Luminescence Spectra of β‑SiAlON/Pr3+ Under High HydrostaticPressureSebastian Mahlik,*,† Agata Lazarowska,† Marek Grinberg,† Tzu-Chen Liu,‡ and Ru-Shi Liu*,‡

†Institute of Experimental Physics, University of Gdansk, Wita Stwosza 57, 80-952 Gdansk, Poland‡Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

ABSTRACT: In this contribution, steady-state and time-resolved photoluminescencespectra of β-Si5.9Al0.1O0.1N7.9/Pr

3+0.016 obtained at high hydrostatic pressures up to 260 kbar

are presented. At ambient pressure, excitation spectra of the β-SiAlON/Pr3+ luminescenceconsist of sharp lines in the spectral region of 20000−22200 cm−1 related to the 3H4 →3P0,1,2 transitions and a broad band with two maxima peaking at 30300 and 35700 cm−1,which are related to the ionization of Pr3+ ions occupying two different crystallographic sitesin the β-SiAlON lattice. In both sites, the luminescence spectra consist of the dominant 1D2red emission and 3P0 blue emission of Pr3+ ions. The intensity of luminescence related totransitions from the 3P0 state and the respective decay time gradually decreases when thepressure increases. The intensity of emission from the 1D2 state is independent of thepressure and remains strong over the entire pressure range. Weak dependence of the Pr3+

emission on applied pressure can be related to the covalent and strong Pr−N bonds and specific location of Pr3+ ions occupyingempty lattice points in β-SiAlON.

■ INTRODUCTION

Nitridosilicate materials, such as Si3N4 and its solid solutions,have found several applications1−6 due to their excellentthermomechanical properties, high chemical stability, highhardness, and so forth. Recently, Si3N4-based derivatives dopedwith RE ions have been widely investigated as phosphors forwhite-emitting diodes (W-LEDs).7−9 Silicon nitride has twomajor crystalline phases, phase α-Si3N4 with a trigonal structureand phase β-Si3N4 with a hexagonal structure. The chemicalformula of α-SiAlON is Mm/vSi12−(m+n)Alm+nOnN16−n, where Mrepresents the cation metals Li, Mg, Ca, and Y or rare earth(RE) elements10 and v is the valence of M. β-SiAlON is derivedfrom β-Si3N4 by equivalent substitution of the Al−O for theSi−N pairs. The chemical composition of β-SiAlON isdescribed by the formula Si6−zAlzOzN8−z, where z representsthe number of Al−O pairs being substituted for Si−N pairs (ztakes the values from 0 to 4.2).1,4 β-SiAlON doped with Eu2+

has been extensively investigated as a green phosphor.11−14

Luminescence properties of β-SiAlON doped with the RE ionsCe3+,15 Yb3+,16 and Pr3+17,20 also have been investigated inprevious literature.The radiuses of four-fold-coordinated Si4+ and Al3+ are equal

to 0.26 and 0.39 Å, respectively, whereas the radius of the Pr3+

changes from 0.99 to 1.179 Å for six-fold and nine-foldcoordination, respectively.18 Therefore, in the case of β-SiAlON/Pr3+, Pr3+ does not substitute lattice ions. Like otherRE ions, Pr3+ions can reside only along the continuous channelsparallel to the c direction.11−13,19 Steady-state and time-resolvedspectroscopy of β-SiAlON/Pr3+ obtained at different temper-atures (10−293 K) have been presented in previous work.20

Luminescence spectra consist of the dominant 1D2 red emissionand 3P0 blue emission of Pr3+ ions. It was found that under 330

nm (30300 cm−1) excitation, additional luminescence peaksappeared beside those earlier observed in the region of the 3P0and 1D2 emission. These additional lines were attributed to thenovel site of Pr3+.The main crystallographic site 2b (Wyckoff notation)18

corresponds to the Pr3+ located at (0,0,0). In this case, the Pr3+

ion is coordinated by six nitrogen/oxygen ions at a distanceequal to 2.511 Å. In the novel site, Pr3+ is located at the (0,0,1/4) position (Wyckoff notation 2a), where it is coordinated bynine nitrogen/oxygen ions, three of them at a distance equal to2.403 Å and another six at a distance of 2.810 Å. Possiblelocations of Pr3+ in β-SiAlON are presented in Figure 1a and b.The respective lattice points are labeled 2b and 2a, respectively.

Received: March 11, 2013Revised: May 29, 2013Published: June 3, 2013

Figure 1. Crystal structure of the 2 × 2 × 2 unit cell ofSi5.9Al0.1O0.1N7.9/Pr0.016 (a) on the 2b site where the Pr3+ ion islocated at (0,0,0) surrounded by six N/O ions and (b) on the 2a sitewhere the Pr3+ ion is located at (0,0,0.25), surrounded by nine N/Oions.

Article

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© 2013 American Chemical Society 13181 dx.doi.org/10.1021/jp402475n | J. Phys. Chem. C 2013, 117, 13181−13186

In this paper, a detailed study of the luminescence andluminescence kinetics of the β-SiAlON/Pr3+ system under highhydrostatic pressure obtained from the diamond anvil cell(DAC) is presented and discussed. The main motivation toperform a high hydrostatic pressure luminescence study of thismaterial was the fact that Pr3+ions in β-SiAlON do not occupyits substitutional sites. In the case when lanthanide ions Ln3+

replace the lattice ions, they provide additional localized statesto the energetic structure of the system that can be described asbelonging to the 4fn, 4fn−15d electronic configuration of Ln3+

and the impurity trapped exciton (ITE) described as Ln4+ thatbinds electrons by means of the Coulomb potential.21 Pressurecauses the increase of the energies of localized states for 4fn,4fn−15d electronic configurations with respect to host bandsenergies.22 This effect is seen as pressure-induced diminishingof the energy of the ITE with respect to the energies of thestates belonging to the 4fn electronic configuration. In such acase, the influence of high hydrostatic pressure on the states of4fn, 4fn−15d electronic configurations can be described using thecrystal field approximation.22 The case when Pr3+ occupies theempty lattice points in β-SiAlON is qualitatively different. Apartfrom the states of 4fn, 4fn−15d electronic configurations, dopantsprovide the bonding−antibonding states that involve electronsfrom the 4f6s2 orbital of Pr3+. As a result, Pr3+ occupying theempty lattice point creates more reach structure than Pr3+ inthe substitutional site. We expect that high-pressure spectros-copy can show this difference.

■ EXPERIMENTThe β-Si5.9Al0.1O0.1N7.9/Pr

3+0.016 sample was prepared from α-

Si3N4, AlN, Al2O3, and Pr2O3 using the gas pressure sintering(GPS) method. The obtained β-SiAlON/Pr3+ has a hexagonalstructure characterized by a P63/m space group. The unit cellparameters are a = b = 7.60953(4) Å and c = 2.91009(6) Å.More details on the preparation and structural investigation aredescribed in the paper by T. C. Liu et al.17

Luminescence excitation spectra were acquired using aFluoroMax-4P TCSPC spectrofluorometer produced byHoriba, containing Czerny−Turner monochromators forexcitation and emission. The excitation source in this systemwas a 150 W ozone-free xenon lamp, which makes it possible tomeasure luminescence from the 220 to 850 nm range. Thefluorescence intensity was measured using a R928 side-onphotomultiplier. Steady-state luminescence spectra were excitedwith the He−Cd and diode lasers with wavelengths of 325(30770 cm−1) and 473 nm (21140 cm−1), respectively. Thephotoluminescence signal was dispersed using a PGS2spectrometer working as a monochromator and detected by aHamamatsu photomultiplier model R943-2 working in thephoton counting regime.The experimental setup for luminescence kinetics and time-

resolved emission spectra consists of a PL 2143 A/SS laser asthe excitation source and a PG 401/SH parametric opticalgenerator. This system can generate 30 ps laser pulses, with afrequency of 10 Hz with wavelengths ranging from 220 to 2200nm. The emission signal was analyzed by a Bruker Optics2501S spectrometer equipped with a 50 grooves/mm gratingand the Hamamatsu Streak Camera model C4334-01 with afinal spectral resolution 0.47 nm. Time-resolved luminescencespectra were obtained by integration of streak camera imagesover time intervals, whereas luminescence decays were obtainedby the integration of streak camera images over the wavelengthintervals. Details of the experimental setup are described in the

ref 23. High hydrostatic pressure was applied in a MerrillBassett type DAC.24 Polydimethylsiloxane oil was used as thepressure-transmitting medium, and the pressure was measuredby the shift of the R1 luminescence line of ruby.

■ RESULTSThe room-temperature excitation spectrum of β-SiAlON/Pr3+

monitored at 15820 cm−1 (1D2 → 3H4 transition of Pr3+) ispresented in Figure 2 (curve a). The spectrum consists of sharp

lines in the spectral region from 20000 to 22220 cm−1

associated with transitions from the ground state to the 3P0,3P1, and

3P2 excited states of Pr3+ ions and a broad band withtwo maxima at about 35700 and 30300 cm−1 that can beattributed to the ionization process of Pr3+ ions (transfer of anelectron from the Pr3+ ion to the conduction band) in sites 2band 2a, respectively.20

The time-resolved emission spectra of β-SiAlON/Pr3+

measured within the range of 0−2 μs after pulse laser excitationare presented in Figure 2 (curve b and c). The emissionspectrum of the system under excitation with energies of 35700(280 nm) and 21140 cm−1 (473 nm) is presented in Figure 2(curve b). The lines peaking between 14900 and 16700 cm−1

are related to the3P0 →3H6 and the1D2 →

3H4 transitions, andthe lines peaking in the regions of 18800−20000 and 14285−14900 cm−1correspond to the 3P0 → 3H4 and 3P0 → 3F2transitions of Pr3+, respectively. Under 280 nm excitation,emission (represented by curve b) that originates from Pr3+ inthe 2b site is seen exclusively. The emission spectrum of thesystem excited in the absorption band with the maximum of30300 cm−1 that corresponds to the ionization of Pr3+ at the 2asite is presented in Figure 2 (curve c). Under this excitation,additional luminescence peaks appeared beside earlier observedpeaks. The new peaks are indicated by arrows in Figure 2c andare related to the luminescence of Pr3+ ions in the 2a site. Theenergies of the localized states of the Pr3+ ion obtained from theexcitation and emission spectra differ from the energies of freeion much more than expected due to the crystal field splitting.Usually, these differences are attributed to the nephelauxeticeffect caused by the larger covalency of Pr−N bonds.25

The steady-state emission spectra of β-SiAlON/Pr3+

obtained at room temperature under different pressures

Figure 2. Excitation spectrum of the emission monitored at 15820cm−1 (632 nm) that corresponds to the maximum emission line fromsite 2a. Time-resolved luminescence spectra under different excitationwavelengths with a time acquisition of 0−2 μs. The strongest linesrelated to the 2a site are marked with black arrows.

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under excitation with 21140 (473 nm) and with 30770 cm−1

(325 nm) are presented in Figure 3a and b, respectively. In

Figure 3a, the emission excited with 21140 cm−1 (3H4 →3P2) is

presented. Under this excitation, only luminescence from the2b site is observed exclusively. In Figure 3b, the emission fromboth 2b and 2a sites excited with the wavelength 30770 cm−1 ispresented. The lines in the spectral range of 18500−20000cm−1 correspond to the emissions from the 3P0 state, whereasthe lines in the spectral region of 15380−16670 cm−1 arerelated to emission from the 1D2 state. One notices that theemission spectra of 2a and 2b Pr3+ sites are dominated by linesrelated to the 1D2 →

3H4 transitions.In Figure 3c, energies of the individual lines versus pressure

are presented. When the pressure increases, all emission linesweakly shift toward the lower energies. The red shift of theemission lines for the Pr3+ ions in 2b and 2a sites are similar.The respective rates range from −0.5 to −0.9 cm−1/kbar andare small in comparison to the values of −1.75−4.1 cm−1/kbarobserved for Pr3+ in oxide matrixes.26−30 Furthermore, in oxidesdoped with Pr3+, the intensity of the luminescence from the 3P0state was strongly quenched with increasing pressure. Thiseffect has been attributed to the decrease of energy of the ITEstate with respect to energies of the states of the 4f2 electronicconfiguration of Pr3+. When the energy of the ITE is betweenthe energies of the 3P0 and 1D2 states, nonradiative energy

transfer from the 3P0 to 1D2 state of the Pr3+ ITE statequenches the 3P0 luminescence.

27−30

A different situation is observed in the case of β-SiAlON/Pr3+, where the 3P0 emission intensity weakly decreases withincreasing pressure (see Figure 3a and b). The intensity ofemission from the 1D2 state is independent of pressure andremains strong over the entire pressure range.In Figure 4a and b, decays of luminescence related to the 3P0

→ 3H4 transition of the Pr3+ ion obtained at different pressures

are presented. The decays of luminescence of Pr3+ in site 2bexcited with 473 nm are presented in Figure 4a. The decaysbecome faster when the pressure increases and remains single-exponential for all pressures. Fitted luminescence lifetimes arepresented in Figure 4c. The decays of luminescence of Pr3+

occupying sites 2b and 2a, excited with 30300 cm−1, arepresented in Figure 4b. The decays are evidently two-exponential. The 2a site luminescence lifetimes were obtainedas the shorter components of the two exponential decays.Obtained lifetimes of the 2b and 2a emission as a function of

Figure 3. Luminescence spectra of β-SiAlON/Pr3+ under differentpressures excited at 21140 (473 nm) (a) and at 30770 cm−1 (325 nm)(b). Position of the selected emission peaks versus pressure (c).

Figure 4. Luminescence decays that correspond to the3P0 →3H4transition of Pr3+ for different pressures under pulse excitation:(a) at 21140 cm−1 (473 nm) monitored at 19608 cm−1 (510 nm) and(b) at 30300 cm−1 (330 nm) monitored at 18660 cm−1 (536 nm). (c)Decay time of the transition from the 3P0 to

3H4 state as a function ofthe pressure. For site 2b, a one-exponential function was used, and forsites 2b+2a, a two-exponential function has been used.

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pressure are presented in Figure 4c. It is observed that thedecay of the luminescence of the 2a site is faster than that ofthe 2b site. Pressure-induced quenching of the 3P0 lumines-cence is accompanied by shortening of luminescence decayswith the same rate for both sites.Luminescence related to emission from the 1D2 state is

strong enough that it is possible to spectrally separate theluminescence decay related to the Pr3+ ion in sites 2b and 2a.Decay profiles of the luminescence related to emission from the1D2 state of Pr

3+ ions in sites 2b and 2a monitored at 623 and632 nm, obtained at different pressures, are presented in Figure5a and b, respectively. Decays appeared nonexponential, andeffective lifetimes were determined using the following relation:

∫∫

τ =·I t t t

I t t

( ) d

( ) d (1)

Obtained effective lifetimes with respect to the pressure arepresented in Figure 5c. In the case of the 1D2 emission, only asmall shortening of the luminescence decays is observed, and itis accompanied by a small decrease of the intensity of emission.The fact that decay profile of the emission is not single-exponential can be related to nonradiative quenching of the1D2

emission arising from the cross relaxation process given by therelation (1D2,

3H4) → (1G4,3F4).

The rise of the 1D2 emission of the Pr3+ ion in site 2b and site

2a is present in Figure 5d. The emission of the Pr3+ ion in site2a rises faster than that in site 2b. The emission related to site2a excited with 30300 cm−1 appeared almost at once afterexcitation, while emission related to site 2b excited with 35700cm−1 is delayed by time equal to approximately 200 ns.

■ DISCUSSION AND CONCLUSIONS

This high-pressure luminescence study of β-SiAlON/Pr3+ hasconfirmed the existence of two different Pr3+ sites, one with six-fold and the other with nine-fold coordination of oxygen/nitrogen ions, labeled according to the Wyckoff notation 2b and2a, respectively. The excitation bands of Pr3+ luminescence insite 2a peaking at 30300 cm−1 and site 2b at 35700 cm−1 arerelated to different ionization energies of Pr3+ ions. Theemission lines of the Pr3+ ion occupying site 2a appear to havelower energies and are characterized by shorter lifetimes thanthose in site 2b. Another difference is related to differentexcitation spectra and different pathways of population of the1D2 state, which result in different luminescence rise times ofthe 1D2 emission.The ionization transition creates the Pr4+ ion and an electron

in the conduction band. This process has been observed earlierin various oxide materials doped with Pr3+.28−30 Afterionization, Coulomb attraction between Pr4+ and ecb createsan additional localized state considered as ITE.21,22 In theequivalent description of this process, one electron istransferred from Pr3+ to other metal cation(s) and is calledthe intervalence charge transfer (IVCT) transition.31 In bothapproaches, the Pr4+ ions attract the negative ligands more thanthose of Pr3+. As a result, the electron−lattice relaxationadditionally diminishes the energy of ITE or IVCT states. Suchsystems can be described by the configuration coordinatediagrams, where electronic energies are given by parabolaswhere the abscissa axis represents the average distance betweenthe Pr3+ ion and ligands, R. Configuration coordinate diagramsrepresenting Pr3+ ions in 2b and 2a sites at ambient pressure arepresented in Figure 6a and b, respectively.The localized states related to the ground 4f2 electronic

configuration of Pr3+ ions (black and blue parabolas in Figure 6a and b) have minima at the same value of R. In the case of theITE state, the minimum energy appears at a shorter distancebetween Pr4+ and its ligands; therefore, the respective parabolasare shifted to the left. The parabola representing the electronicenergy of the ITE in site 2b (Figure 6a) was obtainedconsidering that the broad band in the excitation spectrum ofPr3+ with a maximum at energy 35720 cm−1 corresponding tothe transition from the ground state of Pr3+ to the ITE state(ionization energy of Pr3+ in site 2b) and lattice relaxationenergy Sℏω is equal to 5400 cm−1. To obtain the parabolarepresenting ITE in the case of site 2a (Figure 6b), anionization energy of 30300 cm−1 and electron−lattice coupling(the same as in site 2b) of Sℏω = 5400 cm−1 were used. Themethodology of estimation of the energy of Sℏω is seen inFigure 6. The value of Sℏω was obtained as an energydifference between the energy of the maximum of the excitationband and teh zero-phonon transition energy, considered as theenergy of the long-wavelength end of the excitation band. Theenergy of the band maximum can be easily estimated, whereasthe energy of the zero-phonon line is not seen in our case.

Figure 5. Luminescence decays that correspond to the1D2 → 3H4transition of Pr3+ for different pressures under pulse excitation: (a) at21140 cm−1 (473 nm) with a monitored luminescence at 16050 cm−1

(623 nm) and (b) at 30300 cm−1 (330 nm) with a monitoredluminescence at 15820 cm−1 (632 nm). (c) Average decay time of thetransition from the 1D2 to

3H4 state as a function of pressure. (d) Risecurves collected at the maximum 1D2 → 3H4 luminescence under35715 and 30300 cm−1 excitation.

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Therefore, the value of Sℏω is estimated with a rather largeerror that could be equal to 500 cm−1.The dashed arrows in Figure 6a and b represent the possible

relaxation pathways. In site 2b, after excitation to the ITE state,the system relaxes to the minimum energy of the ITE electronicmanifold and then to the 3P0 state of Pr

3+. In this site, the 1D2state is populated only by nonradiative multiphonon transitionsfrom the 3P0 state. The 1D2 state is not populated bynonradiative transition from the ITE state because crossoverof the ITE and 1D2 electronic manifolds has higher energy thanthe crossover of the ITE and the3P0 electronic manifolds.In the case of Pr3+ ions in the 2a site, the crossover between

the ITE and 3P0 electronic manifolds and the ITE and 1D2 takesplace at the same energy, very close to the minimum energy ofthe ITE electronic manifold. As a result, in the 2a site, it is morelikely that the 1D2 state is populated directly by the energytransfer from the ITE state. This different relaxation pathwayinfluences the luminescence processes in Pr3+ ions at differentsites. Specifically, emission from the 3P0 state in site 2a is muchweaker than that in site 2b. For the same reason the rise of the1D2 emission of Pr3+ in site 2a is faster than that in site 2b.The results of high-pressure spectroscopy of β-SiAlON/Pr3+

should be discussed in the context of other materials dopedwith Pr3+ under high hydrostatic pressure. Typically, at ambientpressure, energies of the 3P0 and

1D2 states are lower than theenergy of the ITE state. When the pressure increases, thedistance between Pr3+ and negatively charged ligand ionsdecreases. As a result, the energy of the states from the groundelectronic configuration 4f2 increases with respect to bandstates of the host.21,22,32−34 Thus, it is expected that underpressure, the energies of the 3P0 and the 1D2 states increasewith respect to the ITE state. Such an effect quenches the 3P0(and then the 1D2) emission quite rapidly at pressures when 3P0and ITE (and then the 1D2 and ITE) crossovers takeplace.28−30 The luminescence spectra obtained at highpressures for β-SiAlON/Pr3+ do not show such an effect.Although the intensity of the emission from the 3P0 statedecreases with increasing pressure and its lifetime becomesshorter, this effect has different characteristics than thosereported in oxides.28−30 The luminescence related to transitionsfrom the 3P0 state is quenched smoothly and exists for allapplied pressures. The decrease of the intensity of emissionfrom the 3P0 state is similar for Pr3+ in the 2a and 2b sites. Theenergy of the ITE state in site 2a is lower than that in site 2b.

Therefore, if the pressure shifts the ground state of the Pr3+

ions toward higher energies, one would expect that theemission from the 3P0 state of the Pr3+ in the 2a site shouldbe quenched at lower pressure than the respective emission ofthe Pr3+ in the 2b site. The results show that the nonradiativeprocesses that depopulated the 3P0 state are similar in both sites(Figure 4c). Even at a pressure of 260 kbar, weak emission fromthe 3P0 in both sites was still observed. An increase ofnonradiative transitions from the 3P0 in both sites can beexplained due to the fact that high pressure causes an increaseof the phonons’ energy, and therefore, the probability ofmultiphonon relaxation from the 3P0 to

1D2 state also increases.The reason why the pressure does not change the energy of

the ground electronic configuration of the Pr3+ ions withrespect to the conduction band in β-SiAlON/Pr3+ is still notwell understood. At the present stage, one can relate it to thedifferences between β-SiAlON/Pr3+ and other oxides dopedwith Pr3+. In β-SiAlON/Pr3+, praseodymium ions occupy thelattice points that are empty in undoped lattices, whereas inother oxides, Pr3+ ions usually substitute the lattice ions. As aresult, Pr3+ in the β-SiAlON/Pr3+ introduces additional Pr−Nbonding and antibonding states, which do not exist in a purelattice. Pr−N bonds are probably very covalent and strong andthus responsible for the large nephelauxetic effect in β-SiAlON/Pr3+. As a result, the application of pressure does not changethe length of Pr−N bonds and therefore does not change theenergies of the states of the 4f2 electronic configuration withrespect to the ITE state.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: s.mahlik@ug.edu.pl. Phone: +48585232182 (S.M.); E-mail: rsliu@ntu.edu.tw. Phone: 886-2-33661169 (R.-S.L.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis paper has been funded by the Polish Ministry of Scienceand Higher Education by a Grant active from 2012 to 2014 andby POIG.01.01.02-02-006/09 project “New efficient phosphorsfor lighting and solar concentrators”. The contribution of A.L.was supported within the International Ph.D. Project “Physicsof future quantum-based information technologies”, GrantMPD/2009-3/4 from the Foundation for Polish Science. T.-

Figure 6. Configurational coordinate diagram representing the β-SiAlON/Pr3+ system on (a) 2b and (b) 2a sites.

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C.L. and R.-S.L. would like to thank the National ScienceCouncil of Taiwan (Contract No. NSC 101-2113-M-002-014-MY3) for financially supporting this research.

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dx.doi.org/10.1021/jp402475n | J. Phys. Chem. C 2013, 117, 13181−1318613186