the lines becomenarrower and their number increases (compare curve c inFigure 2A with curve e in Figure 1).The 9 K measurements confirmed the considerations of

vibrational modes derived from the analysis of the RT spectraand displayed in Figure 1. In Figure 2A, the peak due to anovertone absorption of the O−Si−O asymmetric stretching(shifted from 2260 to ∼2275 cm−1 as a consequence of thetemperature decrease from RT to 9 K) in the undoped sample(curve a) is replaced by a “triplet” (peaks at 2226, 2240, and2257 cm−1) in the 10 mol % Eu3+ sample (curve c). The bandsat ∼3646 and 4521 cm−1 due to the stretching and combination(stretching + bending) mode absorptions of Si−OH groups,respectively,47 monitored in the undoped sample (curve a) arestill present in the spectrum of the 3 mol % Eu3+ sample (curveb); however, the latter is remarkably weaker than thehomologous absorption detected in the corresponding RTspectrum (compare curve b in Figure 2A with curve d in Figure1; also compare curves a and b in the bottom right panel ofFigure 9). Both bands disappear for the 10 mol % Eu3+ sample(Figure 2A, curve c). The 3600 cm−1 bump detected at RT inthe spectrum of the 3 mol % Eu3+ sample (Figure 1, curve d)contributes to the broadening and red shift of the ∼3646 cm−1

band at 9 K (Figure 2A, curve b). No trace of OH belonging towater molecules was monitored, as supported by the absence ofthe related stretching and combination-mode absorptions at∼3440 and 5260 cm−1, respectively, thus proving that the 1050°C annealing (section 2) was effective in removing water fromthe xerogel.47,48 At variance with Gd-doped sol−gel silica,where a peak at 4250 cm−1 was attributed to a Gd−OH

complex,25 no direct evidence of similar absorptions was foundin the spectra of the Eu3+-doped samples.The modifications induced by Eu3+ doping on the intrinsic

silica O−Si−O overtone mode at ∼2260 cm−1 (RT), portrayedin Figures 1 (RT) and 2A (9 K), suggested that such an analysisbe extended to the silica fundamental modes by measuring inthe 200−2000 cm−1 range the RT spectra of CsI pelletscontaining chips of doped and undoped silica samples (Table1). The results are collected in Figure 3, where spectra related

to two different fragments of the 10 mol % Eu3+-doped sample,one (curve c) being partially transparent and the other (curved) being completely milky, are compared to the spectrum of anundoped sol−gel silica (curve a). Strong changes are inducedby Eu3+ doping: additional absorptions appear, and the peaksdue to intrinsic O−Si−O vibrational modes at ∼1100, 800, and470 cm−1 experience a red shift and shape modifications. Onthe contrary, the major changes induced in the 3 mol % Eu3+-doped sample are only broad and weak shoulders peakingaround 240, 540, and 920 cm−1 (curve b).To complement the information acquired from absorption

spectra concerning the changes induced by Eu3+ on the silicaintrinsic fundamental vibration modes, microreflectance meas-urements were performed at RT in the 600−1400 cm−1 range.The spectra are collected in the top panel of Figure 4: curves band c, related to the 1 and 3 mol % Eu3+ samples, respectively,show moderate modifications with respect to the spectrum ofthe undoped sample (curve a). To better catch the shapechanges induced by 1 and 3 mol % Eu3+ doping, the spectrahave been normalized to the reflectivity maximum occurring at∼1123 cm−1. The position is in good agreement with the valueof 1122 cm−1 measured by means of a specular reflectanceaccessory on extended-area, polished, undoped fused silicasamples.46 The main features of the undoped sample (curve a)are a peak at 1123 cm−1 (peak A) and a shoulder at ∼1230cm−1 (peak B), both related to O−Si−O asymmetric stretchingmodes, and a peak at ∼780 cm−1 related to the bending mode(peak C) (see section 4.1.3). The major modifications inducedby Eu3+ doping levels up to 3 mol % are a small red shift of

Figure 2. Optical absorption spectra measured at 9 K in the region ofthe 7F0 →

7F4,7F5,

7F6 Eu3+ transitions. (A) SiO2 samples doped with

0, 3, and 10 mol % Eu3+ (curves a, b, and c, respectively) and CdF2:1mol % Eu3+ (curve d). In curve c, the background due to lightscattering (see curve e in Figure 1) has been subtracted. (B) CsIpellets containing Eu2SiO5 (curve e), Eu2O3 (curve f), and Eu2Si2O7(curve g) powders (see Table 1). For clarity, some spectra have beenvertically shifted. Figure 3. Optical absorption spectra measured at room temperature

on CsI pellets containing chips of samples or reference powders(Table 1). Curve a, undoped SiO2 sample; curve b, SiO2:3 mol % Eu3+

sample; curve c, SiO2:10 mol % Eu3+ sample (transparent fragment);curve d, SiO2:10 mol % Eu3+ sample (milky fragment); curve e,Eu2SiO5 powder; curve f, Eu2O3 powder; curve g, Eu2Si2O7 powder;curve h, smoky quartz sample. For clarity, some spectra have beenvertically shifted.



peak A and a dip occurring between peaks A and B; the shift inpeak A is nearly negligible for Eu concentrations lower than 1mol % (Figure 5). Similar results were recently reported forCe3+-, Gd3+-, Tb3+-, and Yb3+-doped silica.26

The spectrum related to the milky 10 mol % Eu3+ sample(curve d) shows a completely different pattern: peak Aseemingly disappears (or is strongly red shifted; also see

Figure 5) and new peaks grow over the whole rangeinvestigated. Such features seem particular to Eu3+ dopingbecause they do not appear in the spectra of silica doped with10 mol % Ce3+ or Tb3+, for which even the shift in peak A israther modest (see the open diamond and open star in Figure5).A nonuniform Eu3+ distribution within the sample can be

easily monitored by microreflectivity, as proven by Figure 6A

both for very low (0.001 mol %) and high (10 mol %) Eu3+

concentrations. The spectra of a perfectly transparent area anda slightly cloudy area within the 0.001 mol % Eu3+ sample areportrayed by curves b and c, respectively, and compared withthat of the undoped sample (curve a). Curves a and b coincidewithin the experimental error. However, with respect to curve a,curve c shows a meaningful increase in the 700−1100 cm−1

range that in part resembles the increase displayed in curve c inthe top panel of Figure 4, which is related to the 3 mol % Eu3+

sample. In addition, a small red shift of the 1123 cm−1 peak wasdetected (Figure 5). In the case of the 10 mol % sample, thepattern described by curve d in the top panel of Figure 4 andreported again as curve d in Figure 6A is related to the bottom,rough face of the milky sample; curve e monitors the top,smooth face of the same milky piece, while curve f is related toa small, nearly transparent chip (the poor quality of the sampleaccounts for the noise, recorded mainly on the low-wave-number side, where the dedicated detector is less sensitive).The shape of the last curve looks to be intermediate betweenthat described by curve c in the top panel of Figure 4 (3 mol %)and those portrayed by curves d and e in Figure 6A. Furtherevidence of the nonuniform distribution of Eu in sol−gel silicais supplied by Figure 6B, where the 9 K CF spectra related totwo different areas of the same 10 mol % sample are compared.Such a nonuniform Eu distribution might be related to anunpredictable early reaction of Eu during the gelation processfollowed by precipitation.

Figure 4. Top panel: RT microreflectance spectra of SiO2 samplesdoped with different amounts of Eu3+. Curve a, undoped SiO2; curve b,1 mol %; curve c, 3 mol %; curve d, 10 mol %. The spectra arecompared with the microreflectance spectrum of a natural α-quartzsample (curve e) and the diffuse reflectance spectra of Eu2Si2O7 (curvef) and smoky quartz (curve g) powders. All of the spectra arenormalized to their maximum. Bottom panel: difference micro-reflectance spectra (ΔR) relative to the spectrum of the undoped SiO2sample (curve a, top panel) for the 0.01, 1, and 3 mol % Eu3+-dopedsamples (curves h, i, and l, respectively).

Figure 5. RT peak position of the main microreflectivity band as afunction of the Eu3+ concentration in silica samples. The black solidline and open squares are related to as-prepared samples; the dashedline and open circles are for samples submitted to a further RTT (seesection 2). Solid black squares are related to cloudy or milky as-prepared samples and open squares to transparent ones. Straight linesindicate the peak positions in pure samples. For comparison, the opendiamond and open star are related to 10 mol % Tb- and Ce-dopedsilica, respectively. Inset: RT radioluminescence spectra of silica dopedwith different Eu concentrations (0.01, 0.1, 3, and 10 mol %; curves a−d, respectively). The spectra have been vertically shifted for clarity.The wavenumber ranges of the 5D0 →

7FJ (J = 0, 1, ..., 4) transitionsare indicated.

Figure 6. (A) Microreflectance spectra measured at RT on anundoped SiO2 sample (curve a) and on different areas of the 0.001 and10 mol % Eu3+-doped ones. Curve b, transparent 0.001 mol %; curve c,cloudy 0.001 mol %; curve d, bottom surface of milky 10 mol %; curvee, top surface milky 10 mol %; curve f, transparent 10 mol %. (B)Optical absorption spectra measured at 9 K on two different areas ofthe same SiO2:10 mol % Eu3+ sample in the region of the 7F0 →

7F4Eu3+ CF transition. The spectrum measured at 9 K on a Eu2Si2O7pellet (see Table 1) is also reported for comparison (curve c). Thecurves have been vertically shifted for clarity. The scale on the left isfor curves a and b, and that on the right is for curve c.



3.2. Raman Spectroscopy. Raman investigations were alsocarried out to obtain a complementary picture of the vibrationalproperties of glasses. The Raman measurements are reported inFigure 7, where also the spectra of Eu oxide (Eu2O3), Eu silicate(Eu2SiO5), and Eu pyrosilicate (Eu2Si2O7) powders andcrystalline quartz are displayed for comparison.

The Raman profile of the undoped glass (curve a) showstypical modes of amorphous silica, including the structures at440, 800, and 1060 cm−1 ascribed to rocking, bending, andasymmetric stretching of SiO2,

49 respectively, and the peaks at490 and 610 cm−1 related to symmetric stretching of fourfoldand threefold rings of silicon dioxide tetrahedra, respectively.50

These features are also observed in the spectrum of the 3 mol %Eu3+ sample (curve b) which also contains additionalvibrational modes at ∼880 cm−1. When the concentration ofEu rises to 10 mol % (curves c−e), the homogeneity of thesample is not preserved: some portions show a spectrum rathersimilar to that of SiO2:3% mol Eu3+ (curve c), while others(curves d and e) display a more complex spectrum in which amultitude of modes is clearly evidenced together with thedisappearance of the broad low-frequency structure at 440

cm−1. In addition, relative to SiO2:10% mol Eu3+, a strong peakat 465 cm−1 is detected in all three spectra.

3.3. RL Measurements. For all Eu concentrations, the RLspectra measured at RT feature 5D0 →

7FJ transitions of Eu3+,

while no evidence of Eu2+ emissions was found. Up to 3 mol %the lines are broad (curves a−c in the inset of Figure 5), whilefor the highest concentration (10 mol %) the spectrum ischaracterized by more numerous and narrower lines (curve d inthe inset of Figure 5). Such a trend confirms that alreadysuggested by the RT absorption spectra (Figure 1, curves b−e).A common feature displayed by the RT absorption and RLcurves is the transition whose final state is 7F4 (7F0 → 7F4absorption and 5D0 →

7F4 emission in Figure 1 and the Figure5 inset, respectively). The RL intensity, obtained by integrationof the RL signals in the 550−750 nm (18182−13333 cm−1)interval and plotted as a function of the Eu concentration,displays a minor maximum at about 0.01 mol % followed by amonotonic decrease and a sudden sharp increase for SiO2:10mol % Eu. The ratio of the latter intensity to that of the minormaximum is about 50.

3.4. TEM Analysis. The marked differences exhibited by theoptical spectra (absorption, microreflectance, Raman, and RL)of the 3 and 10 mol % Eu-doped silica samples suggested thatsuch samples should be analyzed by means of TEM. Crystallinenanoparticles having sizes in the 10−30 nm range could bedetected in some areas of the 3% mol sample (Figure 8a). Thepoor quality of the related diffraction patterns, due to the smallnanoparticle size, allowed the claim that they are crystalline butcould not be used to determine their crystal phases. Twodifferent characteristic typologies of nanocrystalline areas wereidentified in the 10 mol % sample, giving a direct evidence ofthe inhomogeneity pointed out by the optical spectra. Theformer is characterized by a very dense distribution ofcrystalline nanoparticles, quite homogeneous in size andshape (elongated crystals whose size is in the 30−70 nmrange), embedded in an amorphous matrix made up by lighterelements, as indicated by the darker contrast in the HAADFimages (Figure 8b). The latter displays bigger nanocrystallineparticles (variable size in the 100−500 nm range) embedded ina crystalline matrix formed by nanoparticles of different type(Figure 8c). As in the previous case, the matrix shows a darkerHAADF contrast and therefore hosts lighter chemical elements.

4. DISCUSSION

4.1. FTIR Spectroscopy. 4.1.1. Crystal Field Spectra(Including RL Results). According to the RL results, no Eu2+

Figure 7. RT Raman spectra of SiO2 samples doped with different Euconcentrations and spectra of some reference compounds forcomparison. Curve a, undoped SiO2 sample; curve b, 3 mol % Eu-doped sample; curve c, 10 mol % Eu-doped sample, transparent area;curves d and e, 10 mol % Eu-doped sample, milky areas; curve f,smoky quartz powder; curve g, Eu2SiO5 powder; curve h, Eu2Si2O7powder; curve i, Eu2O3 powder. Some curves have been verticallyshifted for clarity.

Figure 8. (a) Bright-field image of a nanoparticle-rich area in the 3 mol % Eu-doped silica sample. (b, c) HAADF images of two differentnanoparticle-rich areas in the 10 mol % Eu-doped silica sample. The contrast is proportional to the atomic number Z, and thus, the nanoparticleshost heavier elements (as Eu) than the silica matrix does (i.e., ZEu = 63, ZSi = 14, and ZO = 8). In (b) the nanoparticles are small, while those in (c)are larger. The arrow in (c) indicates the crystal on which diffraction tomography was performed.



was detected in the present glasses (section 3.3), but only Eu3+,which is a non-Kramers ion characterized by a 4f6

configuration.51 In silica, f−f electronic transitions are partiallyallowed by the “crystal field” originated by the nearestneighbors and probed by the RE3+ ion. The Eu3+ groundmanifold is the 7F0 singlet (J = 0), and the first three CFtransitions (i.e., 7F0 → 7F1,

7F2,7F3) are expected to lie at

energies below ∼2000 cm−1.51 Thus, they cannot be easilyidentified in the Eu3+-doped silica samples because of overlapwith the dominant O−Si−O vibrational absorption peaks(section 4.1.3). The separations between the 5D0 → 7F0emission and each of the 5D0 →

7FJ (J = 1−3) ones, suggestedby the RL spectra displayed in the inset of Figure 5, confirm theabove hint. Absorptions due to the CF transitions 7F0 →

7F4,7F5,

7F6 were detected, as shown by the 9 K spectra displayed inFigure 2A. They appear as broad bands in both the RT and 9 Kspectra of samples containing Eu3+ concentrations up to 3 mol% and are well-structured in the spectra of 10 mol % Eu3+-doped silica. Such a trend clearly appears even upon inspectionof the RL spectra (Figure 5 inset). The absorption bands growwith increasing Eu3+ nominal concentration, thus supportingonce more that they originate from Eu3+. The supralinear trendreported for the RT peak in the 3020−3050 cm−1 range as afunction of the Eu concentration suggests that clustering mayoccur at high Eu3+ doping levels.34

The well-structured CF spectra at the 10 mol % Eu3+ levelsuggest that RE3+ is probing nearly ordered environments, asfor Er3+ in SnO2 nanocrystals embedded in silica,52 at variancewith the behavior of other RE3+ ions (e.g., Ce3+, Tb3+, andYb3+) in silica, where the RE3+-associated absorptions remainstructureless even for concentrations higher than 3 mol %.26

The results suggest that at 10 mol % Eu3+ doping somesegregation of a Eu-rich, nearly ordered phase takes place, assupported by TEM images (Figure 8b,c).Indirect information about the separation of the lowest

excited manifold 7F1 from the ground manifold 7F0 can be

extracted by comparing curve e in Figure 1 with curve c inFigure 2A, both of which related to the 10 mol % Eu3+-dopedsample. For clarity, a magnification of the spectra in the 4410−5160 cm−1 region is portrayed in the bottom left panel ofFigure 9: the 4579 cm−1 peak appearing in the RT spectrum(curve a) is no longer detectable at 9 K (curve b). A similartrend is observed for the spectra related to the 1 and 3 mol %Eu3+-doped samples: the 4527 cm−1 peak occurring in the RTspectrum of the 3 mol % Eu3+-doped sample is stronglyreduced in the spectrum measured at 9 K (Figure 9, bottomright). Such a feature is typical of Eu-doped silica, since in apure fused silica the difference between the RT and 9 K spectrais absolutely negligible (Figure 9, bottom right, curves c and d):the ∼4520 cm−1 peak, accompanied by a lower-wavenumbershoulder, is due to an Si−OH combination mode (stretching +bending). It should be remarked that the 9 K spectra of the 3and 10 mol % Eu-doped silica confirm a limited and a negligibleconcentration of Si−OH, as more clearly displayed by Figure 1(curves d and e, respectively) and Figure 2A (curves b and c,respectively) in the region of the stronger Si−OH stretchingband (∼3670 cm−1). The results portrayed by curves a and b inthe bottom left and bottom right panels of Figure 9 suggest thatthe starting level of the CF transitions corresponding to theabsorptions at 4579 and 4527 cm−1 in the 10 and 3 mol % Eu-doped silica samples, respectively, is populated at RT but ispractically empty at 9 K. Such a level cannot be a sublevel of theground 7F0 manifold, which is not split by the CF (J = 0) andthus may be a sublevel of the first excited manifold, 7F1. TheRT separation of 251 cm−1 between the 4579 cm−1 absorptionand the first peak on its high-energy side (4830 cm−1) for the10 mol % Eu3+-doped sample provides at least a rough estimateof the position of the 7F1 manifold with respect to the groundone. The values obtained for the separation between the twolowest CF levels are ΔE1−0 = 251, ∼231, and ∼237 cm−1 in thecase of 10, 3, and 1 mol % Eu3+-doped samples, respectively,which account well for the population of the 7F1 manifold at

Figure 9. Absorption spectra at RT (curves a and c) and 9 K (curves b and d) to demonstrate the thermally activated population of the Eu3+7F1 level,as monitored by the 4579 cm−1 peak intensity. Top left: CsI/Eu2Si2O7 pellet (Table 1). Bottom left: 10 mol % Eu-doped silica. Bottom right, 3 mol% Eu-doped silica (curves a and b) and pure fused silica (curves c and d). Top right: 4579 cm−1 peak intensity (normalized to that measured at RT)vs temperature for the CsI/Eu2Si2O7 pellet (open squares) and 10 mol % Eu-doped silica (black stars); the red curve is the fit according to eq 2, andthe green curve is the fit obtained by keeping fixed ΔE1−0 = 251 cm−1 and g1 = 18.



RT and are in agreement with a 240 cm−1 separation betweenthe 7F0 →

5D0 and7F1 →

5D0 fluorescence excitation peaks inthe visible range of sol−gel silica doped with 2 mol % Eu.32

Unfortunately the present RL spectra do not provide a clear-cutvalue of the separation between the 5D0 →

7F0 and5D0 →

7F1emissions because of their partial overlapping in silica samplesdoped at Eu concentrations of ≤3 mol % (curves a−c in theFigure 5 inset) and the uncertain identification of the 5D0 →7F0 emission in the 10 mol % Eu3+-doped sample (curve d).Similar analyses of fluorescence spectra performed on varioussilicates provide values for the first Eu3+ CF transition (7F0 →7F1) that fall in the 175−284 cm−1 range.53−55 Indeed, peaksbetween 250 and 300 cm−1 are monitored in the spectrum ofthe present 10 mol % Eu3+-doped silica pellet (Figure 3, curved), and a broad band appears around 240 cm−1 in the 3 mol %Eu3+ one (Figure 3, curve b); these should be ascribed to Eubecause they are absent in the pure silica pellet (curve a).However, they cannot be unequivocally attributed to a Eu3+ CFtransition (7F0 →

7F1) because they may be hidden under themuch stronger absorptions related to Eu−O group vibrations(section 4.1.3), as shown by the spectra of Eu2O3, Eu2SiO5, andEu2Si2O7 pellets (Figure 3, curves f, e, and g, respectively).According to calculations, the “terminal bending modes” ν6(A1)and ν21(B1) are expected to fall around 273 cm−1 in type-Ipyrosilicates, as Eu2Si2O7 is.

56

The sharpest among the Eu3+-related lines portrayed inFigure 2A by curve c (10 mol % Eu3+) is characterized by a fullwidth at half-maximum (fwhm) of about 10 cm−1, which ismuch less than that of the broad bands displayed by curve b (3mol % Eu3+) but larger than that (∼3.5 cm−1) exhibited bysome of the Eu3+ lines in a single crystal of CdF2 (curve d).This suggests that, in the heavily doped sol−gel silica, Eu3+probes environments that are ordered to some extent but not asordered as those experienced in an extended single crystal, assupported by the TEM results (section 4.3). Moreover, thenumber of lines originated by a given 7F0 →

7FJ transition (J =4−6) is much higher than that expected: the ground state is notsplit by the CF (J = 0), and thus, the maximum number of linespredicted at 9 K for Eu3+ probing a low-symmetry environmentis 2J + 1, where J labels the excited manifold reached by thetransition.57 For example, in the region of the 7F0 → 7F5transition (3700−4000 cm−1),51 many more lines are observedthan the expected number of 11. Similar considerations can beapplied to the lines arising from the 7F0 → 7F4 transition(2600−3200 cm−1):51 the number of lines exceeds thepredicted number of 9 (Figure 6B and Table S2 in theSupporting Information). This means that Eu3+ experiencesdifferent surroundings, as also supported by TEM findings(section 4.3).The supralinear dependence of a Eu3+ band amplitude (in

the region of the 7F0 →7F4 transition) on the nominal Eu3+

concentration indicates that Eu3+ clustering occurs in the 0.1−3mol % Eu doping range,34 as also expected from moleculardynamics simulations29 and proved by the small nanoparticlesevidenced by the TEM image in Figure 8a for a 3 mol % Eu-doped silica sample. The narrow line spectra monitored in the10 mol % Eu-doped silica (Figure 1, curve e and Figure 2A,curve c) suggest that likely a separate Eu-containing phase ispresent, as also supported by the HAADF image in Figure 8c.Possible candidates are Eu oxyorthosilicate (Eu2SiO5), Eudisilicate or pyrosilicate (Eu2Si2O7), and Eu oxide (Eu2O3).Thus, the spectra of CsI pellets containing powderedpolycrystalline Eu2SiO5, Eu2Si2O7, and Eu2O3 (curves e, f,

and g, respectively) are collected in Figure 2B for comparison.The spectra of the three polycrystalline compounds shownarrow lines, as does the spectrum of 10 mol % Eu3+-dopedsilica (Figure 2A, curve c). The Eu3+ line positions in silicashow a good correspondence with those of Eu2Si2O7, and ascarce one with either Eu2SiO5 or Eu2O3 (see Table S2 in theSupporting Information). The root-mean-square (rms) devia-tion of the Eu3+ line positions in silica from the correspondingones in Eu2Si2O7 pyrosilicate is only 2.3 cm−1 for the set of 27lines detected in the 2500−5100 cm−1 range. The similarity ofthe related spectra is impressive: an example is supplied inFigure 6B, where the spectra of the SiO2:10 mol % Eu3+ sample(curve b) and the Eu2Si2O7 pellet (curve c) are compared in theregion of the 7F0 →

7F4 Eu3+ transition. In addition, the two

sets of lines show the same blue shift as a function oftemperature. An example is portrayed in the inset of Figure 10for three lines, while the main frame provides a magnificationfor the 3044.5 cm−1 one.

Moreover, the separation between the two lowest 7F0 and7F1

levels can be estimated as ΔE1−0 = 250.5 cm−1 for the Eupyrosilicate by comparing the RT and 9 K spectra of aEu2Si2O7/CsI pellet (Figure 9, top left, curves a and b), andfollowing the procedure described above for Eu-doped silicasamples. This value practically coincides with the value of 251cm−1 obtained for the 10 mol % Eu3+-doped silica sample(Figure 9, bottom left). Furthermore, the 4579 cm−1 peakintensity at a given temperature T, normalized to that measuredat RT, shows the same temperature behavior for both the Eupyrosilicate pellet and the 10 mol % Eu3+-doped silica sample,as displayed by open squares and black stars, respectively, in thetop right panel in Figure 9.The ratio of the populations of the 7F1 and

7F0 levels, n1/n0,as a function of temperature T can be written as

Figure 10. The inset shows the line positions of three absorptionpeaks common to 10 mol % Eu-doped silica (solid symbols) and theCsI/Eu2Si2O7 pellet (open symbols; see Table 1 for composition) asfunctions of temperature. The solid lines in the inset show the averagepeak positions at 9 K. In the main frame, a magnification of the curvefor the 3044.4 cm−1 peak position ω is shown. The solid line is the fitof ω(T) data according to the single-phonon coupling model (see theformula provided, in which δω is the coupling constant and ωph is thecoupled phonon).



= −Δ −n Tn T

g E k T( )( )

exp( / )1

01 1 0 B

(1)

where g1 is the degeneracy of the 7F1 level (the 7F0 isnondegenerate), kB is the Boltzmann constant, and n0(T) +n1(T) = N is a constant under the assumption that only the 7F0and 7F1 levels are populated over the temperature rangeconsidered (the next level, 7F2, falls ∼1000 cm−1 above 7F0according to Carnall58 for Eu3+(aq) ions and to the present RLspectra, e.g., curve c in the Figure 5 inset). Thus, eq 1 becomes

=−Δ

+ −Δ−

−n T N

g E k T

g E k T( )

exp( / )

1 exp( / )11 1 0 B

1 1 0 B (2)

The red solid curve in the top right panel of Figure 9 provides asatisfactory fit of the experimental data according to eq 2 underthe hypothesis that the normalized 4579 cm−1 peak intensity atgiven temperature T is proportional to n1(T). The parametersΔE1−0 = (240 ± 30) cm−1 and g1 = 11.6 ± 6.1 can be extracted.The ΔE1−0 value of (240 ± 30) cm−1 is in reasonableagreement with those directly obtained from the spectra (i.e.,251 and 250.5 cm−1 for the 10 mol % Eu-doped sample and theEu pyrosilicate pellet, respectively). The g1 value requires amore detailed analysis: (1) both of the natural Eu isotopes,151Eu and 153Eu, whose natural abundances are 47.8 and 52.2%,respectively, are endowed with a nuclear spin I = 5/2, whichcontributes a factor of 2I + 1 = 6 to the degeneracy; (2) J = 1for the 7F1 level, which contributes a factor of 3 to thedegeneracy. Thus, g1 = 18 is expected. In view of (1) the ratherlarge error (±6.1) in the g1 value extracted from the fit, (2) theapproximations introduced, (3) the rather weak signals, mainlyat low T, and (4) the limited number of experimental data, theagreement is not too bad. The green dashed curve in the topright panel of Figure 9, which simulates the trend of eq 2 bykeeping ΔE1−0 fixed at 251 cm−1 (the value deduced from thespectra; see Figure 9, top left panel) and g1 fixed at 18, does notdeviate significantly from the experimental data.These results strongly suggest that phase separation occurs in

the 10% mol Eu3+-doped silica: Eu2Si2O7 crystalline inclusionsrather than generic Eu3+-rich clusters are present in the glass.The finding agrees also with the molecular dynamicssimulations,29 which envisaged in 4−5 mol % Eu-doped silicathe onset of Eu clusters where the number and the distancebetween Eu3+−Eu3+ neighbors have the typical values exhibitedby crystalline Eu2Si2O7. However, at those relatively dilute Euconcentrations, such clusters remain only precursors of smallcrystals, which form in the present 10 mol % Eu-doped sampleand have been identified by CF spectra. Electron diffractionanalysis (see section 4.3) proves that the big nanoparticle in the10% mol Eu3+-doped silica (indicated by the arrow in Figure8c) is indeed a Eu2Si2O7 single crystal. Further support comesfrom the RL efficiency: in amorphous silica, the maximum isreached within a rather low Eu concentration range (between0.01 and 0.1 mol %) and is followed by a monotonic decrease,possibly due to concentration quenching. The crystallizationoccurring in the 10 mol % Eu-doped silica causes an increase ofnearly 2 orders of magnitude in the RL efficiency (section 3.3).Such an increase may be attributed to the higher crystallineorder experienced by Eu3+ ions. In fact, point defects present inthe amorphous silica can favor energy transfer processes fromEu3+ excited levels and, as a consequence, nonradiative de-excitations. The much higher RL efficiency could also be relatedto the remarkably higher density (ρ) of the Eu-rich phase (for

Eu2Si2O7, ρ = 6.23 g/cm3) with respect to that of SiO2 (ρ = 2.2g/cm3). This leads to stronger absorption of X-rays within thenanoparticles than in the matrix, leading to a higher probabilityof excitation of the incorporated Eu3+ ions.The number of CF lines detected in the heavily Eu-doped

silica is, however, larger than that monitored in the pyrosilicatepellet (Table S2 in the Supporting Information), which meansthat an additional minor amount of Eu3+-related species maycontribute to the CF spectra. An example is the set of linesdisplayed in the dashed frame for curve a in Figure 6B, whichwere exhibited by a selected area of the 10% mol Eu3+-dopedsilica but not by the pyrosilicate pellet (curve c). In addition tothe Eu pyrosilicate phase, electron diffraction analysis identifiedthe presence of hexagonal Eu5(SiO4)3O, which might beresponsible for minor contributions to the CF spectrum (seesection 4.3).According to section 4.1.3, in the 10 mol % Eu3+-doped

sample the phase separation does not involve only Eu2Si2O7 butalso α-quartz inclusions; thus, some of the supplementary CFlines might be originated by Eu in quartz. However, thisremains a hypothesis since no reference data are available on Euabsorption lines in quartz in the present considered spectralrange.

4.1.2. Role of Eu3+ Doping Level on OH Concentration. Asthe Eu3+ concentration increases from 0.1 to 10 mol %, the Si−OH peak at 3670 cm−1 decreases, becoming no longerdetectable in the 10 mol % sample (Figure 1). The OHconcentration (COH) was evaluated from the amplitude and/orthe subtended area of the 4521 cm−1 peak,45 and the analysiswas extended to still lower Eu3+ doping levels (down to 10−3

mol %). An initial increase in the OH content with respect tothat monitored in the undoped sample (C0,OH ≈ 0.4 mol %;curve a), is followed by a decrease that becomes more evidentfor Eu3+ concentrations ranging between 1 and 10 mol %(Figure 1). The maximum OH content (Cmax,OH ≈ 1.4 mol %)is found in the 0.03 mol % Eu3+-doped sample. Similar trendshave been detected for sol−gel silica samples doped withincreasing concentrations of Ce3+ and Yb3+.26 However, evenheavy Ce3+ doping (10 mol %) did not cause the completevanishing of the Si−OH band, as in the present case (Figure 1,curve e), confirming the peculiar features of the 10 mol % Eu3+

sample. A decrease in the Si−OH and H2O content as afunction of Pr3+ doping level (in the 2 × 10−4 to 1 mol %range) has also been reported for silica xerogel pellets annealedat 900 °C,36 but such a temperature was apparently too low toremove water completely (section 3.1).The trend of COH versus Eu3+ concentration confirms once

more the interpretation already supplied for other RE3+ ions.26

At low doping levels, Eu3+ is present as an isolated defect in theglassy matrix: it substitutes for Si4+ and thus requires chargecompensation, which may be easily offered by substitution ofOH− for O2−. At higher doping levels, Eu clustering starts untilphase separation may occur, as clearly supported by the CFspectra (section 4.1.1), vibrational spectra (section 4.1.3), andTEM images (Figure 8). Under these circumstances, chargecompensation provided by OH is no longer necessary, andthus, the Si−OH content decreases again because OH groupscan be desorbed during the annealing at 1050 °C. Recentmolecular dynamics modeling of Eu3+ in an ideal OH-free silicashowed that a few Eu3+ pairs are already present at Eu3+

contents as low as 1 mol % in addition to dominating isolatedEu3+; increasing the doping level results in the formation of



large Eu3+ clusters at the expense of both Eu3+ pairs andisolated Eu3+.29

The bump at 3600 cm−1 overlapping the Si−OH peak in theRT spectra of the 1 and 3 mol % Eu3+-doped samples and beingmore marked in the latter (Figure 1, curves c and d) is absent inundoped silica (curve a). This suggests that Eu3+ may beresponsible for it. Two hypotheses may be put forth to explainits origin. The former attributes the bump to a Eu3+ CFtransition, while the latter associates it to an OH (or Si−OH)group perturbed by a neighboring Eu3+. If the bump were dueto a Eu3+ CF transition, it should be present as a well-isolated,rather strong peak in the 10 mol % Eu3+-doped silica spectrum,in which no disturbing overlap with the Si−OH band occurs;however, this is not the case, as proved by curve e in Figure 1and curve c in Figure 2A. Moreover, no Eu3+ CF line wasmonitored around 3600 cm−1 in any of the Eu3+ compoundsinvestigated in the present work (Table S2 in the SupportingInformation). The latter hypothesis requires the simultaneouspresence of Eu3+ and OH (or Si−OH) groups: this is the casefor the 1 and 3 mol % Eu3+-doped samples (Figure 1, curves cand d), but not for the 10 mol % Eu3+-doped silica, where OHis absent (Figure 1, curve e). An interaction between OHgroups and Eu3+ is well-known to take place in sol−gel silica: itcauses Eu3+ fluorescence quenching and favors hole burningwithin the Eu3+ 5D0 → 7F0 emission line.59 Thus, the 3600cm−1 bump monitored in the 1 and 3 mol % Eu3+-dopedsamples can be reasonably attributed to a Si−OH stretchingmode perturbed by one or more neighboring Eu3+ ions. Furthersupport for this interpretation is supplied by the inset in Figure1. The bump present in the 3 mol % Eu3+-doped sample (curvea) vanishes as a consequence of an RTT (curve b): the high-temperature treatment followed by fast quenching (section 2)breaks the Eu−Si−OH complexes responsible for the bump at3600 cm−1 and causes a moderate increase in the Si−OHabsorption at 3670 cm−1 (compare curves a and b in the insetof Figure 1).4.1.3. IR Absorption Spectra: Fundamental Vibrations.

The fundamental vibrational modes of O−Si−O groups areresponsible for the “reststrahlen” monitored by absorption andmicroreflectance spectra of silica samples, as displayed inFigures 3, 4, and 6A. The modes and the related transverseoptical (TO) and longitudinal optical (LO) frequencies inundoped silica are the rocking modes at ∼460 cm−1 (TO1) and∼505 cm−1 (LO1), the bending modes at ∼810 cm−1 (TO2)and ∼820 cm−1 (LO2), and the asymmetric stretching (AS)modes at ∼1075 cm−1 (TO3) and ∼1255 cm−1 (LO3).

60 Twodifferent AS modes have been distinguished according to thetwo Si-adjacent oxygens oscillating either in phase (AS1) or outof phase (AS2), respectively. The disorder typical of a glassmatrix causes a mechanical coupling of the two AS modes, thusintroducing an additional pair of LO and TO modes at ∼1165cm−1 (LO4) and ∼1200 cm−1 (TO4) with an inversion withrespect to the ordinary sequence of the LO and TOfrequencies.61 No meaningful differences in the peak positionsand attributions were found for densified sol−gel silica withrespect to fused silica.62 In the present work, to get a morecomprehensive picture of the vibrational modes in undopedand Eu3+-doped silica, the spectra were taken even in theregions where overtones and combinations of the fundamentalmodes are expected to originate absorptions (Figure 11).Figure 3 summarizes the changes induced by the higher Eu3+

doping levels (i.e., 3 and 10 mol %) in the region offundamental vibration modes of O−Si−O groups, as monitored

by measuring the absorption spectra of CsI/sample pellets.With respect to the undoped sample (curve a), the 3 mol %Eu3+ doping causes a moderate red shift of the main peak at∼1102 cm−1 and the appearance of a shoulder peaking around920 cm−1 (curve b). Such a structureless band closely recallsthose reported for 10 mol % Tb3+-doped and 8 mol % Gd3+-doped sol−gel silicas at ∼940 and 880 cm−1, respectively.25,26

Quite different patterns are exhibited by the spectra measuredon pellets containing chips of 10 mol % Eu3+-doped silica,related to two different portions of the same sample, the formerbeing partially transparent and the latter being completelymilky (curves c and d, respectively). In addition to a moremarked red shift of the main peak from ∼1102 cm−1 in theundoped sample (curve a) to ∼1096 cm−1 in the transparentsample (curve c) and ∼1077 cm−1 in the milky sample (curved), the main changes are represented by the appearance of new,rather narrow peaks. Lowering the temperature of the milkysample caused such peaks to sharpen gradually and look moreresolved, as usually occurs in the absorption spectra of orderedstructures (inset in Figure 11A for the 750−950 cm−1 range).The result agrees with that coming from the analysis of the CFand overtone spectra (sections 4.1.1 and 4.1.4, respectively)and emphasizes once more that at the 10 mol % Eu3+ dopinglevel, segregation of a nearly ordered phase takes place withinthe silica matrix, as supported by TEM analysis (section 4.3).As for the CF spectra, the vibrational ones were compared

with those of CsI pellets containing powdered polycrystallineEu2SiO5, Eu2O3, and Eu2Si2O7 (Figure 3, curves e, f, and g,respectively). The similarity of the additional peaks displayed inthe 290−1130 cm−1 range by 10 mol % Eu3+-doped samples(curves c and d) to those of the Eu2Si2O7 pellet (curve g) isevident, as also supported by considering the correspondencesindicated in Table S3 in the Supporting Information, whichextends to the overtone and combination-mode ranges. Therms deviation of the line positions of 10 mol % Eu3+-dopedsilica from those of Eu pyrosilicate is only 2.9 cm−1 for the setof 26 lines detected in the 290−2500 cm−1 range. Fewercorrespondences or none at all can be found with the Eu2SiO5or Eu2O3 pellets (curves e and f; see Table S3).

Figure 11. Optical absorption spectra measured at 9 K in the regionsof the vibrational overtones and combinations. Curves a, SiO2:10 mol% Eu3+; curves b, Eu2Si2O7 pellet; curves c, smoky quartz pellet (seeTable 1 for the pellet composition). Curves have been vertically shiftedfor clarity. (A) Δn = 2 region; (B) Δn = 3 region. The inset in (A)shows the absorption spectra of the SiO2:10 mol % Eu3+ sample in theregion of fundamental vibrational transitions (Δn = 1) measured atdifferent temperatures (from bottom to top: 9, 80, 150, 220, and 300K). For clarity, some spectra have been vertically shifted.



It should be noted that there is a strong and a prioriunexpected similarity between many peaks displayed in Figure3 in the 265−2000 cm−1 range by pellets of 10 mol % Eu3+-doped silica (curve d) and those of crystalline α-quartz (curveh), as also supported by the correspondences indicated in TableS3 and by TEM analysis (section 4.3). Unequivocal signaturesof the presence of α-quartz are two doublets at 367−394 and780−800 cm−1 and the peak at ∼700 cm−1 (compare curves dand h in Figure 3): such features are absent in IR spectra ofanother form of crystalline SiO2 (i.e., α-cristobalite).

63,64 Thetight correspondence extends to the overtone and combinationmodes, as monitored in massive 10 mol % Eu3+-doped silicaand α-quartz samples (Table S3). The rms deviation of the linepositions in 10 mol % Eu3+-doped silica from the correspondingones in α-quartz is only 2.1 cm−1 for the set of 30 lines detectedin the 265−2450 cm−1 range.The lines attributed to α-quartz show a weak (nearly

negligible) red shift with increasing temperature, at variancewith the blue shift recorded for the CF lines attributed to Eupyrosilicate (Figure 10). These results suggest that at high (10mol %) Eu3+ doping levels, phase separation occurs with theformation of nanocrystals of pyrosilicate and of α-quartz: it ispossible to account for the main features of absorptionspectrum of the milky 10 mol % Eu3+-doped silica (curve d)by superimposing the spectra due to Eu pyrosilicate (curve g)and α-quartz (curve h). The Raman results (section 4.2)portray the same picture, as shown by a comparison of bothcurves d and e with curves h and f in Figure 7.The spectrum related to partially transparent 10 mol % Eu3+-

doped silica (Figure 3, curve c) looks to be intermediatebetween those of 3 mol % Eu3+-doped and milky 10 mol %Eu3+-doped silica (curves b and d, respectively): the onset ofphase separation is monitored by a weak α-quartz doublet at394−460 cm−1 and by Eu silicate absorptions peaking at ∼570cm−1 and in the 750−950 cm−1 range, which overlap thedominant spectrum of pristine silica. The structureless, weakshoulder around 920 cm−1 (curve b) falls in the same range asthe high-energy vibrational modes of Eu2SiO5 and Eu2Si2O7(curves e and g). It may be attributed to generic Eu silicate-based nanoclusters/nanocrystals whose phase could not bedetermined because of their small dimensions (see section 3.4for 3 mol % Eu3+-doped silica), as for heavily Tb- and Gd-doped silica.25,26 In the partially transparent 10 mol % Eu3+-doped silica (curve c), some peaks resembling those of curve dbut less defined are superimposed on a broad shoulder in the750−1000 cm−1 range, recalling that displayed by curve b, asexpected for a sample where the Eu3+ distribution is apparentlyinhomogeneous and where amorphous and nearly orderednanoclusters/nanocrystals of different sizes may coexist. On thelow-energy side, a broad peak at ∼567 cm−1 appears in thespectra of both the 3 mol % Eu3+-doped silica and the partiallytransparent 10 mol % Eu3+-doped one (curves b and c,respectively) and is more marked in the latter. It recalls peaksdisplayed by Eu2SiO5 and Eu2Si2O7 in the 520−600 and 500−620 cm−1 ranges (curves e and g), respectively. Peaks at 530and 551 cm−1 monitored in Eu pyrosilicate are attributed toterminal bending modes [ν5(A1)].

56 It should be remarked thatjust a (554.5 ± 20.2) cm−1 phonon is responsible for thethermally induced shift of the CF line peaking at 3044.5 cm−1,as determined by the fit (Figure 10, black line) to theexperimental data (Figure 10, solid and open triangles for 10mol % Eu-doped silica and Eu pyrosilicate, respectively)according to the single-phonon coupling model65,66 (formula in

Figure 10). A similar feature (at ∼554 cm−1) was reported for10 mol % Tb3+-doped silica and attributed, together with the∼940 cm−1 shoulder, to amorphous Tb clusters with acomposition similar to that of Tb oxyorthosilicate.26

4.1.4. NIR Absorption Spectra: Anharmonicity. Furtherinsight into the stretching modes of SiO4 tetrahedra within theseparate phase inclusions in the milky sample can be derived byinspecting the related spectra at higher wavenumbers, in thespectral regions where the weaker absorptions due tocombination and overtone modes are expected. Figure 11A,Bsummarizes the results in the 1400−2400 and 2400−3000 cm−1

(Δn = 2, 3) ranges, respectively, where n is the quantumnumber labeling the energy levels of an anharmonic oscillator.The anharmonicity of high-energy stretching modes of SiO4tetrahedra has been demonstrated, for example, in sillenitesingle crystals.67 As shown in Figure 11A, at the 9 K the milky10 mol % Eu3+-doped silica displays a well-structured, peak-richspectrum extending below 2300 cm−1 (curve a), which closelyresembles that exhibited by α-quartz (curve c), confirming oncemore the formation of α-quartz inclusions in the 10 mol %Eu3+-doped silica. The peaks below 2300 cm−1 (curve a) aretoo strong to be attributed to the partially forbidden Eu3+ CF7F0 → 7F3 transition (falling in the 1750−1900 cm−1

range);51,58 more likely they are associated with the firstovertones (Δn = 2) of modes whose fundamental absorptionsfall in the 820−980 cm−1 range (Δn = 1) for the CsI/10 mol %Eu3+-doped silica and α-quartz pellets (Figure 3, curves d and hand Figure 11A inset). Also, combinations of two differentmodes may contribute to the 1500−1900 cm−1 spectrum if atleast one of them is IR-active.67 Curve b in Figure 11A, which isrelated to a CsI/Eu2Si2O7 pellet, shows that overtone orcombination modes due to Eu pyrosilicate may also contributeto the spectrum of the milky silica sample (curve a); however,the role of α-quartz modes seems dominant. For clarity andsimplicity, the spectra of CsI/Eu2SiO5 and CsI/Eu2O3 pellets,which were measured as well, are not reported in Figure 11A, asthey do not provide any useful additional information.Higher-order overtones (Δn = 3) and combinations of three

modes (at least one of which is IR-active)67 may be responsiblefor the much weaker absorptions in the 2100−3000 cm−1

region. These partially overlap the peaks originating from theEu3+ CF 7F0 →

7F4 transition, which appear in the 2600−3200cm−1 range according to ref 51 and Table S2 in the SupportingInformation (see curves a, b, and c for massive 10 mol % Eu3+-doped silica, Eu2Si2O7, and smoky quartz pellets, respectively,in Figure 11). The peak at ∼2138 cm−1, which is commonbetween the spectra of the milky silica and the α-quartz pellet(compare curves a and c in Figure 11A), is an example and canbe attributed to a second overtone (Δn = 3) of a quartz modeat ∼780 cm−1 (also see Figure 12A). The set of peaks appearingbetween 2220 and 2700 cm−1 in the milky silica sample cannotbe ascribed to CF transitions but rather are due to Δn = 3vibrational transitions. In fact, the literature data available forEu3+ in a variety of materials (e.g., LiNbO3, YAG, LaCl3,KY3F10, and YVO4,)

68−72 show that no Eu3+ transition occurs inthat range. Furthermore, spectra taken on an undopedpyrosilicate (YAlSi2O7) sample show similar peaks, whichobviously cannot be attributed to Eu3+ CF transitions but ratherare due to pyrosilicate vibrational modes. Curve a in Figure11B, related to the milky silica, resembles quite closely that ofthe CsI/Eu2Si2O7 pellet (curve b), although several minorcontributions due to overtone or combination vibrationalmodes of α-quartz cannot be excluded (curve c). For example



the peak at ∼2595 cm−1, common between milky silica and Eupyrosilicate, can be regarded as a Δn = 3 vibrational transitionof the pyrosilicate mode at ∼935 cm−1 (Figure 11; also seeFigure 12C).Possible associations among the fundamental frequencies

(ω1, Δn = 1, measured on pellets; see Figure 3 and the Figure11 inset) and the related overtones (ω2, Δn = 2; ω3, Δn = 3,measured on the massive sample; see Figure 11) for a set ofselected modes in 10 mol % Eu3+-doped silica, α-quartz, and Eupyrosilicate have been found (Table S4 in the SupportingInformation). If the associations are correct, in the frameworkof the Morse anharmonic oscillator model, a plot of the ratioωn/n versus n should give a straight line67,73 according to theequation

ωω ω= − +

nx n( 1)n

0 0 e (3)

where ω0 is the natural oscillation frequency and xe is theanharmonicity parameter. The linear dependence was foundindeed for a few vibrational modes (Figure 12). Very goodoverlap was found between the data related to α-quartz (at 9 Kand RT) and the milky silica sample (at 9 K) in the case of the∼780 cm−1 mode for Δn = 1, 2, and 3 (Figure 12A). Theanharmonicity parameters are in the (3.5−4) × 10−2 range,which is higher than the value found for SiO4 stretching modesin Bi12SiO20 sillenite (5.9 × 10−3).67 Such a difference can beunderstood as follows: In sillenite, the SiO4 groups behavepractically as noninteracting Einstein oscillators (and thus areless anharmonic) because they are isolated from each other bythe “wall” represented by the heavy Bi network. This is atvariance with silica, where SiO4 groups constitute the backboneof the material. The anharmonicity is also little higher than thatreported for silanol stretching mode in synthetic silica (xe = 2.1× 10−2);74 this is a terminal Si−OH group, thus enabling thelight H atom to vibrate nearly decoupled from the silicanetwork.From the anharmonicity parameter xe, the binding energy De

can be evaluated as

ω= · −D x(4 )e 0 e1

(4)

where De is a parameter appearing in the expression of theMorse potential as a function of distance r:

β− = − − −U r r D r r( ) {1 exp[ ( )]}e e e2

(5)

in which re is the equilibrium distance and β is the widthparameter.73 The De values so obtained (listed in Table S4 inthe Supporting Information) range between 0.6 and 0.9 eV; as aconsequence of eq 4 and the larger anharmonicities, they aremuch lower than both the value of 4.69 eV found for thestretching modes of the well-isolated SiO4 groups in Bi12SiO20sillenite67 and the value of 5.89 eV estimated for the Si−OHgroups in synthetic silica. In molecular dynamics simulations ofthe structural properties of oxides (vitreous silica included) andsilicates, the Morse function (eq 5) has also been used to modelthe short-range contribution of a pairwise interatomic potentialusing the parameter Dij (replacing the above-mentioned De) toindicate the interaction between two atoms i and j. Differentauthors have employed for the Si−O pair Dij values of 0.34

29,75

and ∼2 eV,76 respectively. Such values average all possible Si−O vibrational modes, while in the present case specific modesare selected, for example, the α-quartz bending mode at 780cm−1, for which De = 0.66 eV (Table S4).

4.1.5. IR Microreflectivity Spectra: Fundamental Vibra-tions. Reflectivity spectra have in the recent past been exploitedto monitor the effects produced by increasing doses ofimplanted ions (e.g., Ar, N, Cu) into silica films. They consistof a shift of the vibrational frequencies to lower values and adecrease and a broadening of the peaks; subsequent sampleannealing at high temperature causes a partial recovery of thespectra.60 The shift and the shoulder appearing at ∼1040 cm−1

are attributed to Si−O−Si bond breaking.77 The micro-reflectivity results, summarized in the top panel of Figure 4for samples doped with increasing amounts of Eu3+, suggestthat doping up to 3 mol % Eu3+ causes only gradual changes inthe shape of the spectrum, mainly in the region of AS stretchingmodes. The difference spectra for 0.01, 1, and 3 mol % Eu3+-doped silica with respect to that of the undoped sample (curvea), displayed in the bottom panel of Figure 4, show that withincreasing Eu3+ amount the normalized reflectivity decreasesmainly in two spectral regions: that of the pure silica LO3frequency and that of the TO4 and LO4 frequencies (curves h−l). The LO modes are sensitive to long-range electrostaticinteractions: the presence of the Eu3+ ions (which maysubstitute for Si4+)29 modify such an interaction, decreasingthe number of pristine O−Si−O active modes. The LO3 modecontribution starts to be weakened at Eu3+ concentrations aslow as 0.01 mol % (curve h). In the case of the LO4 and TO4modes, which arise from the mechanical coupling induced bydisorder,61 Eu3+ may play a similar role by interrupting theinteraction sequence, thus decreasing their statistical weight.Similar results were obtained for Tb3+-, Ce3+-, Gd3+-, and Yb3+-doped silica.26

The smooth increase in the normalized microreflectivity inthe 700−1000 cm−1 range induced by increasing the Eu3+

doping level up to 3 mol % (curves b, c, i, and l in Figure 4)recalls the broad shoulder peaking around 920 cm−1 exhibitedby the absorption spectrum of the CsI/3 mol % Eu3+-dopedsilica pellet (curve b in Figure 3). It may be attributed tovibrational modes of Eu-related clusters with a compositionsimilar to that of Eu2Si2O7 (the diffuse reflectance spectrum ofEu2Si2O7 powders is portrayed as curve f in the top panel ofFigure 4 for comparison).

Figure 12. Plots of ωn/n vs n for sets of fundamental (n = 1), firstovertone (n = 2), and second overtone (n = 3) frequencies of a fewvibrational modes monitored at 9 K for the 10 mol % Eu3+-doped silicasample (solid black squares, panels A−C), compared to those of quartz(solid red circles, panel A) and of Eu2Si2O7 pellet (open blue circles,panels B and C). For pellet compositions, see Table 1. The green openstars in panel A are related to quartz modes measured at RT. The full,dashed, dotted, and dot-dashed lines are the linear fits of the solidsquares, solid circles, open stars, and open circles data according to eq3.



As for the absorption, RL, and Raman spectra (Figures 1−3,the Figure 5 inset, and Figure 7, respectively), the micro-reflectivity spectra undergo amazing changes as the silicasamples are doped with a Eu3+ concentration as high as 10 mol% (curve d in Figure 4 and curves d and e in Figure 6A). A setof well-structured peaks appears between 820 and 1500 cm−1

(i.e., in the region of Eu2Si2O7 vibrational modes; comparecurves d and f in Figure 4). The detailed pattern of theseadditional peaks depends on the specific area of the milkysample investigated (compare curves d and e in Figure 6A);such differences can be caught also by examining the CFabsorption spectra in the 3200−3400 cm−1 region (comparecurves a and b in Figure 6B). Although the above-mentionedinhomogeneous dopant distribution in a heavily doped samplecan be easily justified in terms of exceeding the solubility limit,it should be noticed that some departure from uniformity wasoccasionally detected for Eu3+ concentrations as low as 0.001mol % (e.g., compare curves b and c in Figure 6A).As supported by the absorption spectra (Figures 3 and 11)

and Raman spectra (Figure 7), some peaks are common to 10mol % Eu-doped silica and α-quartz (see the correspondencesindicated in Tables S3 and S4 in the Supporting Information).Such coincidences are confirmed by the microreflectancespectra: the dip at ∼1160 cm−1 and the two well-resolvedpeaks at 780 and 796 cm−1 are features common tomicroreflectance spectra of an α-quartz single crystal and 10mol % Eu-doped silica (compare curves d and e in the toppanel of Figure 4). At variance, only a single peak at 790 cm−1

appears in the α-cristobalite reflectance spectrum.78 Since theα-quartz inclusions in 10 mol % Eu-doped silica are expected tobe randomly oriented, the microreflectance spectrum of 10 mol% Eu-doped silica (Figure 4, top panel, curve d) was comparedto the weak diffuse reflectance spectrum of α-quartz powders(Figure 4, top panel, curve g): they look very similar mainly inthe 1130−1220 cm−1 range, where the diffuse reflectanceaccessory exhibits better performance. The results emphasizeonce more that phase separation occurs within the doped silicawith the formation of α-quartz and Eu2Si2O7 nanocrystals. As inthe IR absorption spectrum (Figure 3, curve d), the intrinsicsilica AS modes are heavily modified by the 10 mol % Eu3+

doping: the reststrahlen peak at 1123 cm−1 (undoped sample,Figure 6A, curve a), whose position remains nearly constant forEu concentrations up to 1 mol % (Figure 5), shifts to ∼1115and 1116 cm−1 for the smooth face of the milky piece and thepartially transparent chip (Figure 6A, curves e and f,respectively), while two additional side peaks (a doublet)appear at ∼1076 and 1091 cm−1 (curve e). The micro-reflectivity spectrum of the rough face of the milky piecedisplays a unique broad peak at ∼1078 cm−1 (Figure 6A, curved). In all cases, the high-energy contribution to the pristinesilica reststrahlen appears to be strongly reduced. In addition tothe presence of Eu2Si2O7 inclusions, it seems that even theresidual silica network has been heavily modified. The red shiftsin both the main IR absorption at ∼1100 cm−1 and themicroreflectance peak at ∼1123 cm−1 induced by Eu doping of≥3 mol % may be attributed to both the larger atomic mass ofdispersed Eu3+ ions, which substitute for some of the lighterSi4+ in the Si−O−Si chains, and the more loosely boundoxygens. The role played by the nonbridging oxygens, whoseconcentration may be enhanced by an RTT (section 2), isproved by the microreflectance peak red shift monitored evenin undoped and 1 mol % Eu-doped silica samples (red line andred symbol in Figure 5). In as-prepared samples, the size of the

red shift should depend mainly on the local Eu concentration: itaccounts once more for the inhomogeneous dopant distribu-tion in the sample, as proved by IR absorption (compare curvesc and d in Figure 3), microreflectivity (compare curves d, e, andf in Figure 6A), and Raman spectra (compare curves c, d, and ein Figure 7). Furthermore, curve e in Figure 6A shows that inthe relatively small area investigated, crystalline phases coexistalso with a variety of amorphous silica networks as a result ofdifferent local Eu concentrations, as proved, for example, by thethree peaks shifted to 1115, 1091, and 1076 cm−1 with respectto the 1123 cm−1 peak monitored in the undoped silica sample(curve a in Figure 6A).All of the analyses performed (CF, RL, and vibrational

spectra) emphasize the peculiar effects produced by 10 mol %Eu3+ doping on silica, namely, the growth of ordered phases (α-quartz and Eu2Si2O7), while at lower concentrations (e.g., 3 mol%) the largest part of network and of the Eu-induced clustersremains amorphous (even if some nanocrystalline Eu-relatedclusters are detected by TEM; see Figure 8a and section 4.3).The Eu specificity can also be seen by comparing the effectsproduced by heavy doping (e.g., 10 mol %) with other RE3+

ions, such as Ce3+ and Tb3+.26 The spectra related to 10 mol %Ce3+ and Tb3+ with respect to the undoped sample show amoderate red shift of the AS peak at ∼1123 cm−1 (open starand diamond in Figure 5), a more or less marked dip around∼1200 cm−1, and the growth of a smooth shoulder in the 800−1000 cm−1 range; these features are reminiscent of thosedetected in the spectrum of 3 mol % Eu3+-doped silica (curve cin Figure 4) rather than the rich structure displayed by the 10mol % Eu3+-doped silica (curve d in Figure 4 and curve e inFigure 6A). A tentative explanation may be found byconsidering that both Ce and Tb can also be tetravalent, inwhich case the silicon−oxygen network is not disrupted as aconsequence of Ce (or Tb) substitution for Si4+; in contrast, Eucan also be divalent (although this hypothesis is excluded in thepresent case by RL measurements; see section 3.3), thusincreasing the number of broken Si−O bonds.

4.2. Raman Spectra. Evidence of Eu clustering is clearlydemonstrated also by the evolution of Raman spectra as afunction of Eu concentration (Figure 7). While the spectrumrelated to the 3 mol % Eu3+-doped sample features somemodifications of the silica pattern indicating the onset ofclustering, the spectra collected on 10 mol % Eu3+-doped SiO2evidence clear fingerprints of RE aggregates. The comparisonwith spectra of Eu oxide (Eu2O3), Eu silicate (Eu2SiO5), and Eupyrosilicate (Eu2Si2O7) powders rules out the formation ofEu2O3 (Table S5 in the Supporting Information). In turn,similarities with the spectra of Eu2SiO5 and Eu2Si2O7 areevidenced. The presence in curve d of the main peaks at 740and 998 cm−1 (and the correspondences indicated in Table S5)suggests a more pronounced agreement with the latter, that is,the silicate structure with the highest number of SiO4tetrahedra in its stoichiometry. Moreover, the Raman spectraof 10 mol % Eu3+-doped SiO2 also display very clearly thepresence of crystalline α-quartz (curve f for comparison andcorrespondences listed in Table S5), as demonstrated by thestrong peak at around 465 cm−1 that is unequivocally associatedwith this allotropic form of crystalline SiO2,

79 in agreementwith the IR−NIR absorption and microreflectivity results(sections 4.1.3−4.1.5).

4.3. TEM Analysis. As mentioned in section 3.4, the poorquality of the diffraction patterns recorded on a small area ofthe 3 mol % Eu-doped silica prevented the determination of the



crystal phase of the nanocrystals displayed in Figure 8a. On theother hand, the broad and structureless bands displayed by theCF (Figure 2A, curve b), RL (Figure 5, inset, curve c), andvibrational spectra (Figure 3, curve b; Figure 4, curve c; Figure7, curve b) suggest that such nanocrystals represent a veryminor fraction of the nanoclusters. The TEM results do notcontradict the optical spectroscopy ones because the formergive local information related to a very small area while thelatter gather information from a much larger sample area andthickness.In the case of Figure 8b, related to the 10 mol % Eu-doped

silica, the high density of nanoparticles allowed the collection ofpowder electron diffraction patterns with almost complete rings(Figure 13a). The integrated pattern shows peak positions thatcan be indexed with a hexagonal cell of a = 9.58(5) Å and c =6.96(5) Å and extinction conditions compatible with the P63/mspace group (Figure 13b). This unit cell and symmetry arecompatible with a Sm5(SiO4)3O structure,80 where Eu replacesSm. To confirm this interpretation, a 1 μm × 1 μm area in thethinnest part of a nanocrystal-rich glass particle was scannedusing the ASTAR technique. All of the nanocrystals close to theedge and not overlapping one another could be indexed withthe hexagonal phase, while the matrix between them did notshow any diffraction spots, indicating that it is amorphous(Figure 14).In the case of the bigger nanoparticles embedded in a

crystalline matrix (Figure 8c) the ASTAR technique shows thatthe matrix is formed by α-quartz nanocrystals (Figure 15),while the Eu-rich nanoparticles exhibit diffraction patterns that

cannot be indexed with the hexagonal Eu5(SiO4)3O phase.These Eu nanoparticles were large enough to be investigatedwith standard electron diffraction. The smallest condenseraperture of the Libra microscope allowed an area with adiameter of 150 nm to be illuminated, avoiding collection ofsignal from the surrounding matrix in the case of larger crystals.The 400 nm crystal displayed in Figure 8c was analyzed usingthe ADT technique. Data collection over 100° gave a triclinicunit cell with a = 8.50(7) Å, b = 12.9(1) Å, c = 5.32(5) Å, α =91.9(5)°, β = 92.1(5)°, and γ = 89.9(5)° (Figure 16) which iscompatible to the triclinic high-temperature phase ofEu2Si2O7.

81 Thus, the presence of α-quartz and Eu nanocrystalsin the 10 mol % Eu-doped silica predicted by the spectroscopicmeasurements is nicely supported by direct evidence from theTEM analyses.

4.4. Devitrification Process. Devitrification of silica glassesas a consequence of prolonged (20 h) heat treatment at 1300°C was reported by Crookes already in the early 20th century.82

As a “supercooled liquid”, silica glass is characterized by highviscosity, which prevents the structural rearrangement neededduring a fast cooling to promote the formation of crystallinequartz, which is the thermodynamically stable state. Crystal-lization may occur at elevated temperatures in the presence ofcontaminants that lower the viscosity by breaking up the highlyconnected silicon−oxygen network and/or acting as nucleationcenters. Examples of impurities that catalyze devitrification arealkalis,83,84 OH,85 chlorine, and fluorine.86 In sol−gel silicadoped with some REs (Er, Ce), the fluorine codopant initiatesthe crystallization either by inducing local order around the RE

Figure 13. (a) Electron diffraction pattern of a 10 mol % Eu3+-doped silica sample taken on a 25 μm2 selected area, similar to that in Figure 8b. (b)Plot of the radial integration of the pattern in (a). The ticks at the bottom correspond to the positions of the reflections of the Eu5(SiO4)3Ohexagonal phase.

Figure 14. (a) Bright-field TEM image of a small area of a 10 mol % Eu3+-doped silica sample near the border of the grain displayed in Figure 8b.(b−d) Diffraction patterns (gray/black) collected in ASTAR mode on three different nanoparticles. The best matching pattern, generated accordingto the hexagonal structure of Eu5(SiO4)3O, is displayed in red and superimposed. (e) Pattern collected in ASTAR mode in between the nanoparticlesin an area of image (a) where the contrast is lighter. The absence of any diffraction spot indicates that the embedding matrix is amorphous.



ion or by its partial decoupling from the disordered host matrixdue to the creation of network terminating sites.52,87 Thepresent Eu-doped sol−gel silica samples sintered at 1050 °Cexhibit the preconditions for the onset of crystallization: (1)Eu3+, by substituting for Si4+, favors the formation ofnonbonding oxygen, thus interrupting the continuity of thesilicon−oxygen network. Increasing the Eu concentrationincreases the silica depolymerization as well and is furtheremphasized by the simultaneous presence of Si−OH groups

(section 4.1.2). (2) Eu3+ clusters, which are already present inthe 0.1−3 mol % Eu doping range,29,34 may act as nucleationcenters. Eu3+ catalyzes silica dissolution, as proved byfluorescence analysis of Eu3+ sorption−desorption processeson nonporous silica in aqueous solutions.88 In the initial step, acomplex of Eu3+ with Si−OH groups forms at the silica surface(at the expense of water molecules). The further decrease inthe number of water molecules, the concomitant silicadissolution, and the irreversibility of the Eu3+ sorption suggestthat a new phase grows at the silica surface. The increased Eu3+

fluorescence lifetime during the Eu3+ sorption process rules outthe involvement of Eu3+ hydroxide. Eu3+ silicate has beenproposed as a candidate, but no hint of its specific compositionand structure has been supplied. The above picture, obtained ina completely different framework (i.e., that of geochemistry andradioactive waste management88), can be applied to the stepsthat bring the initial sol to the present 10 mol % Eu3+-dopedsilica and account for the OH removal and the separation ofphases, which CF and vibrational spectroscopies, supported byTEM images, were able to identify as Eu pyrosilicate and α-quartz. A process that occurs during the rather slow coolingfrom the densification temperature (1050 °C; see section 2)may be envisaged: in the early step, nanocrystals of Eupyrosilicate grow, and the two SiO4 tetrahedra may act ascrystallization seeds for the distorted ones belonging to thesurrounding silica, with the final formation of a matrix includingα-quartz, as clearly supported by Figure 8c and section 4.3.Crystallization of undoped sol−gel silica has been already

mentioned as a consequence of heat treatment for 3 h at 1000°C62 and for 2 h at 1300 °C.89 In both cases, the crystallinephase was identified as α-cristobalite and not α-quartz; on thecontrary, the occurrence of α-quartz in the present 10 mol %Eu-doped silica heat-treated at 1050 °C is clearly proved by theTEM and spectroscopic analyses. According to XRD measure-ments performed on sol−gel silica, undoped or doped with RE(Pr, Eu, Ho, Er) amounts increasing from 0.5 to 6 mol % andheat-treated at 1200 °C for 2 h, crystallization takes place inform of α-cristobalite.90 Thus, prolonged thermal treatments atrather high temperatures seem to favor the onset of the silica toα-cristobalite transformation. The rate of subsequent coolingmay also play an important role because at ordinary pressuresα-cristobalite is metastable at temperatures lower than ∼270°C, while α-quartz is the stable form of silica below 573 °C.91

5. CONCLUSIONComplementary spectroscopic techniques have been applied tothoroughly study Eu incorporation into silica samples preparedvia a sol−gel route. The crystal field absorption, radio-luminescence, and vibrational (absorption, Raman, and micro-reflectance) spectra have converged to prove unequivocally (1)the growth of amorphous Eu3+-rich clusters and Eu−OHcomplexes as the dopant concentration increases in the 0.001−3 mol % range and (2) the devitrification of silica, which occursat a Eu concentration of 10 mol %, due to the formation ofnanocrystals of Eu pyrosilicate and (unpredictably in principle)α-quartz.The onset of the ordered nanocrystalline phase is proved by

the following observations: (1) narrowing of the Eu spectrallines with increasing Eu concentration from 3 to 10 mol % anddecreasing temperature over the 300−9 K range; (2) the tightcorrespondence of the line positions in the 10 mol % Eu-dopedsilica with those of (poly)crystalline Eu pyrosilicate and α-quartz as well as the common thermally induced line shift (well-

Figure 15. (a) Virtual bright-field images obtained by integrating thecentral spot of all the patterns recorded in ASTAR mode (see ref 44for details) on the left part of the grain displayed in Figure 8c. (b)Display of the index value for α-quartz. The index value was calculatedthrough a cross-correlation between the collected patterns in ASTARmode and the patterns generated according to the α-quartz structure(see ref 44 for details). An intense green color means a good matchwith some α-quartz crystal orientation. The matching is good in thematrix and bad in the heavy-element-containing nanoparticles. (c)Diffraction patterns (in gray/black) collected in ASTAR mode onthree nanoparticles of the matrix where the index with α-quartz is high.The best matching pattern, generated according to the crystal structureof α-quartz, is displayed in red and superimposed.

Figure 16. (a−c) 3D reconstruction of the reciprocal space ofEu2Si2O7 viewed along the three main zone axis directions. For clarity,16 unit cells of the reciprocal lattice are superimposed in the images.(d) 3D reconstruction viewed along the tilt axis to point out the 100°reciprocal space coverage. The reconstruction was obtained fromelectron diffraction tomography data collected on the crystal indicatedby the arrow in Figure 8c.



described by the single-phonon coupling model), the commonsequences of fundamental vibrations and related overtones, andthe comparable anharmonicity parameters (in the framework ofMorse model); (3) the sharp increase of radioluminescenceintensity in 10 mol % Eu samples and the simultaneoussuppression of OH, a widespread impurity often responsible forluminescence quenching; and (4) further direct evidence byTEM and electron diffraction analysis.The peculiar behavior of Eu in comparison with other REs

(e.g., Ce, Gd, Tb, and Yb), to develop nanocrystal growth at 10mol % doping accompanied by OH removal opens someprospects in the field of lasers and scintillators: the presentapproach may be extended to silica doped with Eu3+

concentrations in the 3−10% range (not yet investigated) toidentify the proper doping level and thermal treatments capableof producing size-controlled, Eu-rich luminescent nanocrystalsembedded in a transparent host matrix.

■ ASSOCIATED CONTENT*S Supporting InformationTables with wave numbers of the main absorption lines, fitresults, and Raman shifts. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected].

Present Addresses∥E.B.: IMEM-CNR Institute, Parco Area delle Scienze 37/A,43124 Parma, Italy.⊥M.M.: ICFO-The Institute of Photonic Sciences, Mediterra-nean Technology Park, Av. Carl Friedrich Gauss, num. 3, 08860Castelldefels, Barcelona, Spain.#A.L.: Laboratory for Multifunctional Materials, Department ofMaterials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093Zurich, Switzerland.∇F.M.: ILM UMR5306 Universite Claude Bernard Lyon 1-CNRS, Batiment Alfred Kastler, 10 rue Ada Byron, 69622Villeurbanne cedex, France.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are indebted to Prof. Roberto Boscaino(University of Palermo, Italy) for supplying samples of purefused silica, to Dr. Pavel Bohacek (Academy of Sciences,Prague, Czech Republic) for supplying some referencecompounds, and to Dr. Norberto Chiodini and Dr. ElenaRizzelli (University of Milano-Bicocca, Italy) for help in sol−gelsample preparation and in RL measurements.

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