Unprecedented High Solubility of Oxygen Interstitial Defects in La

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Unprecedented High Solubility of Oxygen Interstitial Defects in La1.2Sr0.8MnO4+δ up to δ ∼ 0.42 Revealed by In Situ High Temperature Neutron 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 Matériaux, UMR CNRS 6226, Université de Rennes 1, 263 Avenue du Général 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 (RP) La1.2Sr0.8MnO4+δ phases has been monitored in situ in flowing O2 on heating over the temperature range 65 < T/°C < 550 by means of neutron powder diffraction (instrument D20, ILL/Grenoble). Sequential Rietveld refinement showed that the I4/mmm nearly stoichiometric phase undergoes a first oxygen uptake in the temperature range 300−400 °C with phase separation 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 Bmab oxidized phase is present. The orthorhombicity decreases progressively on further heating, up to 510 °C with minor variation in the oxygen content. Above ∼510 °C, the system undergoes a second abrupt oxidation step involving a single phase process to reach after prolonged isothermal heating at 550 °C, δ ∼ 0.42(2). Such a high solubility for the excess-oxygen defects has never been reported so far for a K2NiF4-type structure. The La1.2Sr0.8MnO4+δ system is thus interesting for the fundamental studies of structural distortions induced by the intercalation 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

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INTRODUCTION In recent years, there has been considerable interest in the K2NiF4-type oxides as possible solid oxide fuel cell cathode materials after the discovery of significant oxide ion diffusivity in the La2NiO4+δ compositions.1 The K2NiF4-type oxides are structurally related to the perovskite oxides and consist of alternating ABO3 (perovskite) and AO (rock salt) layers along the c-axis. In these materials, there is sufficient space in the AO layer to accommodate an excess of oxygen as interstitial species, charge-balanced through the oxidation of the B-site cation. This leaves a favorable ab plane pathway for oxygen mobility2 and thus introducing a new type of mixed ionic−electronic conductor (MIEC). Although there has been considerable attention devoted to the redox properties of transition metal ions in perovskite oxides,3 equivalent studies for high oxidation states in K2NiF4-type oxides have not been reported, despite the structural similarity of these phases. Furthermore, several materials with the K2NiF4 structure were found to have mixed conductivity as well as competitive stability and thermal expansion properties, with regards to the existing perovskite-type cathode materials. However, the © XXXX American Chemical Society

majority of these investigations have been concentrated on the development of Ni, Co, and Fe-substituted MIEC4 and their suitability as electrodes materials for high temperature electrochemical devices. On the other hand, little work has been reported on the related layered manganese oxides. These materials have, however, attracted attention following the discovery of a large negative magnetoresistance in alkaline-earth doped Ln−Mn−O systems and significant efforts were directed toward the development of new compositions5 and the determination of their structural, electrical, and magnetic properties below room temperature.6 In particular, the electron-doped material La1.2Sr0.8MnO3.94 prepared by Li and Greaves with the intention of investigating Mn2+−O2p−Mn3+ (dx2−y2)1−O2p−(dx2−y2)0 ferromagnetic interactions has drawn our attention as potential materials for MIEC application. 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, 2013 Revised: September 11, 2013

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dx.doi.org/10.1021/cm402194q | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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temperature range 400−525 °C, the sample appears to experience a slight mass loss (0.03 oxygen/f.u.). 1.4. Iodometric Titration. Iodometric titration has been performed on the as-prepared La1.2Sr0.8MnO4+δ material as well as on two oxidized samples heated under flowing O2 at 350 or 550 °C for 15 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 of Mn2O3 was performed to confirm the reliability of the experimental protocol and estimate the standard deviations. 1.5. Neutron Powder Diffraction. The Neutron Powder Diffraction (NPD) experiments were carried out on the high-flux two-axis neutron powder diffractometer D208 at the Institut Laue Langevin (ILL, Grenoble, France) according to the strategy described previously.9 A takeoff angle of 118° from the (117) plane of a germanium monochromator was chosen, giving a wavelength of λ = 1.3594(1) Å and a resolution optimum of Δd/d ∼ 1.6 × 10−3, while retaining 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 flow over the sample, while also being the mechanical support. Two K-type thermocouples were placed in the quartz tube: one just above the sample to monitor the temperature and another just below the sample to regulate the temperature. The as-prepared sample was heated to ∼950 °C (the highest temperature than can be reached in the quartz tube) in a 5% H2−He flow then isothermally held at 950 °C for 80 min before cooling. The gas was then switched from hydrogen to oxygen, and the sample was heated to ∼550 °C at 1 °C min−1 and held at 550 °C for ∼3 h 45 min. A representation of the experimental setup and thermal treatment is displayed in Figure 2.

For an electrode material in a SOFC, it is essential to study the behavior under operating conditions (high temperature under reducing and/or oxidizing gases) to get insight into the kinetics and thermodynamics of the oxygen-exchange processes and structural changes to define the limits for application. It is worthwhile mentioning that no neutron diffraction data was reported so far on La1.2Sr0.8MnO4+δ. The large amount of excess lattice oxygen (δ ∼ 0.27) that can be accommodated in this material could be not only a valuable asset for ionic conductivity properties but also a unique opportunity to fully investigate the structural response of a K2NiF4 structure-type material to exceptionally wide range of oxygen composition.

1. EXPERIMENTAL SECTION 1.1. Synthesis. A powder sample with the nominal composition La1.2Sr0.8MnO4 was prepared via the citrate/ethylene glycol sol−gel route. La2O3, MnO (Aldrich, 99%), and SrCO3 (Aldrich, ≥ 99.9%) was dissolved in dilute nitric acid warmed to 80 °C. Citric acid (Acros Organic, 99%) and ethylene-glycol (Sigma-Aldrich, > 99%) were added to the solution to give an ethylene-glycol/citric-acid/metal ion molar ratio of 4:2:1. The mixture was then stirred and heated until a gel and subsequently a resin was formed. The resin was calcined overnight at ∼600 °C in air to decompose the organic components. This precursor was then ground and pressed into pellets (2 mm thickness, 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 room temperature under hydrogen. 1.2. Preliminary Analysis of the Product. The reaction product was characterized by X-ray powder diffraction (XRPD) using a Bruker AXS D8 Advance diffractometer equipped with a Ge primary monochromator (selecting Kα1 radiation) and a Lynxeye detector. The as-prepared La1.2Sr0.8MnO4+δ consists in a single pure phase and crystallizes in the K2NiF4 structure with space group I4/mmm. The cell parameters a = 3.82702(2) Å and c = 13.06869(7) Å are consistent with those determined by Li and Greaves.7 1.3. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed using a NETZSCH STA 449F3 thermal analyzer. 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 °C for 6 h. Figure 1 shows that the sample undergoes a two-step oxidation process. The first one proceeds over the temperature range 250−400 °C and involves significant oxygen uptake of 0.35 oxygen atoms per formula unit (f.u.), resulting in the composition La1.2Sr0.8MnO4.35(1), whereas the second one with an onset at T ∼ 525 °C corresponds to moderate oxygen uptake (0.05 oxygen/f.u.), leading to the final La1.2Sr0.8MnO4.40(1) stoichiometry. Between the two steps, in the

Figure 2. Schematic representation of the temperature cycle used in the in situ NPD experiment. Full diffraction patterns (10−140°, 2θ) were collected every minute on heating under hydrogen and every 5 min on cooling under hydrogen and heating under O2 flow. Since a significant background was observed, originating from the amorphous sample environment (quartz tube and quartz wool plugs) diffraction patterns were collected from the empty quartz tube for 50 and 20 min at RT and 400 °C, respectively, in order to ensure a background correction. These contributions were then normalized to the incident monitor count so that they could be subtracted from the raw data without introducing significant noise. The diffraction patterns were analyzed by the Rietveld method10 using the FullProf program11 and, whenever possible, the sequential refinement procedure available in WinPLOTR.12 Refinements were essentially carried out in the tetragonal space group I4/mmm. Some data, particularly for the oxidized phases at 400 and 550 °C, were also refined in the Bmab space group13 consistent with a transformation toward a √2a × √2a × c orthorhombic unit cell. The background remaining after subtraction of the sample environment contribution was determined using linear interpolation then modeled by the Fourier filtering technique. The peak profile was modeled using a Thompson−Cox−Hastings pseudoVoigt function;14 two asymmetry parameters were refined below 2θ ∼ 55°. Anisotropic strain peak broadening parameters were refined according to the Stephen’s phenomenological model.15 1.6. Magnetic Measurements. Magnetic data were collected in the temperature range 5−300 K using a Quantum Design MPMS

Figure 1. Oxygen content in La1.2Sr0.8MnO4+δ estimated from TGA in O2 flow (40 mL/min). The sample was heated at 1 °C min−1 then held for 6 h at 550 °C under the gas flow. Black line represents the temperature profile. B



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

initially suspected.7 The structural parameters are listed in Table 1. 2.2. Thermal Oxidation Monitored by NPD. The chemical oxidation of La1.2Sr0.8MnO4+δ is much more complex than the behavior observed throughout the reduction cycle. An overall view is represented in Figure 4. More detailed

2. RESULTS AND DISCUSSION 2.1. Reduction Cycle Monitored by NPD. Sequential refinements have been performed on data collected during thermal cycle under hydrogen flow in I4/mmm space group indicating that all sites remain fully occupied with the exception of the equatorial oxygen site (Oeq), which seems to be slightly deficient (occupancy factor ∼0.98(1)) confirming that La1.2Sr0.8MnO3.96(2) is stable and does not intercalate oxygen under ambient conditions. In order to achieve a good agreement between the observed and calculated diffraction profiles, the hkl-dependence of the anisotropic strain broadening was modeled using Stephens’ formulation15 resulting in a satisfactory fit (Figure 3). Such

Figure 4. Surface plot of the diffraction patterns versus temperature for La1.2Sr0.8MnO4+δ during oxidation. Darker pixels correspond to higher intensity.

description of the patterns and their refinements will be discussed later. The set of patterns can be divided into three main regions. In the first region, spanning from 65 °C to around 310 °C, all the peaks could be ascribed to the initial reduced phase La1.2Sr0.8MnO3.96(2). In the second region the initial phase is converted to a new phase around 310 °C and such phase remains stable till around 520 °C when its cell parameters suddenly change along with a significant increase of the background at 2θ ≈ 34° and 2θ ≈ 70°. In the third and final part of the thermal cycle, the last phase remains stable for more than 3 h all along the final plateau at 550 °C. 2.2.1. Oxidation Monitored by NPD from 65 to 410 °C. In the region 65−310 °C, all the peaks could be ascribed to the initial phase La1.2Sr0.8MnO3.96(2). Sequential refinements show a continuous and monotonically minor expansion of the cell parameters. The equatorial site (Oeq) remains deficient as long as the temperature is below 250 °C then slowly increases till reaching full chemical occupation at 300 °C. While equatorial oxygen site is replenishing no evidence of interstitial oxygen has been 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 peaks are 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 an anisotropic model of strains.27 S.G. I4/mmm, RBragg = 2.43%; Rp = 7.27%, Rwp = 6.06%, χ2 = 3.82.

strains, which can be interpreted as static fluctuations of cell parameters and their correlations,16 often occur in layered oxides 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 The slightly larger lattice parameters, a = 3.83188(6) Å and c = 13.0863(3) Å, reflect thermal expansion due to the different temperatures at which the neutron and X-ray data were collected. In addition, NPD showed that the slight deviation of the oxygen occupancy from unity affects the equatorial anion position rather than the axial one in contrast to what has been

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

a

atom

x

y

z

β11 × 104

β22 × 104

β33 × 104

occupancy

La/Sr Mn Oax Oeq

0 0 0 0

0 0 0 0.5

0.3571(1) 0 0.1745(2) 0

118(7) 80(9) 378(9) 234(9)

118(7) 80(9) 378(9) 106(8)

9(1) 15(2) 13(1) 21(1)

0.6/0.4* 1.00* 1.00* 0.98(1)

Space 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. C



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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 be directly followed via the growth of the peaks ascribed to the oxidized phase (blue curve) and the simultaneous decrease of peaks ascribed to the stoichiometric 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.

phase La1.2Sr0.8MnO4+δ (δ ≤ 0.02) to an orthorhombic phase indexed on the space group Bmab, related to the initial tetragonal 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 in Figure 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 be ascribed to a partial conversion of the strong Jahn−Teller distorted Mn3+ ion to the undistorted Mn4+ ion. Consequently, the Bmab phase is assigned to an oxygen rich phase incorporating interstitial oxygen. In the temperature range 310−410 °C, two-phase sequential Rietveld refinements were attempted using the initial tetragonal phase and an orthorhombic Bmab phase, but the fits were unstable and unable to reach convergence. Due to the small intensity of superlattice reflections and little difference between the a and b cell parameters in the oxidized Bmab phase, the contribution of this phase to the diffraction patterns could be reasonably approximated assuming a tetragonal I4/mmm model with anisotropic strain broadening, as shown in Figure 6 for the data collected at 360 °C. Consequently, sequential refinements

in this temperature range have been performed using two tetragonal I4/mmm phases constraining the isotropic thermal displacement of the interstitial oxygen in the two phases to be equal. The variation of cell parameters and evolution of the weight fractions and interstitial oxygen occupancy assuming this approximation are displayed in Figure 7 (red and blue curves for stoichiometric and oxidized phases respectively). As mentioned, the oxidized phase undergoes an axial deformation of the unit cell. For instance, if we consider the pattern at 360 °C where the ratio of the two phases is ∼50:50, the lattice parameters of the stoichiometric phase are a = 3.8514(5) Å and c = 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/mmm symmetry). Isothermal chemical expansions are generally measured by dilatometry. The oxygen partial pressure p(O2) is varied, and the expansion is measured when the sample equilibrated to constant length.17 The chemical expansion18 can also be estimated from the relative difference in the unit cell (ΔV/V) and the corresponding change in oxygen stoichiometry (Δδ) at constant temperature. A rough estimation of the average chemical expansion coefficient of La1.2Sr0.8MnO4+δ at 360 °C calculated as 1/3[ΔV/(V0 × Δδ)], where ΔV is the difference in the 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 in the oxidized one. Paying attention that the refinements in the extremes temperature range are not reliable due to a negligible contribution to the pattern from the minor phase, we assume a confidence interval 350 −370 °C where the contribution for each phase is above 20%. In such an interval, the occupancy factor of Oint in the initial phase is not larger than 0.04(5) while in the oxidized phase it remains constant at ∼0.13(1) consistent with an oxygen composition δ ∼ 0.26(2). As a consequence, the phase separation that occurs over the temperature range 300−400 °C could be ascribed to the existence of a miscibility gap of the oxygen stoichiometry in the range 0.08 < δ < 0.26. Such phase separation is consistent with the structural behavior of other R-P type oxides, such as La2NiO4+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 6. Rietveld refinement for La1.2Sr0.8MnO4+δ at 360 °C assuming two 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. D



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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 heating in flowing O2 between 310 and 410 °C. The results are obtained from sequential two-phase refinements assuming the coexistence of two I4/mmm phases; (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

a

atom

x

y

z

β11 × 104

β22 × 104

β33 × 104

La/Sr Mn Oax Oeq Oint

0 0 0 0.25 0.25

0 0 0.027(1) 0.25 0.25

0.3558(1) 0 0.1589(2) −0.0029(8) 0.263(1)

144(10) 81(20) 409(27) 145(14)

128(11) 69(22) 496(34) 121(15)

23(1) 18(2) 30(1) 60(1)

β23 × 104

β12 × 104

Biso

occupancy

1.4(3)

0.6/0.4# 1.00# 1.00# 1.00# 0.148(4)

−13(10) 11(5)

Space 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.

cycle in flowing 5% H2 where the sample did not experience any change in its oxygen stoichiometry. They indicate that the oxidation induced substantial axial compression of the octahedra although the Mn−Oax bond length remains slightly larger than Mn−Oeq (2.017(2) vs 1.9664(2) Å). Moreover, one of the Oax−Oint distances is significantly increased (2.371(5) vs 2.184(4) Å) allowing topotactic intercalation of the interstitial oxygen. Representation of the oxidized phase at 410 °C is depicted in Figure 8. Large anisotropic displacement parameters are observed for Oax, listed in Table 2. Similar anisotropy has been reported for the interstitial oxygen defect in La1.7Sr0.3Co0.5Ni0.5O4.1223 and has been attributed to distortion of the Oax sites due to steric and electronic repulsion effects related to the occupation of the neighboring Oint sites. This interaction is consistent with the very short average distance between 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 Rietveld refinement in the Bmab space group. The variation of the cell parameters in this temperature range is shown in Figure 9; the c

2.2.2. Oxidation Monitored by NPD from 410 to 520 °C. At T ∼ 410 °C, only the peaks ascribed to the oxidized orthorhombic phase are present. The presence of a single phase allows more accurate analysis of the patterns and the Rietveld refinement became possible using the correct space group Bmab. The high occupancy of the interstitial oxygen site promotes orthorhombic symmetry in this temperature range due to the steric effect between the apical and interstitial oxygen, as observed in the intercalated cuprates and nickelates.13,19a,21 As we will see, the high temperature tetragonal phase (HTT) of the oxidized compound is stabilized above ∼500 °C.22 The structural parameters for the Bmab phase at 410 °C are listed in Table 2. At the end of the first oxidation step, the stoichiometry corresponds to La1.2Sr0.8MnO4.30(1), and although the orthorhombicity is still detectable, it is quite limited: 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 same temperature, the variations are larger than expected from pure thermal expansion, estimated at ∼13 × 10−6 K‑1 from the E



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axis exhibits a minor expansion ascribed to thermal expansion, while more interestingly the a and b parameters gradually tend be equal reducing the orthorhombicity. The tilt of the octahedra is also gradually reduced, as shown in Figure 9, consistent with decreased intensity of the superstructure peaks at 32.1° and 33.7° 2θ. At 520 °C, the transformation is completed and the phase is assumed tetragonal or pseudotetragonal and indexed on I4/mmm. It is worth underlining that such phase transitions are quite common in K2NiF4 structure type oxides; the tetragonal symmetry at high temperature being favored by hardening of a transverse-optic-phonon mode at the X-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, sequential refinements have been carried out using the I4/mmm space group and constraining the thermal displacement of the interstitial and equatorial oxygen to be equal. This constraint was imposed to eliminate the high correlation between the occupancy and thermal displacement of Oint and provides occupancies 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 of the Bragg peaks (Figure 10). Indeed, if we consider Figure 11a,

Table 3. Interatomic Distances Determined from Single Phase Rietveld Refinement of a Stoichiometric I4/mmm Phase at 300 °C and an Oxidized Bmab Phase at 410 °C atoms La/ Sr− Oint La/ Sr− Oax La/ Sr− Oeq Oax− Oint Oeq− Oint Mn− Oax Mn− Oeq

interatomic distances in La1.2Sr0.8MnO4.00(2) (Å) 2.3777(6)

a

interatomic distances in La1.2Sr0.8MnO4.30(1) (Å)

variation

2.3780(9)

+ 0.1%

2.403(2)b

2.490(3)

+3.6%

2.6876(7)

2.658(7)

−1.1%

2.184(4)a

2.371(5) and 2.189(4)

3.2809(1)a

3.193(1)

+8.6% and +0.2% −2.7%

2.279(1)

2.017(2)

−11.5%

1.92095(2)

1.9664(2)

+2.4%

a

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

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

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

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

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



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

°C, δ ∼ 0.42(2) with a rate k550 ∼ 1.50 × 10−4 min−1. Such a high δ value, in remarkable agreement with the TGA experiment (Figure 1) and iodometric titration, has never been reported so far for any n = 1 R-P phases, even prepared under high oxygen pressures or obtained by electrochemical oxidation (up to δ ∼ 0.18 for La2NiO4+δ,13 δ ∼ 0.25−0.32 for La2CoO4+δ,20,25 and δ ∼ 0.10−0.18 for La2CuO4+δ26). The origin of the diffuse background remains unclear. The change in the cell parameters suggests that it relates to the bulk phase rather than reaction with the wall of the quartz tube; no crystalline impurities are observed. The undulating background remains constant after prolonged heating at 550 °C. The more plausible option seems the existence of short-range order. The occurrence of this effect at high oxygen concentrations suggests that short-range order may occur within the interstitial oxygen sublattice. Clearly, to elucidate such kind of disorder, additional experimental 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 relevant interatomic distances for the δ ∼ 0.30(1) at 520 °C and δ ∼ 0.42(2) at 550 °C are compared in Table 5. Although they relate to slightly different temperatures, they clearly show that the incorporation of additional oxygen defects results in further increase of Oax−Oint (+ 0.6%) and a reduction of the distortion (Mn−Oax ∼ −1.6% and Mn−Oeq ∼ + 0.9%) leading to regular octahedra (Mn−Oax/Mn−Oeq ∼ 1.00). The average chemical expansion coefficient at 550 °C can be evaluated from the change 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 corresponding value of Δδ ∼ 0.46. Calculations using the approximate formula 1 /3[ΔV/(V0 × Δδ)], where V0 is the volume in 5% H2 gives an average chemical expansion of ∼0.0044. This value is smaller than that determined for the La1−xSrxMnO3+δ perovskite

The total scattering intensity remains constant while the scale factor in the Rietveld refinement decreases. Moreover, the progressive reduction in the scale factor has a strong covariance with the evolution in a and c lattice parameters, as shown in Figure 11. Effectively, the a axis increases while the c axis decreases with the latter following the same trend as the scale factor. Such variation in the cell parameters is ascribed to further oxidation as confirmed by the evolution of the interstitial oxygen occupancy (Figure 12) and is consistent with additional conversion of the strong Jahn−Teller distorted Mn3+ ion to the undistorted Mn4+ ion.

Figure 12. Evolution of the Oint occupancy factor in the oxidized La1.2Sr0.8MnO4+δ phase above 400 °C assuming a tetragonal symmetry. The straight line corresponds to the temperature profile.

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

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 O2a atom

x

y

z

β11 × 104

β22 × 104

β33 × 104

occupancy

La/Sr Mn Oax Oeq Oint

0 0 0 0 0

0 0 0 0.5 0.5

0.3556(6) 0 0.1591(8) 0 0.25

410(26) 2860(300) 1206(80) 451(70)* 451(70)*

410(26) 2860(300) 1206(80) 857(80) 451(70)*

39(4) 19(16) 29(8) 59(6)** 59(6)**

0.6/0.4# 1.00# 1.00# 1.00# 0.21(1)

Space 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 be equal, (#) fixed. S220 = 42 × 10−4, S202 = −2 × 10−4. a

G



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utes to the slow oxidation kinetics of La1.2Sr0.8MnO4+δ with respect to BaYMn2O5+δ and Ca2AlMnO5+δ. 2.3. NPD General Considerations. NPD of La1.2Sr0.8MnO4+δ heated in flowing O2 provides evidence of a complex structural behavior as a function of temperature and oxygen stoichiometry as indicated by the variation of unit cell parameters with composition, Figure 13. The oxygen uptake

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

520 °C

550 °C

variation

La/Sr−Oint La/Sr−Oax La/Sr−Oeq Oax−Oint Oeq−Oint Mn−Oax Mn−Oeq

2.3807(8) 2.479(2) 2.6850(9) 2.273(1) 3.1673(1) 2.029(2) 1.96756(3)

2.383(4) 2.453(2) 2.688(3) 2.286(4) 3.1299(3) 1.996(5) 1.9853(2)

+0.1% −1.0% +0.1% +0.6% −1.2% −1.6% +0.9%

materials from in situ X-ray diffraction (0.05 ± 0.01)27 but comparable to those determined for La2NiO4+δ.17b Structural refinements reveal that the anisotropic displacement parameters (ADPs) have strongly increased in La 1.2 Sr 0.8 MnO 4.42(2) at 550 °C in comparison to the intermediate La1.2Sr0.8MnO4.30(1) phase at 520 °C. This anisotropy is particularly striking for the manganese ion which exhibits unusual high in-plane βii value (∼ 0.28) whose evolution as a function of temperature and δ is strongly correlated with the scale factor and probably related to the increase of the noncrystalline contribution. It is likely that the effects observed in the ADPs relates to the occurrence of the diffuse background. Short-range order of the interstitial oxygen ions within the ab plane would be expected to result in consequential displacements of neighboring ions, and the effect could translate also to the Mn ions. In addition, we note that the effect is seen for the most oxidized sample, so small displacements from the center of octahedra may occur for the small Mn4+ cations. It is interesting to mention the recent investigations on the oxygen storage capability of the ordered perovskite manganites BaYMn2O5+δ28 and Ca2AlMnO5+δ.29 These manganites exhibit the widest variation in the oxygen content among the double perovskite materials. The magnitude and sharpness of oxygen intake in BaYMn2O5+δ (∼3.7 wt % in 12 s representing the total amount of oxygen that can be involved) is observed on switching from 5% H2 to O2 and reflects the large variation in δ (from 0.0 to 1.0). The authors attribute the fast oxygen intake to several important structural features: (i) the oxygen-deficient structure in which an ionic conduction pathway within the yttrium plane inherently exists and (ii) the existence of only three distinct values for the oxygen content, that is, δ = 0.0, 0.5, and 1.0, presumably due to a strong tendency of oxygen ordering which prohibits a gradual change in the oxygen content inducing a steep chemical-potential gradient between oxygen-deficient (O5/O5.5) and fully oxygenated (O6) domains, which enhances the oxygen intake kinetics. In the present study, we monitored the evolution of the Oint occupancy in La1.2Sr0.2MnO4+δ on continuous heating in O2. The variation in δ from 0.30 to 0.42 (Figure 12) accompanied by the change in the average oxidation state of Mn from +3.40 to +3.64 involves a smaller oxygen uptake (∼0.54 wt %) thus resulting in slower kinetics in comparison to the double perovskite materials. Moreover, the fact that oxygen is accommodated in interstitial positions in La1.2Sr0.8MnO4+δ whereas it inserts in lattice oxygen-deficient positions in the ordered perovskites contrib-

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

mainly occurs in two steps: (i) a two-phase process between 300 and 400 °C involving an almost stoichiometric phase and an oxygen-rich orthorhombic phase with a gap in oxygen composition 0.02 ≤ δ ≤ 0.32; (ii) a single-phase process for the second oxidation step above 500 °C involving an oxidized phase La1.2Sr0.8MnO4+δ with δ ranging from 0.32 to 0.42. The symmetry of the oxidized phase is deeply affected by temperature; at 400 °C, δ = 0.32 and the phase is orthorhombic while on further heating, the stoichiometry remains either roughly constant (NPD refinements) or slightly decreases (TGA) while the orthorhombicity decreases until 510 °C to give a tetragonal (or pseudotetragonal) phase. At higher temperature, a further increase in the oxygen stoichiometry is concomitant with the presence of diffuse peaks in the background and major changes in the cell parameters. The phase and stoichiometry evolution is strongly correlated with the evolution of the lattice parameters derived from Rietveld refinements (Figure 13). Both a and c parameters undergo strong variations at the onset of the two intercalation steps, ∼ 300 and ∼500 °C. The c axis contracts between 300 and 400 °C then exhibits pure thermal expansion on crossing the 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 expand significantly. It is interesting to notice that the variations in the lattice parameters for the manganite system are similar to those observed in La2CoO4+δ upon oxygen uptake20 (c contraction and a expansion) but opposite to those reported for the isostructural nickelates19b,30 and cuprates31 where the a and c lattice parameters decreases and expands, respectively. Such variation has to be ascribed mainly to individual Mn−O bond lengths that probably provide the most sensitive probe for the electronic effects of the oxygen defects in the two phases. Indeed, the two-step oxidation process is reflected by the variation of the axial (decrease) and equatorial (increase) Mn− H



Article

O bond lengths as well as evolution of the bond length ratio (Mn−Oax/Mn−Oeq). The overall behavior of the Mn−O and Mn−Oax/Mn−Oeq ratio reflects a decrease of the Jahn−Teller (J-T) distortion of the MnO6 octahedra consistent with ordering 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 at 350 °C to finally reach ∼1.0 in La1.2Sr0.8MnO4.42(2) at 550 °C after 3 h 45 min of isothermal heating. The above description is confirmed by the evolution of the oxidation states of the manganese ions throughout the oxidation process (Figure 14) assuming a mixture of Mn3+ 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; for T > 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 20 250 K (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 are indicative of higher moments of 8.10 μB and 6.43 μB. As previously discussed,6 the enhanced magnetic moments suggest that FM clusters occur, and the size increases as the temperature decreases. Although spins within the clusters are aligned parallel, the resultant moments on individual clusters are randomly distributed.

3. CONCLUSION In the course of the investigation of the in situ high temperature oxidation under oxygen flow of La1.2Sr0.8MnO4+δ (prepared under 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 twophase region consisting of an almost stoichiometric tetragonal (I4/mmm) phase La1.2Sr0.8MnO4.00(2) and an oxygen-rich orthorhombic (Bmab) phase La1.2Sr0.8MnO4.30(1) is found I



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between 300−400 °C. From 400 °C to ∼510 °C the orthorhombicity of the system decreases until it disappears with minor variation in δ. Finally, above 510 °C the system undergoes 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 for the first uptake at 300 °C and a single-phase process for the second uptake at 510 °C. An accurate determination of the structures of these phases and understanding the origin of the noncrystalline contribution at high temperature (550 °C) must await data taken at higher resolution and over a much broader d spacing to detect subtle structural response of the La1.2Sr0.8MnO4+δ system to oxygen exchange. In particular, the presence of complex structural modulations as observed in La2CoO4.1433 could be considered. With respect to the high temperature properties in air, Munnings et al.5a reported on the stability of La2−x SrxMnO4+δ solid solution series (0.6 ≤ x ≤ 2) through the use of in situ high temperature X-ray diffraction in air up to 800 °C. They found that the overall stability of the materials below 800 °C was good with no decomposition products observed. However, since neutron diffraction data are ideally required, the stability in air at the high temperatures required for SOFC application and oxygen composition change will be the subject of future neutron studies.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Daramsy for assistance with the collection of the NPD data at the Institut Laue Langevin (ILL), Grenoble, J. Hanlon (Post-Doc, ISCR), and T. Roisnel (CDIFX, ISCR) for fruitful discussions. We are grateful to T. Guizouarn (ISCR) for allowing us access to the SQUID magnetometer. We thank the Région Bretagne for the provision of a postdoctoral funding (SAD call) to D.S. This work has been supported by the French National Agency for Research (ANR) through the IDEA-MAT project.

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