Unprecedented High Solubility of Oxygen Interstitial Defects inLa1.2Sr0.8MnO4+δ up to δ ∼ 0.42 Revealed by In Situ High TemperatureNeutron Powder Diffraction in Flowing O2

Thibault Broux,† Carmelo Prestipino,† Mona Bahout,*,† Olivier Hernandez,† Diptikanta Swain,†

Serge Paofai,† Thomas C. Hansen,‡ and Colin Greaves*,§

†Institut des Sciences Chimiques de Rennes, Equipe Chimie du Solide et Materiaux, UMR CNRS 6226, Universite de Rennes 1, 263Avenue du General Leclerc, 35042 Rennes, France‡Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex, France§School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom

ABSTRACT: The structural behavior of the n = 1 Ruddlesden−Popper (R-P) La1.2Sr0.8MnO4+δ phases has been monitored in situ in flowing O2 onheating over the temperature range 65 < T/°C < 550 by means of neutronpowder diffraction (instrument D20, ILL/Grenoble). Sequential Rietveldrefinement showed that the I4/mmm nearly stoichiometric phase undergoes afirst oxygen uptake in the temperature range 300−400 °C with phaseseparation into an oxygen-rich orthorhombic (Bmab) La1.2Sr0.8MnO4.30(1)phase and a stoichiometric tetragonal (I4/mmm) La1.2Sr0.8MnO4.00(2) phase.At T ∼ 410 °C, the first oxidation step is complete and only the Bmaboxidized phase is present. The orthorhombicity decreases progressively onfurther heating, up to 510 °C with minor variation in the oxygen content.Above ∼510 °C, the system undergoes a second abrupt oxidation stepinvolving a single phase process to reach after prolonged isothermal heating at550 °C, δ ∼ 0.42(2). Such a high solubility for the excess-oxygen defects has never been reported so far for a K2NiF4-typestructure. The La1.2Sr0.8MnO4+δ system is thus interesting for the fundamental studies of structural distortions induced by theintercalation of a large amount of oxygen defects owing to the flexibility of the R-P structure and high stability of the Mn4+

oxidation state in an oxygen octahedral environment.

KEYWORDS: manganese oxides, K2NiF4-type structure, in situ neutron powder diffraction, oxygen nonstoichiometry,Solid Oxide Fuel Cell- SOFC

■ INTRODUCTION

In recent years, there has been considerable interest in theK2NiF4-type oxides as possible solid oxide fuel cell cathodematerials after the discovery of significant oxide ion diffusivityin the La2NiO4+δ compositions.1 The K2NiF4-type oxides arestructurally related to the perovskite oxides and consist ofalternating ABO3 (perovskite) and AO (rock salt) layers alongthe c-axis. In these materials, there is sufficient space in the AOlayer to accommodate an excess of oxygen as interstitial species,charge-balanced through the oxidation of the B-site cation. Thisleaves a favorable ab plane pathway for oxygen mobility2 andthus introducing a new type of mixed ionic−electronicconductor (MIEC). Although there has been considerableattention devoted to the redox properties of transition metalions in perovskite oxides,3 equivalent studies for high oxidationstates in K2NiF4-type oxides have not been reported, despitethe structural similarity of these phases.Furthermore, several materials with the K2NiF4 structure

were found to have mixed conductivity as well as competitivestability and thermal expansion properties, with regards to theexisting perovskite-type cathode materials. However, the

majority of these investigations have been concentrated onthe development of Ni, Co, and Fe-substituted MIEC4 andtheir suitability as electrodes materials for high temperatureelectrochemical devices. On the other hand, little work hasbeen reported on the related layered manganese oxides. Thesematerials have, however, attracted attention following thediscovery of a large negative magnetoresistance in alkaline-earthdoped Ln−Mn−O systems and significant efforts were directedtoward the development of new compositions5 and thedetermination of their structural, electrical, and magneticproperties below room temperature.6

In particular, the electron-doped material La1.2Sr0.8MnO3.94prepared by Li and Greaves with the intention of investigatingMn2+−O2p−Mn3+ (dx

2−y

2)1−O2p−(dx2−y2)0 ferromagnetic inter-actions has drawn our attention as potential materials for MIECapplication. As reported in ref 7, this material can be oxidized(on heating in air at 550 °C) to La1.2Sr0.8MnO4.27 (δ = 0.27).

Received: July 4, 2013Revised: September 11, 2013

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For an electrode material in a SOFC, it is essential to studythe behavior under operating conditions (high temperatureunder reducing and/or oxidizing gases) to get insight into thekinetics and thermodynamics of the oxygen-exchange processesand structural changes to define the limits for application. It isworthwhile mentioning that no neutron diffraction data wasreported so far on La1.2Sr0.8MnO4+δ. The large amount of excesslattice oxygen (δ ∼ 0.27) that can be accommodated in thismaterial could be not only a valuable asset for ionicconductivity properties but also a unique opportunity to fullyinvestigate the structural response of a K2NiF4 structure-typematerial to exceptionally wide range of oxygen composition.

1. EXPERIMENTAL SECTION1.1. Synthesis. A powder sample with the nominal composition

La1.2Sr0.8MnO4 was prepared via the citrate/ethylene glycol sol−gelroute. La2O3, MnO (Aldrich, 99%), and SrCO3 (Aldrich, ≥ 99.9%)was dissolved in dilute nitric acid warmed to 80 °C. Citric acid (AcrosOrganic, 99%) and ethylene-glycol (Sigma-Aldrich, > 99%) wereadded to the solution to give an ethylene-glycol/citric-acid/metal ionmolar ratio of 4:2:1. The mixture was then stirred and heated until agel and subsequently a resin was formed. The resin was calcinedovernight at ∼600 °C in air to decompose the organic components.This precursor was then ground and pressed into pellets (2 mmthickness, 13 mm diameter) that were heated under a flow of 5% H2−N2 at 1200 °C for 12 h and subsequently slowly cooled down to roomtemperature under hydrogen.1.2. Preliminary Analysis of the Product. The reaction product

was characterized by X-ray powder diffraction (XRPD) using a BrukerAXS D8 Advance diffractometer equipped with a Ge primarymonochromator (selecting Kα1 radiation) and a Lynxeye detector.The as-prepared La1.2Sr0.8MnO4+δ consists in a single pure phase andcrystallizes in the K2NiF4 structure with space group I4/mmm. The cellparameters a = 3.82702(2) Å and c = 13.06869(7) Å are consistentwith those determined by Li and Greaves.7

1.3. Thermogravimetric Analysis. Thermogravimetric analysis(TGA) was performed using a NETZSCH STA 449F3 thermalanalyzer. The sample (∼ 30 mg) was heated at 1 °C/min in flowing O2(40 mL/min) up to 550 °C before being isothermally held at 550 °Cfor 6 h. Figure 1 shows that the sample undergoes a two-step oxidationprocess. The first one proceeds over the temperature range 250−400°C and involves significant oxygen uptake of 0.35 oxygen atoms performula unit (f.u.), resulting in the composition La1.2Sr0.8MnO4.35(1),whereas the second one with an onset at T ∼ 525 °C corresponds tomoderate oxygen uptake (0.05 oxygen/f.u.), leading to the finalLa1.2Sr0.8MnO4.40(1) stoichiometry. Between the two steps, in the

temperature range 400−525 °C, the sample appears to experience aslight mass loss (0.03 oxygen/f.u.).

1.4. Iodometric Titration. Iodometric titration has beenperformed on the as-prepared La1.2Sr0.8MnO4+δ material as well ason two oxidized samples heated under flowing O2 at 350 or 550 °C for15 h. The oxygen stoichiometries determined correspond to δas‑prepared= −0.04(2), δ350°C = 0.29(2), and δ550°C = 0.39(2). An analysis ofMn2O3 was performed to confirm the reliability of the experimentalprotocol and estimate the standard deviations.

1.5. Neutron Powder Diffraction. The Neutron PowderDiffraction (NPD) experiments were carried out on the high-fluxtwo-axis neutron powder diffractometer D208 at the Institut LaueLangevin (ILL, Grenoble, France) according to the strategy describedpreviously.9 A takeoff angle of 118° from the (117) plane of agermanium monochromator was chosen, giving a wavelength of λ =1.3594(1) Å and a resolution optimum of Δd/d ∼ 1.6 × 10−3, whileretaining still a high flux on the sample (∼ 107 n cm2 s−1). The sample(∼ 3 g) was loaded in the center of a quartz tube (8 mm diameter)between two plugs of quartz wool, which allowed unrestricted gas flowover the sample, while also being the mechanical support. Two K-typethermocouples were placed in the quartz tube: one just above thesample to monitor the temperature and another just below the sampleto regulate the temperature. The as-prepared sample was heated to∼950 °C (the highest temperature than can be reached in the quartztube) in a 5% H2−He flow then isothermally held at 950 °C for 80min before cooling. The gas was then switched from hydrogen tooxygen, and the sample was heated to ∼550 °C at 1 °C min−1 and heldat 550 °C for ∼3 h 45 min. A representation of the experimental setupand thermal treatment is displayed in Figure 2.

Full diffraction patterns (10−140°, 2θ) were collected every minuteon heating under hydrogen and every 5 min on cooling underhydrogen and heating under O2 flow. Since a significant backgroundwas observed, originating from the amorphous sample environment(quartz tube and quartz wool plugs) diffraction patterns were collectedfrom the empty quartz tube for 50 and 20 min at RT and 400 °C,respectively, in order to ensure a background correction. Thesecontributions were then normalized to the incident monitor count sothat they could be subtracted from the raw data without introducingsignificant noise. The diffraction patterns were analyzed by theRietveld method10 using the FullProf program11 and, wheneverpossible, the sequential refinement procedure available in Win-PLOTR.12 Refinements were essentially carried out in the tetragonalspace group I4/mmm. Some data, particularly for the oxidized phasesat 400 and 550 °C, were also refined in the Bmab space group13

consistent with a transformation toward a √2a × √2a × corthorhombic unit cell. The background remaining after subtractionof the sample environment contribution was determined using linearinterpolation then modeled by the Fourier filtering technique. Thepeak profile was modeled using a Thompson−Cox−Hastings pseudo-Voigt function;14 two asymmetry parameters were refined below 2θ ∼55°. Anisotropic strain peak broadening parameters were refinedaccording to the Stephen’s phenomenological model.15

1.6. Magnetic Measurements. Magnetic data were collected inthe temperature range 5−300 K using a Quantum Design MPMS

Figure 1. Oxygen content in La1.2Sr0.8MnO4+δ estimated from TGA inO2 flow (40 mL/min). The sample was heated at 1 °C min−1 then heldfor 6 h at 550 °C under the gas flow. Black line represents thetemperature profile.

Figure 2. Schematic representation of the temperature cycle used inthe in situ NPD experiment.

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SQUID magnetometer. Zero-field-cooled (ZFC) and field-cooled(FC) measurements were performed on warming using an appliedfield of 0.1 T.

2. RESULTS AND DISCUSSION2.1. Reduction Cycle Monitored by NPD. Sequential

refinements have been performed on data collected duringthermal cycle under hydrogen flow in I4/mmm space groupindicating that all sites remain fully occupied with the exceptionof the equatorial oxygen site (Oeq), which seems to be slightlydeficient (occupancy factor ∼0.98(1)) confirming thatLa1.2Sr0.8MnO3.96(2) is stable and does not intercalate oxygenunder ambient conditions.In order to achieve a good agreement between the observed

and calculated diffraction profiles, the hkl-dependence of theanisotropic strain broadening was modeled using Stephens’formulation15 resulting in a satisfactory fit (Figure 3). Such

strains, which can be interpreted as static fluctuations of cellparameters and their correlations,16 often occur in layeredoxides due to the presence of local defects, oxygen vacancies,stacking faults, etc.Refinements at 65 °C yield the same composition

La1.2Sr0.8MnO3.96(2) as that determined previously.15 Theslightly larger lattice parameters, a = 3.83188(6) Å and c =13.0863(3) Å, reflect thermal expansion due to the differenttemperatures at which the neutron and X-ray data werecollected. In addition, NPD showed that the slight deviation ofthe oxygen occupancy from unity affects the equatorial anionposition rather than the axial one in contrast to what has been

initially suspected.7 The structural parameters are listed inTable 1.

2.2. Thermal Oxidation Monitored by NPD. Thechemical oxidation of La1.2Sr0.8MnO4+δ is much more complexthan the behavior observed throughout the reduction cycle. Anoverall view is represented in Figure 4. More detailed

description of the patterns and their refinements will bediscussed later. The set of patterns can be divided into threemain regions. In the first region, spanning from 65 °C toaround 310 °C, all the peaks could be ascribed to the initialreduced phase La1.2Sr0.8MnO3.96(2). In the second region theinitial phase is converted to a new phase around 310 °C andsuch phase remains stable till around 520 °C when its cellparameters suddenly change along with a significant increase ofthe background at 2θ ≈ 34° and 2θ ≈ 70°. In the third and finalpart of the thermal cycle, the last phase remains stable for morethan 3 h all along the final plateau at 550 °C.

2.2.1. Oxidation Monitored by NPD from 65 to 410 °C. Inthe region 65−310 °C, all the peaks could be ascribed to theinitial phase La1.2Sr0.8MnO3.96(2). Sequential refinements show acontinuous and monotonically minor expansion of the cellparameters. The equatorial site (Oeq) remains deficient as longas the temperature is below 250 °C then slowly increases tillreaching full chemical occupation at 300 °C. While equatorialoxygen site is replenishing no evidence of interstitial oxygen hasbeen detected.In the range between T ∼ 310−400 °C, a clear deterioration

of the fits occurs. Close inspection of the patterns (Figure 5)reveals the presence of additional reflections. The extra peaksare straightforwardly ascribed to the conversion of the initial

Figure 3. Portion of the Rietveld profile for La1.2Sr0.8MnO3.96(2) at 65°C after a heating/cooling cycle under 5%H2−He modeled using ananisotropic model of strains.27 S.G. I4/mmm, RBragg = 2.43%; Rp =7.27%, Rwp = 6.06%, χ2 = 3.82.

Table 1. Structural Parameters of La1.2Sr0.8MnO3.96(2) Determined from NPD at 65 °C after a Heating/Cooling Cycle under 5%H2−Hea

atom x y z β11 × 104 β22 × 104 β33 × 104 occupancy

La/Sr 0 0 0.3571(1) 118(7) 118(7) 9(1) 0.6/0.4*Mn 0 0 0 80(9) 80(9) 15(2) 1.00*Oax 0 0 0.1745(2) 378(9) 378(9) 13(1) 1.00*Oeq 0 0.5 0 234(9) 106(8) 21(1) 0.98(1)

aSpace group I4/mmm (No. 139): a = 3.83188(6) Å, c = 13.0863(3) Å, (*) fixed, RBragg = 2.43%; Rp = 7.27%, Rwp = 6.06%, χ2 = 3.82.

Figure 4. Surface plot of the diffraction patterns versus temperaturefor La1.2Sr0.8MnO4+δ during oxidation. Darker pixels correspond tohigher intensity.

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phase La1.2Sr0.8MnO4+δ (δ ≤ 0.02) to an orthorhombic phaseindexed on the space group Bmab, related to the initialtetragonal unit cell by a √2a × √2a × c transformation.Indexation on Bmab is consistent with the raise of the (121)and (212) peaks respectively at 32.1° and 33.7°, 2θ (labeled *in Figure 5a) and splitting of the peaks (h h l) as shown inFigure 5b (shoulder on the left of the (220) reflection).The Bmab phase exhibits clear expansion of the basal plane

and contraction along the c axis. Such variations could beascribed to a partial conversion of the strong Jahn−Tellerdistorted Mn3+ ion to the undistorted Mn4+ ion. Consequently,the Bmab phase is assigned to an oxygen rich phaseincorporating interstitial oxygen.In the temperature range 310−410 °C, two-phase sequential

Rietveld refinements were attempted using the initial tetragonalphase and an orthorhombic Bmab phase, but the fits wereunstable and unable to reach convergence. Due to the smallintensity of superlattice reflections and little difference betweenthe a and b cell parameters in the oxidized Bmab phase, thecontribution of this phase to the diffraction patterns could bereasonably approximated assuming a tetragonal I4/mmm modelwith anisotropic strain broadening, as shown in Figure 6 for thedata collected at 360 °C. Consequently, sequential refinements

in this temperature range have been performed using twotetragonal I4/mmm phases constraining the isotropic thermaldisplacement of the interstitial oxygen in the two phases to beequal.The variation of cell parameters and evolution of the weight

fractions and interstitial oxygen occupancy assuming thisapproximation are displayed in Figure 7 (red and blue curvesfor stoichiometric and oxidized phases respectively). Asmentioned, the oxidized phase undergoes an axial deformationof the unit cell. For instance, if we consider the pattern at 360°C where the ratio of the two phases is ∼50:50, the latticeparameters of the stoichiometric phase are a = 3.8514(5) Å andc = 13.054(2) Å while those of the oxidized one are a =3.9116(5) Å and c = 12.733(2) Å (assuming the I4/mmmsymmetry).Isothermal chemical expansions are generally measured by

dilatometry. The oxygen partial pressure p(O2) is varied, andthe expansion is measured when the sample equilibrated toconstant length.17 The chemical expansion18 can also beestimated from the relative difference in the unit cell (ΔV/V)and the corresponding change in oxygen stoichiometry (Δδ) atconstant temperature. A rough estimation of the averagechemical expansion coefficient of La1.2Sr0.8MnO4+δ at 360 °Ccalculated as 1/3[ΔV/(V0 × Δδ)], where ΔV is the difference inthe unit cell of the oxidized and reduced phases and Δδ ∼ 0.34,results in ∼ −0. 6%.As visible in Figure 7d, the occupation factor of the

interstitial oxygen in the original phase remains lower than inthe oxidized one. Paying attention that the refinements in theextremes temperature range are not reliable due to a negligiblecontribution to the pattern from the minor phase, we assume aconfidence interval 350 −370 °C where the contribution foreach phase is above 20%. In such an interval, the occupancyfactor of Oint in the initial phase is not larger than 0.04(5) whilein the oxidized phase it remains constant at ∼0.13(1)consistent with an oxygen composition δ ∼ 0.26(2). As aconsequence, the phase separation that occurs over thetemperature range 300−400 °C could be ascribed to theexistence of a miscibility gap of the oxygen stoichiometry in therange 0.08 < δ < 0.26. Such phase separation is consistent withthe structural behavior of other R-P type oxides, such asLa2NiO4+δ with 0.05 < δ < 0.12,19 La2CoO4+δ with 0.18 < δ <0.25,20 and La2CuO4+δ cuprates superconductors with 0.02 < δ< 0.12.

Figure 5. Selected angular ranges of the NPD data collected on heating La1.2Sr0.8MnO4+δ in flowing O2. Progress of the oxidation reaction can bedirectly followed via the growth of the peaks ascribed to the oxidized phase (blue curve) and the simultaneous decrease of peaks ascribed to thestoichiometric phase (red curve). The orthorhombicity of the oxidized phase is detected by the presence of additional reflections labeled (*) in (a)and splitting of the peak (220) in (b). All the peaks are indexed on I4/mmm for clarity.

Figure 6. Rietveld refinement for La1.2Sr0.8MnO4+δ at 360 °C assumingtwo tetragonal phases. Upper marks stand for the stoichiometric phase,and lower marks for the orthorhombic oxidized one. Observed(markers), calculated (line), and difference profiles are shown.

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2.2.2. Oxidation Monitored by NPD from 410 to 520 °C.At T ∼ 410 °C, only the peaks ascribed to the oxidizedorthorhombic phase are present. The presence of a single phaseallows more accurate analysis of the patterns and the Rietveldrefinement became possible using the correct space groupBmab. The high occupancy of the interstitial oxygen sitepromotes orthorhombic symmetry in this temperature rangedue to the steric effect between the apical and interstitialoxygen, as observed in the intercalated cuprates and nickel-ates.13,19a,21 As we will see, the high temperature tetragonalphase (HTT) of the oxidized compound is stabilized above∼500 °C.22 The structural parameters for the Bmab phase at410 °C are listed in Table 2. At the end of the first oxidationstep, the stoichiometry corresponds to La1.2Sr0.8MnO4.30(1), andalthough the orthorhombicity is still detectable, it is quitelimited: a = 5.5514(2) Å, b = 5.5712(2) Å.Table 3 lists relevant interatomic distances in the

stoichiometric and oxidized materials at 300 and 410 °C,respectively. Although they do not relate to the sametemperature, the variations are larger than expected frompure thermal expansion, estimated at ∼13 × 10−6 K‑1 from the

cycle in flowing 5% H2 where the sample did not experienceany change in its oxygen stoichiometry. They indicate that theoxidation induced substantial axial compression of theoctahedra although the Mn−Oax bond length remains slightlylarger than Mn−Oeq (2.017(2) vs 1.9664(2) Å). Moreover, oneof the Oax−Oint distances is significantly increased (2.371(5) vs2.184(4) Å) allowing topotactic intercalation of the interstitialoxygen.Representation of the oxidized phase at 410 °C is depicted in

Figure 8. Large anisotropic displacement parameters areobserved for Oax, listed in Table 2. Similar anisotropy hasbeen reported for the interstitial oxygen defect inLa1.7Sr0.3Co0.5Ni0.5O4.12

23 and has been attributed to distortionof the Oax sites due to steric and electronic repulsion effectsrelated to the occupation of the neighboring Oint sites. Thisinteraction is consistent with the very short average distancebetween Oax and the interstitial oxygen site (2.189(4) Å).The evolution of the oxidized phase in the temperature range

410−520 °C has been investigated by sequential Rietveldrefinement in the Bmab space group. The variation of the cellparameters in this temperature range is shown in Figure 9; the c

Figure 7. Evolution of the (a) a and (b) c cell parameters, (c) weight fractions, and (d) interstitial oxygen occupancy in La1.2Sr0.8MnO4+δ on heatingin flowing O2 between 310 and 410 °C. The results are obtained from sequential two-phase refinements assuming the coexistence of two I4/mmmphases; (red) stoichiometric phase, (blue) oxidized phase.

Table 2. Structural Parameters of La1.2Sr0.8MnO4.30(1) Determined from NPD at 410 °C in Flowing O2a

atom x y z β11 × 104 β22 × 104 β33 × 104 β23 × 104 β12 × 104 Biso occupancy

La/Sr 0 0 0.3558(1) 144(10) 128(11) 23(1) 0.6/0.4#

Mn 0 0 0 81(20) 69(22) 18(2) 1.00#

Oax 0 0.027(1) 0.1589(2) 409(27) 496(34) 30(1) −13(10) 1.00#

Oeq 0.25 0.25 −0.0029(8) 145(14) 121(15) 60(1) 11(5) 1.00#

Oint 0.25 0.25 0.263(1) 1.4(3) 0.148(4)aSpace group Bmab (No. 64): a = 5.5514(2) Å, b = 5.5712(2) Å, c = 12.6364(2) Å, RBragg = 5.55%; Rp = 9.42%, Rwp = 7.1%, χ2 = 3.24; (#) fixed.

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axis exhibits a minor expansion ascribed to thermal expansion,while more interestingly the a and b parameters gradually tendbe equal reducing the orthorhombicity. The tilt of theoctahedra is also gradually reduced, as shown in Figure 9,consistent with decreased intensity of the superstructure peaksat 32.1° and 33.7° 2θ. At 520 °C, the transformation iscompleted and the phase is assumed tetragonal or pseudote-tragonal and indexed on I4/mmm. It is worth underlining thatsuch phase transitions are quite common in K2NiF4 structuretype oxides; the tetragonal symmetry at high temperature beingfavored by hardening of a transverse-optic-phonon mode at theX-point of the Brillouin zone of the HTT symmetry.22,24

2.2.3. Oxidation Monitored by NPD from 520 to 550 °C.For the last temperature segment 520−550 °C, sequentialrefinements have been carried out using the I4/mmm spacegroup and constraining the thermal displacement of theinterstitial and equatorial oxygen to be equal. This constraintwas imposed to eliminate the high correlation between theoccupancy and thermal displacement of Oint and providesoccupancies that are in reasonable agreement with TGA.In this temperature range, the patterns exhibit a progressive

variation of the background along with a decrease in intensity ofthe Bragg peaks (Figure 10). Indeed, if we consider Figure 11a,

we notice that the rise of the two broad bumps at ∼34 and 72°2θ (related to the background component of the pattern) isaccompanied by reduction in intensity of the Bragg reflections.

Table 3. Interatomic Distances Determined from SinglePhase Rietveld Refinement of a Stoichiometric I4/mmmPhase at 300 °C and an Oxidized Bmab Phase at 410 °C

atomsinteratomic distances inLa1.2Sr0.8MnO4.00(2) (Å)

interatomic distances inLa1.2Sr0.8MnO4.30(1) (Å) variation

La/Sr−Oint

2.3777(6)a 2.3780(9) + 0.1%

La/Sr−Oax

2.403(2)b 2.490(3) +3.6%

La/Sr−Oeq

2.6876(7) 2.658(7) −1.1%

Oax−Oint

2.184(4)a 2.371(5) and 2.189(4) +8.6% and+0.2%

Oeq−Oint

3.2809(1)a 3.193(1) −2.7%

Mn−Oax

2.279(1) 2.017(2) −11.5%

Mn−Oeq

1.92095(2) 1.9664(2) +2.4%

aSince no interstitial oxygen is detected in phase 1, a dummy atom wasintroduced to calculate the interatomic distances. bConcerns theshortest La/Sr−Oax bond (i.e., along the c-axis).

Figure 8. Bmab unit cell of La1.2Sr0.8MnO4.30(1) at 410 °C highlightingthe tilt of the octahedra and the environment of the interstitial oxygendefect.

Figure 9. Evolution of the (left) cell parameters and (right) octahedra tilt in the Bmab space group.

Figure 10. Evolution of raw NPD pattern between 510 and 550 °Chighlighting the significant increase of the background at 2θ ≈ 34° and2θ ≈ 72°. Temperature increases upwards.

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The total scattering intensity remains constant while the scalefactor in the Rietveld refinement decreases. Moreover, theprogressive reduction in the scale factor has a strong covariancewith the evolution in a and c lattice parameters, as shown inFigure 11.Effectively, the a axis increases while the c axis decreases with

the latter following the same trend as the scale factor. Suchvariation in the cell parameters is ascribed to further oxidationas confirmed by the evolution of the interstitial oxygenoccupancy (Figure 12) and is consistent with additionalconversion of the strong Jahn−Teller distorted Mn3+ ion tothe undistorted Mn4+ ion.

Indeed, the interstitial oxygen occupancy increases drasticallyat ∼520 °C to reach after prolonged isothermal heating at 550

°C, δ ∼ 0.42(2) with a rate k550 ∼ 1.50 × 10−4 min−1. Such ahigh δ value, in remarkable agreement with the TGAexperiment (Figure 1) and iodometric titration, has neverbeen reported so far for any n = 1 R-P phases, even preparedunder high oxygen pressures or obtained by electrochemicaloxidation (up to δ ∼ 0.18 for La2NiO4+δ,

13 δ ∼ 0.25−0.32 forLa2CoO4+δ,

20,25 and δ ∼ 0.10−0.18 for La2CuO4+δ26). The

origin of the diffuse background remains unclear. The change inthe cell parameters suggests that it relates to the bulk phaserather than reaction with the wall of the quartz tube; nocrystalline impurities are observed. The undulating backgroundremains constant after prolonged heating at 550 °C. The moreplausible option seems the existence of short-range order. Theoccurrence of this effect at high oxygen concentrations suggeststhat short-range order may occur within the interstitial oxygensublattice. Clearly, to elucidate such kind of disorder, additionalexperimental data are necessary.Details of the refinement in I4/mmm at 550 °C for

La1.2Sr0.8MnO4.42(2) are given in Table 4 and relevantinteratomic distances for the δ ∼ 0.30(1) at 520 °C and δ ∼0.42(2) at 550 °C are compared in Table 5. Although theyrelate to slightly different temperatures, they clearly show thatthe incorporation of additional oxygen defects results in furtherincrease of Oax−Oint (+ 0.6%) and a reduction of the distortion(Mn−Oax ∼ −1.6% and Mn−Oeq ∼ + 0.9%) leading to regularoctahedra (Mn−Oax/Mn−Oeq ∼ 1.00). The average chemicalexpansion coefficient at 550 °C can be evaluated from thechange in the unit cell volume ΔV between 5% H2 and O2

atmospheres (a = 3.85570(5), c = 13.1667(2) and a =3.9520(1), c = 12.6095(4), respectively) and the correspondingvalue of Δδ ∼ 0.46. Calculations using the approximate formula1/3[ΔV/(V0 × Δδ)], where V0 is the volume in 5% H2 gives anaverage chemical expansion of ∼0.0044. This value is smallerthan that determined for the La1−xSrxMnO3+δ perovskite

Figure 11. Evolution of (left) scale factor compared to the total scattering intensity in correlation with (right) the variation in cell parameters.

Figure 12. Evolution of the Oint occupancy factor in the oxidizedLa1.2Sr0.8MnO4+δ phase above 400 °C assuming a tetragonal symmetry.The straight line corresponds to the temperature profile.

Table 4. Structural Parameters of La1.2Sr0.8MnO4.42(2) Derived from Rietveld Refinement in I4/mmm (5 min data set, 550 °C)Collected after 3 h 45 min of Isothermal Heating in Flowing O2

a

atom x y z β11 × 104 β22 × 104 β33 × 104 occupancy

La/Sr 0 0 0.3556(6) 410(26) 410(26) 39(4) 0.6/0.4#

Mn 0 0 0 2860(300) 2860(300) 19(16) 1.00#

Oax 0 0 0.1591(8) 1206(80) 1206(80) 29(8) 1.00#

Oeq 0 0.5 0 451(70)* 857(80) 59(6)** 1.00#

Oint 0 0.5 0.25 451(70)* 451(70)* 59(6)** 0.21(1)

aSpace group I4/mmm (No. 139): a = 3.9707(4) Å, c = 12.520(1) Å, RBragg = 7.81%; Rp = 23.2%, Rwp = 12.7%, χ2 = 3.54 ; (*, **) constrained to beequal, (#) fixed. S220 = 42 × 10−4, S202 = −2 × 10−4.

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materials from in situ X-ray diffraction (0.05 ± 0.01)27 butcomparable to those determined for La2NiO4+δ.

17b

Structural refinements reveal that the anisotropic displace-ment parameters (ADPs) have strongly increased inLa1.2Sr0.8MnO4.42(2) at 550 °C in comparison to theintermediate La1.2Sr0.8MnO4.30(1) phase at 520 °C. Thisanisotropy is particularly striking for the manganese ionwhich exhibits unusual high in-plane βii value (∼ 0.28) whoseevolution as a function of temperature and δ is stronglycorrelated with the scale factor and probably related to theincrease of the noncrystalline contribution. It is likely that theeffects observed in the ADPs relates to the occurrence of thediffuse background. Short-range order of the interstitial oxygenions within the ab plane would be expected to result inconsequential displacements of neighboring ions, and the effectcould translate also to the Mn ions. In addition, we note thatthe effect is seen for the most oxidized sample, so smalldisplacements from the center of octahedra may occur for thesmall Mn4+ cations.It is interesting to mention the recent investigations on the

oxygen storage capability of the ordered perovskite manganitesBaYMn2O5+δ

28 and Ca2AlMnO5+δ.29 These manganites exhibit

the widest variation in the oxygen content among the doubleperovskite materials. The magnitude and sharpness of oxygenintake in BaYMn2O5+δ (∼3.7 wt % in 12 s representing the totalamount of oxygen that can be involved) is observed onswitching from 5% H2 to O2 and reflects the large variation in δ(from 0.0 to 1.0). The authors attribute the fast oxygen intaketo several important structural features: (i) the oxygen-deficientstructure in which an ionic conduction pathway within theyttrium plane inherently exists and (ii) the existence of onlythree distinct values for the oxygen content, that is, δ = 0.0, 0.5,and 1.0, presumably due to a strong tendency of oxygenordering which prohibits a gradual change in the oxygencontent inducing a steep chemical-potential gradient betweenoxygen-deficient (O5/O5.5) and fully oxygenated (O6) domains,which enhances the oxygen intake kinetics. In the presentstudy, we monitored the evolution of the Oint occupancy inLa1.2Sr0.2MnO4+δ on continuous heating in O2. The variation inδ from 0.30 to 0.42 (Figure 12) accompanied by the change inthe average oxidation state of Mn from +3.40 to +3.64 involvesa smaller oxygen uptake (∼0.54 wt %) thus resulting in slowerkinetics in comparison to the double perovskite materials.Moreover, the fact that oxygen is accommodated in interstitialpositions in La1.2Sr0.8MnO4+δ whereas it inserts in latticeoxygen-deficient positions in the ordered perovskites contrib-

utes to the slow oxidation kinetics of La1.2Sr0.8MnO4+δ withrespect to BaYMn2O5+δ and Ca2AlMnO5+δ.

2.3. NPD General Considerations. NPD ofLa1.2Sr0.8MnO4+δ heated in flowing O2 provides evidence of acomplex structural behavior as a function of temperature andoxygen stoichiometry as indicated by the variation of unit cellparameters with composition, Figure 13. The oxygen uptake

mainly occurs in two steps: (i) a two-phase process between300 and 400 °C involving an almost stoichiometric phase andan oxygen-rich orthorhombic phase with a gap in oxygencomposition 0.02 ≤ δ ≤ 0.32; (ii) a single-phase process for thesecond oxidation step above 500 °C involving an oxidizedphase La1.2Sr0.8MnO4+δ with δ ranging from 0.32 to 0.42. Thesymmetry of the oxidized phase is deeply affected bytemperature; at 400 °C, δ = 0.32 and the phase is orthorhombicwhile on further heating, the stoichiometry remains eitherroughly constant (NPD refinements) or slightly decreases(TGA) while the orthorhombicity decreases until 510 °C togive a tetragonal (or pseudotetragonal) phase. At highertemperature, a further increase in the oxygen stoichiometry isconcomitant with the presence of diffuse peaks in thebackground and major changes in the cell parameters.The phase and stoichiometry evolution is strongly correlated

with the evolution of the lattice parameters derived fromRietveld refinements (Figure 13). Both a and c parametersundergo strong variations at the onset of the two intercalationsteps, ∼ 300 and ∼500 °C. The c axis contracts between 300and 400 °C then exhibits pure thermal expansion on crossingthe metastability range of the δ ∼ 0.32 phase (400−500 °C)and finally contracts again at the onset of the second oxidation(∼ 500 °C). During both oxidative steps, a and b expandsignificantly. It is interesting to notice that the variations in thelattice parameters for the manganite system are similar to thoseobserved in La2CoO4+δ upon oxygen uptake20 (c contractionand a expansion) but opposite to those reported for theisostructural nickelates19b,30 and cuprates31 where the a and clattice parameters decreases and expands, respectively. Suchvariation has to be ascribed mainly to individual Mn−O bondlengths that probably provide the most sensitive probe for theelectronic effects of the oxygen defects in the two phases.Indeed, the two-step oxidation process is reflected by thevariation of the axial (decrease) and equatorial (increase) Mn−

Table 5. Interatomic Distances in La1.2Sr0.8MnO4+δDetermined from Rietveld Refinement in I4/mmm (5 minData Set) Collected in Flowing O2 at 520 °C (i.e., afterCompletion of the First Oxidation Step) and 550 °C after 3h 45 min of Isothermal Heating (i.e., after Completion of theSecond Oxidation Step)

interatomic distance (Å) 520 °C 550 °C variation

La/Sr−Oint 2.3807(8) 2.383(4) +0.1%La/Sr−Oax 2.479(2) 2.453(2) −1.0%La/Sr−Oeq 2.6850(9) 2.688(3) +0.1%Oax−Oint 2.273(1) 2.286(4) +0.6%Oeq−Oint 3.1673(1) 3.1299(3) −1.2%Mn−Oax 2.029(2) 1.996(5) −1.6%Mn−Oeq 1.96756(3) 1.9853(2) +0.9%

Figure 13. Evolution of the oxygen content in La1.2Sr0.8MnO4+δ heatedin flowing O2 obtained from (blue) TGA and (black and red) unit cellparameters refined from NPD data.

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O bond lengths as well as evolution of the bond length ratio(Mn−Oax/Mn−Oeq). The overall behavior of the Mn−O andMn−Oax/Mn−Oeq ratio reflects a decrease of the Jahn−Teller(J-T) distortion of the MnO6 octahedra consistent withordering of the hole in the dz2 orbital of the eg1 configuration.Indeed, this ratio varies from ∼1.19 in La1.2Sr0.8MnO3.96 at 20°C to ∼1.04 in the intermediate La1.2Sr0.8MnO4.30(1) phase at350 °C to finally reach ∼1.0 in La1.2Sr0.8MnO4.42(2) at 550 °Cafter 3 h 45 min of isothermal heating.The above description is confirmed by the evolution of the

oxidation states of the manganese ions throughout theoxidation process (Figure 14) assuming a mixture of Mn3+

and Mn2+ in the reduced phase and mixture of Mn3+ and Mn4+

in the oxidized phase. The composition of the original reducedmaterial, 72% Mn3+; 28% Mn2+, varies up to more than 90% forMn3+, a point at which the material undergoes the first phasetransition. This implies that the finite solubility for excess latticeoxygen (up to δ ∼ 0.02) is strongly related to the limitedcoexistence of Mn3+ and Mn4+ cations in the same lattice. Theintermediate oxidized phase appear with δ ∼ 0.30(1)corresponding to 60% Mn3+; 40% Mn4+ to finally reach 36%Mn3+; 64% Mn4+ in the δ ∼ 0.42(2) terminal material withinwhich the J-T distortion is no more observable.2.4. Magnetic Characterization. While the main focus of

the present paper is the structural response to oxygen uptake,we also wished to investigate the effects of the oxygen contenton the magnetic behavior. Figure 15 shows the temperaturedependence of the reciprocal magnetic susceptibility of the twosamples heated for 15 h under O2 at 350 and 550 °Ccorresponding to the δ = 0.32(2) and δ = 0.42(2) compositionsas determined by iodometric analysis. A cusp is seen at lowtemperature in the zero-field cooled (ZFC) at Tf ∼ 18.7 Kbelow which deviation between ZFC and field-cooled (FC)susceptibility characteristic of spin glass behavior32 is observed(Figure 16).The magnetic data are very similar to those previously

reported for La1.2Sr0.8MnO4.27.7 The materials show complex

behavior that is incompatible with the Curie−Weiss law, andthe inverse susceptibility plots (Figure 15) show a decrease ingradient on cooling. Figure 15 indicates values for the Weissconstant (θ) and effective moment for high and lowtemperature regions assuming applicability of the Curie−

Weiss law. The high temperature θ value suggests thatferromagnetic (FM) interactions are dominant, which iscompatible with strong Mn3+(dx2 − y2)−O2p−Mn4+(dx2 −y2)0 superexchange and double exchange interactions. Theeffective magnetic moments extracted for the data with T > 250K (μeff ∼ 5.64 and 4.60 μB for δ = 0.32 and 0.42, respectively)are larger than those expected from the spin-only values (μeff ∼4.47 and ∼4.26 μB). Moreover, the lower temperature data areindicative of higher moments of 8.10 μB and 6.43 μB. Aspreviously discussed,6 the enhanced magnetic moments suggestthat FM clusters occur, and the size increases as thetemperature decreases. Although spins within the clusters arealigned parallel, the resultant moments on individual clustersare randomly distributed.

3. CONCLUSIONIn the course of the investigation of the in situ high temperatureoxidation under oxygen flow of La1.2Sr0.8MnO4+δ (preparedunder reducing atmosphere), a new solid solution with 0.30 < δ< 0.42 was obtained and characterized.After oxygen intercalation at ∼300 °C at the equatorial site

yielding the stoichiometric material La1.2Sr0.8MnO4.00(2), a two-phase region consisting of an almost stoichiometric tetragonal(I4/mmm) phase La1.2Sr0.8MnO4.00(2) and an oxygen-richorthorhombic (Bmab) phase La1.2Sr0.8MnO4.30(1) is found

Figure 14. Evolution of the mixed oxidation states Mn2+ (blue), Mn3+

(red), and Mn4+ (green) in La1.2Sr0.8MnO4+δ heated under O2 flow.The region 300−400 °C has been omitted due to the coexistence oftwo phases. Straight line represents the temperature profile.

Figure 15. Inverse of the magnetic susceptibility (zero-field cooled)for La1.2Sr0.8MnO4+δ: (blue) oxidized at 350 °C under O2 for 15 h; forT > 250 K, μeff = 5.64 μB, θ ∼ 90 K and for 20 < T/K < 150 K, μeff =8.10 μB, θ ∼ −2.7 K; (red) oxidized at 550 °C under O2 for 15 h; for T> 250 K, μeff = 4.60 μB, θ ∼ 90 K and for 20 <T/K < 150 K, μeff = 6.43μB, θ ∼ 8.5 K.

Figure 16.Magnetic susceptibility of La1.2Sr0.8MnO4+δ; (blue) oxidizedunder O2 at 350 °C and (red) 550 °C; ratio Mn3+/Mn4+ determinedfrom iodometric titration.

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between 300−400 °C. From 400 °C to ∼510 °C theorthorhombicity of the system decreases until it disappearswith minor variation in δ. Finally, above 510 °C the systemundergoes further oxygen intercalation to reach δ ∼ 0.42(2).An important observation from this work is that the excess

oxygen incorporated into the δ ∼ 0.30(1) and δ ∼ 0.42(2)phases involves different mechanisms: a two-phase process forthe first uptake at 300 °C and a single-phase process for thesecond uptake at 510 °C. An accurate determination of thestructures of these phases and understanding the origin of thenoncrystalline contribution at high temperature (550 °C) mustawait data taken at higher resolution and over a much broader dspacing to detect subtle structural response of theLa1.2Sr0.8MnO4+δ system to oxygen exchange. In particular,the presence of complex structural modulations as observed inLa2CoO4.14

33 could be considered.With respect to the high temperature properties in air,

Munnings et al.5a reported on the stability of La2−x SrxMnO4+δsolid solution series (0.6 ≤ x ≤ 2) through the use of in situhigh temperature X-ray diffraction in air up to 800 °C. Theyfound that the overall stability of the materials below 800 °Cwas good with no decomposition products observed. However,since neutron diffraction data are ideally required, the stabilityin air at the high temperatures required for SOFC applicationand oxygen composition change will be the subject of futureneutron studies.

■ AUTHOR INFORMATIONNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank A. Daramsy for assistance with the collection of theNPD data at the Institut Laue Langevin (ILL), Grenoble, J.Hanlon (Post-Doc, ISCR), and T. Roisnel (CDIFX, ISCR) forfruitful discussions. We are grateful to T. Guizouarn (ISCR) forallowing us access to the SQUID magnetometer. We thank theRegion Bretagne for the provision of a postdoctoral funding(SAD call) to D.S. This work has been supported by the FrenchNational Agency for Research (ANR) through the IDEA-MATproject.

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