Effect of Oxygen Non Stoichiometry and Oxidation State of Transition

Chem. Mater. 2009, 21, 5307–5318 5307 DOI:10.1021/cm902443n

Effect of Oxygen Non Stoichiometry and Oxidation State of Transition Elements on High-Temperature Phase Transition in A-Site Deficient La0.95Ni0.6Fe0.4O3-δ Perovskite Elena Konysheva,*,† Emmanuelle Suard,‡ and John T. S. Irvine† †

School of Chemistry, University of St. Andrews, St. Andrews, Fife, KY 16 9ST, U.K., and ‡ Institut Laue-Langevin, BP 156X, 38042 Grenoble Cedex 9, France Received August 10, 2009. Revised Manuscript Received September 23, 2009

In situ high-temperature neutron powder diffraction, dilatometry, X-ray photoelectron spectroscopy (XPS), and thermal gravimetrical analysis (TGA) were applied to study crystal structure and phase transformations in A-site deficient nominal composition La0.95Ni0.6Fe0.4O3-δ with temperature and oxygen partial pressure variation. Neutron diffraction demonstrates that at room temperature the A-site deficient nominal composition La0.95Ni0.6Fe0.4O3-δ can be represented as a mixture of a stoichiometric phase with perovskite structure and NiO as a secondary phase. At room temperature the perovskite exhibits a rhombohedral structure with space group R3c (167), a = 5.4491(1) A˚, β = 60.761(1), and V = 116.372(3) A˚3. NiO secondary phase was refined to rhombohedral structure with space group R3m (No. 166), a = 2.9560(5) A˚, β = 59.99(1), and V = 18.260(2) A˚3. High-temperature phase transformation of the perovskite phase from rhombohedral (R3c) to cubic (Pm3m) symmetry was observed above 550 C. According to the refinement, the high temperature perovskite phase with cubic structure possesses minimal deviation from the ABO3 oxygen and cation stoichiometries. In the reduced form of the La0.95Ni0.6Fe0.4O3-δ, the cubic phase (Pm3m) appears above 700 C. Several factors seem to be responsible for the increase in the temperature of the R3c f Pm3m phase transition in the reduced form of the La0.95Ni0.6Fe0.4O3-δ: the increase in oxygen nonstoichiometry and the decrease in the fraction of Ni cations in high oxidation state (3þ) in the perovskite constituent. X-ray photoelectron spectroscopy indicates surface heterogeneity in the reduced form of La0.95Ni0.6Fe0.4O3-δ at room temperature. 1. Introduction Complex perovskites with the general formula A1-xA0 xNi1-yByO3 (A and A0 are rare-earth and alkaline-earth elements, correspondingly; and B is a transition metal) show mixed ionic conductivity and electronic conductivity. In addition, some of them exhibit metallic conductivity in a wide temperature range. As a consequence, these perovskites have been considered for numerous applications in solid state devices operating at high and intermediate temperatures: in heterogeneous catalysis,1,2 cathodes for solid oxide fuel cells3-7 and membrane reactors.8 Members of the LaNi1-yFeyO3-δ series with *Corresponding author. E-mail: [email protected]. Tel.: þ44-1334463844. Fax: þ44-1334-463808.

(1) Falcon, H.; Martinez-Lope, M. J.; Alonso, J. A.; Fierro, J. L. G. Solid State Ionics 2000, 131, 237. (2) Pena, M. A.; Fierro, J. L. G. Chem. Rev. 2001, 101, 1981. (3) Chiba, R.; Orui, H.; Komatsu, T.; Tabata, Y.; Nozawa, K.; Arakawa, M.; Sato, K.; Arai, H. J. Electrochem. Soc. 2008, 155, B575. (4) Knudsen, J.; Friehling, P. B.; Bonanos, N. Solid State Ionics 2005, 176, 1563. (5) Swierczek, K.; Marzec, J.; Pazubiak, D.; Zaja-c, W.; Molenda, J. Solid State Ionics 2006, 177, 1811. (6) Bevilacqua, M.; Montini, T.; Tavagnacco, C.; Fonda, E.; Fornasiero, P.; Graziani, M. Chem. Mater. 2007, 19, 5926. (7) Konysheva, E.; Irvine, J. T. S. J. Mater. Chem. 2008, 18, 5147. (8) Kharton, V. V.; Viskup, A. P.; Naumovich, E. N.; Tikhonovich, V. N. Mater. Res. Bull. 1999, 34, 1311.

r 2009 American Chemical Society

intermediate compositions exhibit high electronic conductivity and sufficient stability at elevated temperatures.9 They also show very good catalytic activity with respect to peroxide decomposition and oxygen reduction.10,11 X-ray powder diffraction (XRD) showed that the crystal structure of the compositions in the LaNi1-yFeyO3-δ series varies depending on Ni/Fe ratio.5,9,12 At room temperature in air, perovskites with a high Fe content, y g 0.6, exhibit orthorhombic structure with space group Pnma (No. 62), which is typical of LaFeO3. The stoichiometric and A-site deficient compositions with a moderate Ni content, 0.3 e y < 0.6, show rhombohedral structure R3c (No. 167). The coexistence of phases with different structure (orthorhombic/tetragonal or rhombohedral/ traces of orthorhombic) were observed in the region with high Ni content, y < 0.3.9,12 It is known from the literature13 that LaNiO3-δ perovskite is the most (9) Chiba, R.; Yoshimura, F.; Sakurai, Y. Solid State Ionics 1999, 124, 281. (10) Falcon, H.; Carbonio, R. E. J. Electroanal. Chem. 1992, 339, 69. (11) Carbonio, R. E.; Fierro, C.; Tryk, D.; Scherson, D.; Yeager, E. J. Power Sources 1988, 22, 387. (12) Falcon, H.; Goeta, A. E.; Punte, G.; Carbonio, R. E. J. Solid State Chem. 1997, 133, 379. (13) Nakamura, T.; Petzow, G.; Gauckler, L. J. Mater. Res. Bull. 1979, 14, 649.

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unstable among LaBO3 perovskites (B = Cr, Mn, Fe, Co, and Ni), and it starts decomposing to La2NiO4 and NiO in air at 1000 C. According to XRD, LaFeO3 exhibits orthorhombic structure up to 800 C.5 Phase transformations in the LaNi1-yFeyO3-δ series with a high Fe content were investigated by high temperature XRD only at 600 and 800 C.5 R3c rhombohedral structure was observed at 800 and 600 C for LaFe0.8Ni0.2O3 and LaFe0.6Ni0.4O3, respectively, indicating the Pnma f R3c phase transformation. In general, perovskites may undergo different phase transitions depending on the tilting of the anion octahedra or displacement of cations.14-16 Hightemperature phase transitions have been reported for simple perovskites (BaCeO3,17 La0.9Sr0.1Ga0.8Mg0.2O2.85,18 La0.75Sr0.25Cr0.5Mn0.5O3-δ,19 and Pr1-xSrxMnO320) and for double perovskites Sr2MWO6 (M = Ni, Zn, Co, Cu)21 characterized by neutron powder diffraction. Recently A-site deficient perovskites have been actively considered as cathode or anode materials due to good electrochemical characteristics and better chemical compatibility with other components in fuel cells.22-24 However, limited structural characterization studies have been carried out for this group of materials.19,25 Moreover, there is no information about crystal structure of compounds in the LaNi1-yFeyO3-δ series under moderate reducing condition at intermediate and high temperatures although it is important for their application as ceramic membrane reactors. Therefore, in the present study we have carried out systematically a structural characterization of A-site deficient La0.95Ni0.6Fe0.4O3-δ (parent and reduced under argon atmosphere) by neutron powder diffraction. In addition, several methods such as X-ray photoelectron spectroscopy, dilatometry, and thermal gravimetrical analysis were applied (i) to characterize the oxidation state of transition elements; (ii) to obtain additional evidence for structural and phase transformations which could take place with temperature and oxygen partial pressure variation; and (iii) to predict surface and grain boundary chemical composition, which plays vital functions in transport properties of perovskites. 2. Experimental Section 2.1. Sample Preparation. An initial composition La0.95Ni0.6Fe0.4O3-δ (LNF) produced by combustion spray pyrolysis (14) Glazer, A. M. Acta Crystallogr., Sect. A 1975, 31, 756. (15) Ritter, C.; Radaelli, P. G.; Lees, M. R.; Barratt, J.; Balakrishnan, G.; Paul, D. McK. J. Solid State Chem. 1996, 127, 276. (16) Howard, C. J.; Stokes, H. T. Acta Crystallogr., Sect. B 1998, 54, 782. (17) Knight, K. S. Solid State Ionics 1994, 74, 109. (18) Slater, P. R.; Irvine, J. T. S.; Ishihara, T.; Takita, Y. J. Solid State Chem. 1998, 139, 135. (19) Tao, S.; Irvine, J. T. S. Chem. Mater. 2006, 18, 5453. (20) Knizek, K.; Hejtmanek, J.; Jirak, Z.; Martin, C.; Hervieu, M.; Raveau, B.; Andre, G.; Bouree, F. Chem. Mater. 2004, 16, 1104. (21) Gateshki, M.; Igartua, J. M.; Hernandez-Bocanegra, E. J. Phys.: Condens. Matter 2003, 15, 6199. (22) Tao, S.; Irvine, J. T. S. Nat. Mater. 2003, 2, 320. (23) Konysheva, E.; Laatsch, J.; Wessel, E.; Tietz, F.; Christiansen, N.; Singheiser, L.; Hilpert, K. Solid State Ionics 2006, 177, 923. (24) Knudsen, J.; Friehling, P. B.; Bonanos, N. Solid State Ionics 2005, 176, 1563. (25) Konysheva, E.; Irvine, J. T. S. J. Power Sources 2009, 193, 175.

Konysheva et al. was supplied by PRAXAIR Inc., U.S.A. In addition, La0.95Ni0.6Fe0.4O3-δ powder was mildly reduced under argon (p(O2) ∼ 10-3 atm) at 800 C for 12 h. This material was assigned as LNF-red. For the sake of convenience, the abbreviation “LNF-red” will also be used further in the text for all experiments with a powder or a pellet carried out under argon atmosphere. 2.2. Characterization Methods. Neutron powder diffraction (NPD) measurements were carried on a D1A instrument, using a wavelength of λ = 1.909 A˚ (ILL, Grenoble, France). Measurements were carried out in a temperature range of 25-800 C under air in a quartz tube for the parent LNF powder. The LNFred powder was placed in the vanadium container and measured under secondary vacuum to avoid the reoxidation at high temperatures. The diffraction spectra were registered in the angular range 0 e 2Θ e 158 with a step size of 0.05. X-ray powder diffraction (XRD) data were recorded in air at room temperature (RT) in transmission mode on a Stoe Stadi-P diffractometer with Cu KR radiation (Stoe & Cie GmbH, Germany) and in reflection mode on a Philips analytical X-ray PW1710 diffractometer with Cu KR radiation (Nederlandse Philips Bedrijven B.V., The Netherlands). Si powder (Alfa Aesar, Karlsruhe, Germany) was used as the external standard for the calibration of the diffractometer. The diffraction data were refined by the Rietveld method,26,27 using the program General Structure Analysis System (GSAS).28 X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical state of elements in the solid solutions. La 3d, Fe 2p, O 1s, and C 1s core level spectra were recorded on powders with an ESCALAB II spectrometer (V.G., UK) using Al (hν = 1486.4 eV) radiation. The analyzing aperture was about 3 mm in diameter. Data were collected at a takeoff angle of 45, allowing the characterization of the surface and several near surface layers. Thermal gravimetric analysis (TGA) was carried out on NETZSCH TG 209 instrument (NETZSCHGeraetebau GmbH, Selb, Germany) to evaluate the total oxygen content in the LNF and LNF-red compositions as well as to estimate the ratio of Ni3þ and Ni2þ cations in the LNF-red. The initial weight of powders was about 60 mg. The effect of buoyancy was corrected using blank runs with alumina crucibles under corresponding flow rates and gas atmospheres: air, argon, and 5%H2-95%Ar (H2-Ar). The dilatometry investigation was carried out on a NETZSCH DIL 402C dilatometer (alumina holder) with a TASC 414/4 controller. The LNF pellet with a relative density of about 93% was tested in air and argon in the temperature range from room temperature to 900 C at a rate of 3 C/min. The Brunauer-Emmett-Teller surface area, SBET, was measured with nitrogen on a TriStar II Instrument (Micrometrics Instrument Corporation, Norcross, U.S.A.).

3. Results and Discussion 3.1. Phase Composition and Structure of A-Site Deficient La0.95Ni0.6Fe0.4O3-δ (LNF) at Room Temperature. According to XRD recorded in transmission mode at room temperature, A-site deficient LNF has rhombohedral structure.7 However, two very weak diffraction peaks corresponding to NiO were found in the XRD pattern of the LNF recorded in the reflection mode before (26) Rietveld, H. M. Acta Crystallogr. 1967, 22, 151. (27) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (28) Larson, A. C.; von Dreele, R. B. GSAS - Generalised Structure Analysis System; Los Alamos National Laboratory Report LAUR-86748; Los Alamos National Laboratory: Los Alamos, NM, 1994.

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Figure 1. (a) X-ray powder diffraction pattern of the LNF recorded in the reflection mode and (b) neutron diffraction pattern of the LNF at room temperature: observed (cross symbols), calculated (continuous line), and difference profiles (bottom line). Vertical bars show calculated reflections for different phases: upper, NiO; down, the stoichiometric composition LaNi0.579Fe0.421O3-δ (R3c, 167); Rwp = 5.93%, Rp = 4.82%, χ2 = 3.20.

and after calcination at 1350 C (Figure 1a). Refinement of the neutron powder diffraction pattern, which contains six clearly distinguished diffraction peaks of NiO, showed that the nominal LNF composition contains about 5 mol % NiO as a secondary phase. It could indicate that there is no A-site deficiency in this perovskite. The initial material LNF was further represented as a mixture of a stoichiometric phase LaNi0.579Fe0.421O3-δ and NiO. An initial refinement of B-site occupancy in LaNi0.579Fe0.421O3-δ showed the following values: 0.592(27) for Ni and 0.408(27) for Fe. The B-site refinement cannot be certain, however, because of the small difference in the neutron scattering lengths of Ni and Fe (10.3 fm and 9.45 fm, respectively). Therefore, the Ni/Fe ratio in the stoichiometric composition LaNi0.579Fe0.421O3-δ was fixed to 0.579/0.421. Figure 1b illustrates the NPD pattern of the nominal LNF composition recorded at room temperature, which also includes the Rietveld refinement calculated data and the difference plot between the experimental and calculated data. The final refined structural parameters obtained from neutron powder diffraction data for the LNF are listed in Table 1. The perovskite constituent with the stoichiometric composition LaNi0.579Fe0.421O3-δ exhibits rhombohedral structure: space group R3c(167), with La in 2(a), Ni/Fe in 2(b) and O in 6(e) sites; and a = 5.4491(1) A˚, β = 60.761(1), V = 116.372(1) A˚3, and Z = 2. The unit cell of NiO was refined as rhombohedral with space group R3m(166), with Ni in 1(a) and O in 1(b) sites; and a = 2.9560(5) A˚, β = 59.99(1), V = 18.260(2) A˚3, and Z = 1. 3.2. Evolution of Phase and Structure with Temperature. Initially we would like to check whether there is a change in space group symmetry of the rhombohedral


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phase with temperature variation or not. This could take place due to local displacements. The changes of space group symmetry to R3c (No. 161) and R3 (No. 148) were considered. The R-factors and χ2 evaluated for the twophase models {R3c/NiO}, {R3c/NiO}, and {R3/NiO} are presented in Table 2. Smaller R-factors and χ2 at 200 C were achieved for the initial model {R3c/NiO}: the stoichiometric composition LaNi0.579Fe0.421O3-δ with rhombohedral structure, R3c (No. 167), and NiO. Figure 2 illustrates the neutron diffraction patterns of the LNF (range 2Θ: 38.4-40.9) measured in a temperature range of 25-800 C. The rhombohedral distortion of the perovskite structure is characterized by splitting of the main reflections (Figure 2). In a temperature range up to 400 C, the splitting of the main reflections is clearly observed. The neutron diffraction data obtained at 200 and 400 C can be refined well with a two-phase model: the rhombohedral perovskite phase and NiO (Table 1). The R3c/NiO phase ratio was not fixed during the refinement. It was found that the concentration of NiO secondary phase gradually increases, and at 400 C it equals about 6.1 mol %. In this case, the nominal LNF composition can be represented as a mixture of LaNi0.568Fe0.421O3-δ and 6 mol % NiO. The Ni/Fe ratio for the perovskite constituent was fixed to 0.568/0.421, formally introducing B-site deficiency. In this case, χ2 was 1.82. The refinement of the La site occupancy was, further, carried out. Slightly better refinement factors and χ2 were obtained for a composition with La site occupancy lower than 1 and proportional to the sum of Ni and Fe site occupancies. Therefore, the final refinement of the NPD pattern obtained at 400 C was performed for the mixture of a stoichiometric phase LaNi0.574Fe0.426O3-δ and NiO (Table 1). It might also indicate that at intermediate temperatures the initial LNF composes of the three phases: a perovskite phase with minimal oxygen and cation nonstoichiometry, NiO and La2O3. The splitting of the reflection at about 37.2-42.6 gradually decreases at temperatures above 400 C. Notice that at 800 C there is no clear splitting of the peak, but it has an asymmetrical shape (Figure 2). This could indicate the appearance of a new phase with higher symmetry. The gradual decrease in the splitting could also indicate that the second order transformation takes place. In general, several possible symmetry transformations (to Pm3m; I2/ a f I4/mcm; and C2/c) could take place with the R3c perovskite structure according to the group-subgroup relationships.14-16 However, only the transformation R3c f Pm3m will meet the requirements: for a second order transformation to higher symmetry. The NPD pattern obtained at 800 C was refined with different models: the {Pm3m/NiO} two phase model, the {R3c/ NiO} two phase model, and the combined {R3c/Pm3m/ NiO} three phase model. In addition, the {R3c/NiO} two phase model and the combined {R3c/Pm3m/NiO} three phase model were also considered. According to the initial refinement carried out with the stoichiometrical phase LaNi0.579Fe0.421O3-δ (assuming the presence of

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Table 1. Refined Structure Parameters for the LNF between 25 and 800 C atom

parameter

25 C

200 C

400 C

600 C

700 C

800 C

Perovskite constituent La x y z Uiso (A˚2) occupancy La

R3c 1/4 1/4 1/4 0.0016(3) 1

R3c 1/4 1/4 1/4 0.0053(3) 1

R3c 1/4 1/4 1/4 0.0092(3) 1

R3c 1/4 1/4 1/4 0.0128(3) 1

Pm3m 0 0 0 0.0128 1

R3c 1/4 1/4 1/4 0.0145(4) 1

Pm3m 0 0 0 0.021(3) 1

R3c 1/4 1/4 1/4 0.0127(4) 1

Pm3m 0 0 0 0.053(4) 1

Ni/Fe

x y z Uiso (A˚2) Ni Fe

0 0 0 0.0013(3) 0.579 0.421

0 0 0 0.0032(3) 0.579 0.421

0 0 0 0.0051(3) 0.574 0.426

0 0 0 0.0069(3) 0.574 0.426

1/2 1/2 1/2 0.0069 0.574 0.426

0 0 0 0.0049(3) 0.570 0.430

1/2 1/2 1/2 0.011(2) 0.570 0.430

0 0 0 0.0060(3) 0.570 0.430

1/2 1/2 1/2 0.027(2) 0.570 0.430

x y z Uiso (A˚2) O

0.6975(2) -0.1975(2) 1/4 0.0056(2) 0.998(2)

0.6986(1) -0.1986(1) 1/4 0.0099(2) 0.997(1)

0.7007(2) -0.2007(2) 1/4 0.0149(2) 1

0.7016(2) -0.2016(2) 1/4 0.0198(3) 1

0 1/2 1/2 0.0198 1

0.7019(2) -0.2019(2) 1/4 0.0207(3) 1

0 1/2 1/2 0.042(2) 1

0.7040(3) -0.2040(3) 1/4 0.0202(3) 1

0 1/2 1/2 0.085(4) 1

5.4491(1) 60.761(1) 116.372(3)

5.4636(1) 60.680(1) 117.093(1)

5.4799(1) 60.590(1) 117.912(4)

5.4966(1) 60.500(1) 118.746(4)

3.9018(10) 90 59.40(5)

5.5046(1) 60.460(1) 119.168(1)

3.9078(3) 90 59.68(1)

5.5153(1) 60.413(1) 119.735(1)

3.9161(3) 90 60.04(1)

R3m 0 0 0 0.0002 0.96(2)

R3m 0 0 0 0.0014 0.98(1)

R3m 0 0 0 0.0044 0.97(1)

R3m 0 0 0 0.011(3) 0.98(1)

R3m 0 0 0 0.025(2) 1

R3m 0 0 0 0.030(3) 1

occupancy

1/2 1/2 1/2 0.0002 1

1/2 1/2 1/2 0.0010 1

1/2 1/2 1/2 0.0030 1

1/2 1/2 1/2 0.005(2) 1

1/2 1/2 1/2 0.012(4) 1

1/2 1/2 1/2 0.023(5) 1

a (A˚) β (deg) V (A˚3)

2.9560(5) 59.99(1) 18.260(2)

2.9638(6) 59.97(2) 18.394(2)

2.9717(4) 60.00(1) 18.556(2)

2.9774(3) 60.06(1) 18.688(2)

2.9813(3) 60.05(1) 18.759(2)

2.9855(4) 60.09(3) 18.852(1)

Rwp (%) Rp (%) χ2

5.93 4.82 3.2

5.52 4.54 2.07

5.17 4.30 1.79

occupancy O

occupancy a (A˚) β (deg) V (A˚3) NiO Ni

x y z Uiso (A˚)

occupancy O

x y z Uiso (A˚)

5.24 4.48 1.64

4.78 4.28 1.54

5.24 4.51 1.69

Table 2. Comparison of the Refinements with Different Models of the NPD Patterns Obtained at 200 and 800 C for the LNFa refinement factors temperature, C 200 800

phase model {R3c/NiO} {R3c/NiO} {R3/NiO} {R3c/Pm3m/NiO} {R3c/NiO} {R3c/Pm3m/NiO} {R3c/NiO} {Pm3m/NiO}

a

[La]/[Ni]/[Fe] ratio 1/0.579/0.421 1/0.579/0.421 1/0.579/0.421 1/0.558/0.421 1/0.558/0.421 1/0.570/0.430 1/0.558/0.421 1/0.558/0.421 1/0.558/0.421

Rwp, %

Rp, %

χ2

5.52 16.78 30.01 5.46 6.03 5.83 9.34 11.70 38.71

4.54 10.66 22.02 4.73 5.11 5.08 7.36 7.52 30.58

2.07 7.24 17.85 1.75 1.85 1.78 2.70 3.31 26.37

Refinement factors are presented after the subtraction of the background.

5 mol % NiO), the fraction of NiO further increases up to 7.5 mol % at 800 C, which is higher than A-site deficiency in the nominal LNF composition. La2O3 or related phases were not observed in the NPD patterns. The Ni/Fe ratios in both R3c and Pm3m perovskite constituents were fixed to 0.558/0.421 (LaNi0.558Fe0.421O3-δ and 7 mol % NiO). The R-factors and χ2 obtained for different models are presented in Table 2. The accuracy of the refinement of the {Pm3m/NiO} two phase model was very

poor: χ2 = 26.37, Rwp = 38.71% and Rp = 30.58%. For the {R3c/NiO} two phase model, the accuracy of refinement was much better: χ2 = 1.85, Rwp = 6.03%, and Rp = 5.11%. Slightly better fitting was achieved when the combined {R3 c/Pm3m/NiO} three phase model was used for the refinement: χ2 = 1.75, Rwp = 5.46%, and R p = 4.73%. In the case of the {R3c/Pm3m/NiO} model, however, the total number of variable parameters is more compared to those in the {R3c/NiO} model, and a small

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Figure 2. Evolution of neutron diffraction patterns of the LNF nominal composition with temperature variation.

Figure 3. Neutron diffraction pattern of the LNF at 800 C: observed (cross symbols), calculated (continuous line), and difference profiles (bottom line). Vertical bars show calculated reflections for different phases: upper, R3c (No. 167); middle, NiO; and lower, Pm3m (No. 221); Rwp = 5.24%, Rp = 4.51%, and χ2 = 1.69.

decrease in the refinement factors (R wp and Rp) and χ2 observed for the {R3c/ Pm3m/NiO} model can be considered as a significant improvement of the refinement carried out. The refinement of the La site occupancy was, further, carried out. Better refinement factors and χ2 were obtained for a composition with La site occupancy lower than 1 and proportional to the sum of Ni and Fe site occupancies. Therefore, the final refinement of the NPD pattern obtained at 800 C was carried out for the mixture of the perovskite phase with the stoichiometric composition LaNi0.570Fe0.430O3-δ (for both R3c and Pm3m constituents) and NiO with the combined {R3c/ Pm3m/NiO} three phase model (Tables 1 and 2). The refinement of the NPD pattern of the LNF obtained at 800 C is shown in Figure 3. The same procedure was applied for the refinement of the NPD patterns obtained at 600 and 700 C (Table 1). Figure 4 illustrates fraction of different phases in the LNF at different temperatures. The fraction of the perovskite constituent with cubic structure (Pm3m) increases, whereas that of the perovskite phase with rhombohedral structure (R3c) decreases above 400 C. Figure 5 and shows the variation in the lattice parameters of different phases in the nominal LNF composition as a function of temperature. The a lattice parameters of both R3c and Pm3m phases increase with the rise in the temperature. The beta angle of the perovskite constituent


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Figure 4. Fraction of different phases in the A-site deficient nominal composition LNF at different temperatures.

with rhombohedral structure (R3c) decreases from ∼60.76 to 60.40 between 25 and 800 C, indicating the decrease in the rhombohedral distortion. The oxygen occupancy in the perovskites with rhombohedral and cubic symmetry was refined. It was noticed that the oxygen occupancy in the perovskite constituent with rhombohedral structure (R3c) is very close to 1 in the whole temperature range investigated, indicating a very low concentration of oxygen vacancies (Table 1). The oxygen site occupancy in the phase with cubic structure (Pm3m) was estimated to be complete within the detection limit of Rietveld analysis. Figure 5d illustrates the variation in the normalized unit cell volume as a function of temperature for the perovskite constituents with rhombohedral and cubic symmetries. The normalized unit cell volumes of the perovskite constituents with rhombohedral and cubic symmetries are very close, indicating a high probability of a reversible phase transformation R3c T Pm3m at high temperatures. The properties of materials are inherently related to their structure. Figure 6 illustrates the linear thermal expansion of the LNF in air. The experimental data are presented in the coordinates “Technical alpha” vs “Temperature”. The linear thermal expansion coefficient (TEC) of the LNF changes with temperature variation. One can distinguish two regions between 300 and 900 C: 11.9 10-6 K-1 (300-550 C) and 13.4 10-6 K-1 (600-900 C). The change in slope at 550-600 C may relate to a phase transformation behavior. Figure 7 illustrates the structure of AB1-xB0 xO3 perovskite constituents (A = La; B and B0 =Ni, Fe) with rhombohedral symmetry (R3c) and cubic symmetry (Pm3m). The high temperature phase has cubic symmetry Pm3m (221), with La in 1(a), Ni/Fe in 1(b), and O in 3(e); with a = 3.9161(3) A˚, V = 60.04(1) A˚3, and Z = 1 at 800 C. Information about crystal structure of the compositions in the LaNi1-yFeyO3-δ series with y g 0.3 and phase transformations observed under air with the temperature variation is summarized in Figure 8. The main distortion from the ideal cubic perovskite structure could be due to the tilting of the [Ni/Fe]O6 octahedra in the perovskite constituent with the rhombohedral symmetry (R3c). The tilts occur about the a, b, and c axes in antiphase; in terms of the notation of Glazer,14 it can be represented as aaa . The average tilting angle of the [Ni/Fe]O6 octahedra, Æωæ, can be defined as (Figure 7c) Æωæ ¼ ð180 - ÆNi=Fe-O-Ni=FeæÞ=2

ð1Þ

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Figure 5. (a-c) Variation of the lattice parameters of different phases in the LNF as a function of temperature; (d) change in the normalized unit cell volume as a function of temperature for perovskite constituents with rhombohedral (R3c) and cubic (Pm3m) symmetries.

Figure 6. Thermal expansion of the LNF in air. The heating rate is 3 C/min.

where ÆNi/Fe-O-Ni/Feæ is the average superexchange angle ÆNi/Fe-O-Ni/Feæ, which can be evaluated from the refinement of the crystal structure of the phase with the rhombohedral symmetry. The average superexchange angle, the average tilting angle of the [Ni/Fe]O6 octahedra, and interatomic distances are presented in Table 3. The tilting angle decreases gradually, whereas the average Ni/Fe-O and Ni/Fe-Ni/Fe distances increase gradually with the temperature variation (Table 3). Local deformations of the [Ni/Fe]O6 octahedra could be an additional reason for the distortion from the ideal cubic perovskite structure, but they cannot be identified by NPD for the LNF because of the small difference in the neutron scattering lengths of Ni and Fe. The distortion of the La-O12 polyhedra decreases with the rise in the temperature. According to the refinement, the fraction of NiO gradually increases with the temperature variation (Figure 4). Ni3þ cations are situated on the B sublattice in the ABO3 perovskite structure. However, Ni3þ cations can reduce to Ni2þ with the increase in temperature.

Figure 7. Structure of AB1-xB0 xO3 perovskite constituents at high temperature: (a) rhombohedral symmetry (R3c) and (b) cubic symmetry (Pm3m). A, La, large dark spheres; O, small gray spheres; B/B0 -Ni/Fe cations are situated inside of the octahedra; (c) a fragment of the rhombohedral structure with a schematic representation of the average superexchange angle, ÆNi/Fe-O-Ni/Feæ, and the average tilting angle, Æωæ.

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Figure 8. Phase diagram of LaNiO3-δ-LaFeO3-δ in air. Crystal symmetry: Rh, rhombohedral; OR, orthorhombic; C, cubic; superscripts near the crystal symmetry symbols (for instance Rh “numbers”) are the references, and the superscript “P” means the data were obtained in the present study.

A fraction of Ni2þ cations remain on the B sublattice in the perovskite structure. Another fraction of the reduced Ni2þ cations could be displaced from the perovskite structure and segregate as the NiO secondary phase, further increasing concentration of NiO. Thermal gravimetrical analysis, however, did not show any weight change in air with the temperature variation up to 800 C, 7 which seems to be reasonable due to the presence of iron cations and their ability to change oxidation state through the double exchange mechanism.29 The presence of Fe5þ cations in compounds in the LaNi1-yFeyO3-δ series with y g 0.6 was demonstrated by M€ ossbauer spectroscopy.5 It was assumed that the presence of Fe5þ related to a charge disproportionate reaction induced by increasing concentration of Ni2þ. The nickel occupancy in NiO varies between 0.96 and 0.98 in a temperature range of 25 and 600 C, indicating the presence of Ni vacancies in nickel oxide secondary phase (Table 1). The existence of Ni vacancies in a Ni1-yO film with the cation deficiency in the range of 0 < y < 0.2 was reported in the literature.30 According to the theoretical studies the surface layer of nickel oxide is enriched in Ni vacancies by about a factor of 40.31 However, a deviation from cation stoichiometry in NiO was not revealed in the refinement of the data recorded at 700 and 800 C (Table 1). Both facts, that (i) the concentration of NiO phase increases with temperature (Figure 3) and (ii) nickel oxide reaches the nominal stoichiometry at 700-800 C, are in good agreement with each other. They can indicate that slight reduction of Ni3þ cations to lower oxidation state (2þ) takes place in air with the temperature variation although the weight change in this case is too small to be detected by TGA. The unit cell volume of NiO increases gradually in a temperature range of 25-800 C (Table 1). The beta angle of the rhombohedral unit cell (R3m) remains nearly constant at ∼60.0 in a temperature range of 25-400 C followed by a slight increase up to ∼60.1 above 400 C (29) Cox, P. A. Transition Metal Oxides; Clarendon Press: Oxford, 1995; p 283. (30) Lunkenheimer, P.; Loidl, A.; Ottermann, C. R.; Bange, K. Phys. Rev. B 1991, 44, 5927. (31) Duffy, D. M.; Tasker, P. W. Philos. Mag. 1984, 50, 143.


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(Figure 5c). It is accompanied by the gradual increase in the a lattice parameter of NiO (Figure 5c). 3.3. Evolution of Phase Composition and Structure with the Lowering of Oxygen Partial Pressure. NPD study was carried out for the La0.95Ni0.6Fe0.4O3-δ reduced at 800 C for 12 h under argon (p(O2) ∼ 10-3 atm). The reduced material was assigned as LNF-red. The LNF-red is composed of a perovskite constituent and NiO, as for the initial LNF material. According to the initial refinement, where the LNF-red was presented as a mixture of a stoichiometric phase LaNi0.579Fe0.421O3-δ and NiO, the concentration of NiO increases up to about 7 mol %. This is higher than the A-site deficiency in the nominal composition. In this case, diffraction lines related to La2O3 were not also observed. The Ni/Fe ratio in the perovskite constituent was, then, fixed to 0.558/0.421, introducing formally B-site deficiency. The refinement of the La site occupancy was further carried out. However, better refinement factors and χ2 were obtained for a composition with La site occupancy lower than 1 and proportional to the sum of Ni and Fe site occupancies (0.558 and 0.421, respectively). Therefore, the values of Ni and Fe site occupancies were normalized to that of La, and the final refinement of the NPD pattern obtained at 55 C was performed for the mixture of a stoichiometric phase LaNi0.570Fe0.430O3-δ and NiO. The final structural parameters obtained from neutron powder diffraction data are listed in Table 4. The perovskite constituent with the stoichiometric composition LaNi0.570Fe0.430O3-δ exhibits rhombohedral structure at 55 C: space group R3c (No. 167), a = 5.4513(1) A˚, β = 60.751(1), V = 116.486(1) A˚3, and Z = 2. The lattice parameters are slightly larger compared to the initial LNF. The unit cell of NiO was refined as rhombohedral with space group R3m (166), a = 2.9548(5) A˚, β = 60.03(1), V = 18.255(2) A˚3, and Z = 1. The same procedure was applied for data recorded for the LNF-red at high temperatures. According to the refinement, the concentration of the NiO did not change and was about 7 mol % in the whole temperature range. The change in space group symmetry of the perovskite phase with rhombohedral symmetry with temperature variation was not observed (Table 5). A high-temperature phase transition from rhombohedral symmetry (R3c) to cubic symmetry (Pm3m) in the LNF-red was observed above 700 C. Figure 9 illustrates the NPD pattern of the LNF-red obtained at 800 C. According to the refinement, the concentration of the perovskite with the cubic symmetry (Pm3m) is about 6.5 mol %, which is less by about a factor of 3 compared to that in the parent LNF at 800 C. This correlates with the results of the dilatometry measurements (Figure 10). The LNF-red shows linear expansion with temperature variation between 300 and 725 C with TEC = 14.2 10-6 K-1 under argon atmosphere. The linear expansion coefficient increases up to 15.4 10-6 K-1 in a temperature range of 750-900 C. The same behavior was observed after exposure for about 15 h to argon atmosphere. The crystal structure of the R3c phase in the LNF-red is similar to that in the initial LNF

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Table 3. Interatomic Distances and Angles in the LNF for the Perovskite Constituent with Rhombohedral Symmetry (R3c) and with Cubic Symmetry (Pm3m) temperature, C Perovskite phase symmetry R3c

parameter dLa-O (A˚) ÆdLa-Oæ (A˚) distortiona (103 Δ) ÆdNi/Fe-Oæ (A˚) ÆNi/Fe-O-Ni/Feæ b (deg) Æωæ c (deg) ÆdNi/Fe-Ni/Feæ (A˚) dLa-O (A˚) ÆdNi/Fe-Oæ (A˚) ÆdNi/Fe-Ni/Feæ (A˚)

25

200

400

600

700

800

2.46612(5) 3 3.04561(6) 3 2.73993(7) 6 2.74790 5.57 1.95920(4) 162.991

2.47601(4) 3 3.04366(5) 3 2.74649(6) 6 2.75316 5.32 1.96224(3) 163.367

2.49160(4) 3 3.03711(5) 3 2.75352(5) 6 2.75894 4.89 1.96511(3) 164.043

2.50101(3) 3 3.03709(3) 3 2.76134(2) 6 2.76520 4.70 1.96902(1) 164.352

2.50782(2) 3 3.03503(3) 3 2.76491(2) 6 2.76817 4.54 1.97066(1) 164.626

2.51975(3) 3 3.02991(3) 3 2.76940(2) 6 2.77212 4.23 1.97259(1) 165.140

8.505 3.8753

8.317 3.8832

7.979 3.8922

7.824 3.9014 2.7590(6) 12 1.9509(5) 3.9018

7.687 7.430 3.9059 3.9121 Pm3m 2.7632(2) 12 2.7691(2) 12 1.9539(1) 1.9581(1) 3.9078 3.9161 P a Δ is a measure of the distortion of the La-O12 polyhedra with the average La-O distance Æd æ estimated as Δ = (1/N) n=1,N{(dn - Ædæ)/Ædæ}2. b ÆNi/Fe-O-Ni/Feæ is the average superexchange angle. c Æωæ is the average tilt angle of the [Ni/Fe]O6 octahedra. Table 4. Refined Structure Parameters for the LNF-Red between 55 and 800 C atom

parameter

55 C

215 C

415 C

600 C

700 C

800 C

Perovskite constituent La x y z Uiso (A˚2) occupancy La

R3c 1/4 1/4 1/4 0.0030(3) 1

R3c 1/4 1/4 1/4 0.0061(3) 1

R3c 1/4 1/4 1/4 0.0101(3) 1

R3c 1/4 1/4 1/4 0.0158(3) 1

R3c 1/4 1/4 1/4 0.0196(3) 1

R3c 1/4 1/4 1/4 0.0215(5) 1

Pm3m 0 0 0 0.023(4) 1

Ni/Fe

x y z Uiso (A˚2) Ni Fe

0 0 0 0.0013 0.570 0.430

0 0 0 0.0022(2) 0.570 0.430

0 0 0 0.0039(3) 0.570 0.430

0 0 0 0.0091(3) 0.570 0.430

0 0 0 0.0102(3) 0.570 0.430

0 0 0 0.0123(4) 0.570 0.430

1/2 1/2 1/2 0.010(2) 0.570 0.430

x y z Uiso (A˚2) O

0.8028(2) -0.3028(2) 1/4 0.0071(2) 0.996(3)

0.8016(2) -0.3016(2) 1/4 0.0107 (2) 0.992(2)

0.7992(2) -0.2992(2) 1/4 0.0147(3) 0.985(3)

0.7976(2) -0.2976(2) 1/4 0.0217(4) 0.972(3)

0.7961(3) -0.2961(3) 1/4 0.0243(4) 0.957(3)

0.7940(3) -0.2940(3) 1/4 0.0287(5) 0.942(3)

0 1/2 1/2 0.032(5) 1

5.4513(1) 60.751(1) 116.486(1)

5.4664(1) 60.680(1) 117.272(1)

5.4807(1) 60.579(1) 117.931(1)

5.5042(1) 60.481(1) 119.198(1)

5.5148(1) 60.432(1) 119.755(1)

5.5221(1) 60.367(1) 120.062(2)

3.9219(1) 90 60.32(1)

R3m 0 0 0 0.009(1) 1

R3m 0 0 0 0.012(2) 1

R3m 0 0 0 0.018(3) 1

R3m 0 0 0 0.024(2) 1

R3m 0 0 0 0.028(2) 1

R3m 0 0 0 0.027(3) 1

occupancy

1/2 1/2 1/2 0.0002 0.81(2)

1/2 1/2 1/2 0.013(2) 1

1/2 1/2 1/2 0.013(5) 0.97(3)

1/2 1/2 1/2 0.016(3) 0.97(2)

1/2 1/2 1/2 0.024(4) 0.96(3)

1/2 1/2 1/2 0.024(4) 0.96(3)

a (A˚) β (deg) V (A˚3)

2.9548(5) 60.03(2) 18.255(2)

2.9624(3) 60.02(1) 18.391(3)

2.9701(5) 60.01(2) 18.531(2)

2.9803(4) 60.02(1) 18.727(2)

2.9868(5) 60.02(1) 18.850(2)

2.9890(4) 60.01(1) 18.887(4)

Rwp (%) Rp (%) χ2

6.86 5.88 1.15

7.12 4.98 1.00

7.46 6.27 1.08

7.95 5.25 1.01

7.45 5.79 1.01

occupancy O

occupancy a (A˚) β (deg) V (A˚3) NiO Ni

x y z Uiso (A˚)

occupancy O

x y z Uiso (A˚)

(Tables 1 and 4). The a lattice parameter of R3c phase in the LNF-red increases, whereas the β angle decreases with the rise in the temperature (Table 4). The unit cell volume of R3c phase in the LNF-red is slightly larger compared to

9.41 7.51 1.08

that in the initial LNF (Tables 1 and 4). The values of the average Ni/Fe-O and Ni/Fe-Ni/Fe distances are also slightly larger compared to that in the initial LNF (Tables 3 and 6). The average tilting angle is comparable

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Table 5. Comparison of the Refinement with Different Models of the NPD Patterns Obtained at 200 and 800 C for the LNF-Reda refinement factors temperature, C

phase model

200

{R3c/NiO} {R3c/NiO} {R3c/Pm3m/NiO} {R3c/NiO}

800 a

[La]/[Ni]/[Fe] ratio 1/0.570/0.430 1/0.570/0.430 1/0.570/0.430 1/0.570/0.430

Rwp, %

Rp, %

χ2

7.12 19.49 9.41 9.46

4.98 12.23 7.51 7.61

1.00 4.61 1.08 1.13

Refinement factors are presented after the subtraction of the background.

Figure 10. Thermal expansion of the LNF-red in argon. The heating rate is 3 C/min.

Figure 9. Neutron diffraction pattern of the LNF-red at 800 C: observed (cross symbols), calculated (continuous line), and difference profiles (bottom line). Vertical bars show calculated reflections for different phases: upper, R3c (No. 167); middle, NiO; and lower, Pm3m (No. 221); Rwp = 9.41%, Rp = 7.51%, and χ2 = 1.08.

for both LNF and LNF-red; it decreases with the rise in the temperature (Figure 11). The distortion of La-O12 polyhedra in the LNF-red is slightly less (Tables 3 and 6). However, the deviation from oxygen stoichiometry increases in the perovskite constituent of the LNF-red with the temperature variation in contrast to that in the initial LNF (Figure 12a). Nickel oxide in the LNF-red has also small oxygen nonstoichiometry unlike Ni1-yO in the nominal LNF composition (Tables 1 and 4). The refinement of the data obtained for the nominal LNF composition showed that the high temperature perovskite phase with cubic symmetry did not exhibit any deviation from the nominal oxygen and cation stoichiometries (Table 1). The deviation from the oxygen stoichiometry in the perovskite with rhombohedral phase seems to be one of the main reasons leading to the increase in the temperature of the R3c f Pm3m phase transition in the LNF-red. It was reported in the literature32 that the value of oxygen nonstoichiometry influences phase relationships in the La1-xSrxFeO3-δ system. A change in the Ni3þ/Ni2þ ratio in the perovskite constituent can be another reason. The above-mentioned aspects will be explored in more detail in next section. 3.4. Oxygen Nonstoichiometry and Ratio of Ni3þ/Ni2þ Cations in the Perovskite Constituent of the LNF-Red. Thermal gravimetric analysis (TGA) is widely used for the characterization of the total oxygen content per formula unit (3-δ) and overall oxidation state of transition metals in complex oxides. Previous investigation (32) Dann, S. E.; Currie, D. B.; Weller, M. T.; Thomas, M. F.; Al-Rawwas, A. D. J. Solid State Chem. 1994, 109, 134.

Figure 11. Evolution of [Ni/Fe]O6 octahedra tilting angle with temperature for the perovskite constituent with rhombohedral structure in the LNF and LNF-red.

showed that the LNF has good thermochemical stability (within the accuracy of TGA) in air during thermal cycling up to 800 C.7,33 Very slight reversible weight loss (up to 0.13 ( 0.02 wt %) was revealed at high temperatures during thermal cycling in argon atmosphere (Figure 12b). These values are very small and very close to the detection limit of TGA. However, data presented for three independent measurements can illustrate a general tendency, and they are consistent with the results of the NPD data refinement (Figure 12). The LNF decomposes to La2O3 and FeNix during exposure to H2-Ar atmosphere at 800 C.34 The (3-δ) value and the average oxidation state of the transition metal in the composition can be estimated from the weight loss after long-term exposure to H2-Ar (up to 10 h). The (3-δ) value for the perovskite phase in the LNF {LaNi0.579Fe0.421O3-δ/5 mol % NiO} and LNF-red {LaNi0.570Fe0.43O3-δ /7 mol % NiO/2 mol % La2O3} were estimated. The (3-δ) values for the perovskite phase in the LNF and LNF-red equal to 2.971 and 2.945, respectively. These values are slightly (33) Konysheva, E.; Irvine, J. T. S. ECS Trans. 2008, 13(26), 115. (34) Konysheva, E.; Irvine, J. T. S. Chem. Mater. 2009, 21, 1514.

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Table 6. Interatomic Distances and Angles in the LNF-Red for the Perovskite Constituent with Rhombohedral Symmetry (R3c) temperature, C

parameters

dLa-O (A˚)

55

215

415

600

2.46554(2) 3 3.04751(3) 3 2.74111(2) 6 2.7488 5.61 1.95998(1) 162.924 8.538 3.8765

2.47642(2) 3 3.04601(3) 3 2.74798(2) 6 2.7546 5.35 1.96335(1) 163.319 8.341 3.8852

2.49206(2) 3 3.03649(3) 3 2.75382(2) 6 2.7591 4.89 1.96514(1) 164.075 7.963 3.8924

2.50794(3) 3 3.03619(4) 3 2.76474(2) 6 2.7684 4.55 1.97088(1) 164.597 7.702 3.9062

700

800

2.51929(3) 3 2.53195(3) 3 3.03149(3) 3 3.02084(4) 3 2.76926(2) 6 2.77188(3) 6 2.7723 2.7741 ÆdLa-Oæ (A˚) a 3 4.27 3.88 distortion (10 Δ) ÆdNi/Fe-Oæ (A˚) 1.97284(2) 1.97300(2) 165.082 165.766 ÆNi/Fe-O-Ni/Feæb (deg) c 7.459 7.117 Æωæ (deg) 3.9123 3.9156 ÆdNi/Fe-Ni/Feæ (A˚) P a Δ is a measure of the distortion of the La-O12 polyhedra with the average La-O distance Æd æ estimated as Δ = (1/N) n=1,N{(dn - Æd æ)/Æd æ}2. b ÆNi/Fe-O-Ni/Feæ is the average superexchange angle. c Æωæ is the average tilt angle of the [Ni/Fe]O6 octahedra.

Figure 12. (a) Oxygen nonstoichiometry in the perovskite constituents of the LNF and LNF-red as a function of temperature from the refinement of the neutron diffraction data; (b) weight change of the LNF-red during thermal cycling under argon atmosphere, data presented for three independent measurements after the subtraction of the buoyancy effect.

lower compared to those obtained from the refinement of the NPD data (Tables 1 and 4, Figure 12). However, it is consistent with the XPS results, indicating a decrease in oxygen content at the surface of the LNF-red (cf. section 3.5). The average oxidation state of Ni and Fe cations in the perovskite phase in the LNF-red decreased to 2.89þ compared to 2.94þ for that in the initial LNF. However, Ni3þ and Fe3þ cations undergo reduction under different oxygen partial pressures.13 The reduction of the LNF in H2-Ar atmosphere is a multistep process.34 Figure 13 illustrates the weight change of the LNF and LNF-red for periods up to 3 h under H2-Ar atmosphere. As was shown elsewhere,34 the initial fast weight loss (up to 6-12 mol % of the lattice oxygen) observed under H2-Ar atmosphere was nearly proportional to the nickel content in La1-xSrxMn1-y-zNiyFezO3(δ complex perovskites independent of the specific surface area of powders (0.27-14.13 m2/g) and H2-Ar flow rate. Nearly the same weight loss was observed for a dense LNF pellet (93% theoretical density). The initial fast weight loss was attributed to both surface and bulk Ni reduction, if the following process Ninþ f Ni2þ mainly takes place, where “n” varies between 2.8þ and 3þ. Therefore, the decrease in a fraction of N3þ cations in the LNF-red powder can be estimated through the comparison of the initial fast weight loss observed for the LNF and LNF-red. To avoid the reoxidation, the LNF-red sample was heated up to 800 C in an argon atmosphere with a rate of 50 C/min, and then, the gas flowing over the sample was changed to H2-Ar. The LNF sample was heated up to 800 C in air, and then, the gas atmosphere was changed consecutively to argon and H2-Ar. More detail on this procedure can

Figure 13. Mass change of the LNF and LNF-red during exposure to H2-Ar at 800 C.

be found elsewhere.34 In the case of the LNF-red, the weight loss at the initial stage was about 1.9 wt %, which is less than the 2.24 wt % observed for the initial LNF (Figure 13). The estimation was carried out with the assumption that (i) both LNF and LNF-red powders are mixtures with the following compositions {LaNi0.579Fe0.421O3-δ/5 mol % NiO} and {LaNi0.570Fe0.430O3-δ/ 7 mol % NiO/2 mol % La2O3}, and (ii) only Ni3þ cations (and not Ni2þ cations) are situated on the B-site of the perovskite constituent in the parent LNF. According to the estimation, about 7% of the sites on the B-sublattice of the perovskite phase in the LNF-red are occupied by Ni2þ cations. The effective ionic radii of Ni3þ and Fe3þ differ compared to that for Ni2þ (rNi2þ = 0.70 A˚; rNi3þ(LS) = 0.56 A˚; rNi3þ(HS) = 0.60 A˚ and rFe3þ(LS) = 0.55 A˚).35 The presence of Ni2þ cations in a partially reduced crystal lattice of the perovskite phase in the LNFred could induce the appearing of Fe5þ cations as it was shown for the compositions in the LaNi1-yFeyO3-δ series with a high Fe content, y g 0.6.5 Therefore, [Ni/Fe]O6 octahedra of the perovskite phase in the LNF-red could be much more locally distorted. However, this type of distortion could be more precisely defined by EXAFS or Moessbauer spectroscopy. 3.5. Chemical States and Surface Composition. X-ray photoelectron spectroscopy (XPS) was used to study the surface composition and the chemical states of La, O, and (35) Shannon, R. D.; Rewitt, C. T. Acta Crystallogr., Sect. B 1969, 25, 925.

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Figure 14. Fe 2p, La 3d, and O1s core level spectra recorded for the LNF and LNF-red.

Fe at the surface of the parent LNF and LNF-red powders. It should be mentioned that it was not possible to investigate the oxidation state of Ni due to the interference with La (La 3d3/2 as split doublet together with Ni 2p3/2, both at 853 eV; other Ni signals are weak and/or broad). Applying the procedure based on the fixed ratio between the La 3d5/2 and La 3d3/2 peak areas (theoretical value is 1.5) also did not allow Ni contribution in the La 3d3/2 peak to be distinguished. The La 3d, O 1s, and Fe 2p core level spectra are shown in Figure 14. The La 3d5/2 spectrum of the LNF is split into two components at binding energies 833.5 (LaI) and 837.1 (LaII). La 3d3/2 is also split and overlapped with Ni 2p3/2 region. O 1s spectrum is complex as well, and the peak shapes are similar to those observed for complex oxides.36,37 The peak at a lower binding energy (EB ∼ 528.17 eV) can be assigned to the oxygen in the lattice.38-40 The peaks at higher binding energy (EB ∼ 530.64 eV) are close to the 531-532 eV range which surface states are responsible for.40 Notice that both peaks in the O 1s spectrum of the LNF are broad. The Fe 2p3/2 peak can be centered at about 710.4 eV, indicating the presence of both Fe3þ (major fraction) and Fe2þ cations.38,41 In the case of the reduction of perovskites, one can, in general, expect a regular shift of XPS spectra toward low binding energies, as was observed during moderate reduction of LaNiO3-δ, LaCr1-xNixO3-δ, La0.6Sr0.4CoO3(δ, and La0.8Sr0.2MnO3(δ.37,42 However, unusual features were revealed in the XPS spectra recorded for the LNFred (Figure 14). La 3d5/2 doublet shifted slightly toward a high binding energy (ΔEB ∼ 0.3 eV), which is a little larger shift than the accuracy of the XPS measurement (∼0.2-0.1 eV). During exposure to argon atmosphere, it is unlikely that La3þ cations undergo the reduction. As a result, a shift of the La 3d5/2 spectrum reflects a change in a local environment caused by a change in (36) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of x-ray photoelectron spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992; p 261. (37) Vovk, G.; Chen, X.; Mims, C. A. J. Phys. Chem. B 2005, 109, 2445. (38) Database for Surface Spectroscopy. http://www.lasurface.com (accessed July 2009). (39) Wang, P.; Yao, L.; Wang, M.; Wu, W. J. Alloys Compd. 2000, 311, 53. (40) van der Heide, P. A. W. Surf. Interface Anal. 2002, 33, 414. (41) Pai, M. R.; Wani, B. N.; Sreedhar, B.; Singh, S.; Gupta, N. M. J. Mol. Catal. A 2006, 246, 128. (42) Konysheva, E.; Francis, S. M.; Irvine, J. T. S. J. Electrochem. Soc. 2010, 157 (1).

chemical states of surrounding cations or anions. Reduction of Ni3þ cation to a low oxidation state (2þ) seems to be the only process which takes place under moderate reducing conditions, although this could also lead to exsolution of NiO from the perovskite, further changing the environment. According to the literature, LaFeO3 starts decomposing to Fe and La2O3 at a very low oxygen partial pressure: p(O2) ∼ 10-16-10-17 atm at 1000 C.13 Further reduction of NiO secondary phase to Ni can be excluded because much lower oxygen partial pressure (p(O2) ∼ 10-11) is required.13 A very wide peak at EB ∼ 848.3 eV appeared in the La 3d3/2 and Ni 2p region (Figure 14), which is difficult to assign. It cannot be ascribed to Ni 2p3/2 in NiO or Ni because the value of the binding energy is too low.43 On the other hand, LaI peak in the La 3d3/2 spectra of LaFeO3 can be centered at EB ∼ 849.0 eV.44 The Fe 2p3/2 peak in the XPS spectrum of the LNF-red slightly shifted toward a high binding energy, like La 3d5/2, indicating the shortening of the Fe-O or Fe-Fe distances on the surface of grains. However, this is in contradictory with the results obtained from the neutron diffraction study (Table 5): the average Ni/Fe-O and Ni/Fe-Ni/Fe distances in the reduced sample are slightly longer than in the parent LNF. The full width at half-maximum value (FWHM) of both La 3d5/2 and Fe 2p3/2 peaks decreased slightly. The binding energy of oxygen in the LNF-red slightly increases. In addition, a small broad peak appeared at ∼532.9 eV, which could be assigned to a chemisorbed dioxygen species, as it was noticed in the literature for La2O3.45 It was also reported46 that an additional O 1s peak at ∼532.7 eV appeared in the O 1s XPS spectra recorded on the nickel rod after exposure in air at 250 C for 1 h, and it was interpreted as a formation NiOads. The presence of H2O or (OH)- can be excluded because of the moderate reduction was carried out under dry argon atmosphere. The enrichment of the surface by La compared to iron or oxygen was observed for the LNF-red (Table 7). At the same time, the surface concentration of oxygen in the (43) Crist, B. V. Handbook of Monochromatic XPS spectra; XPS International, Inc.: Mountain View, CA, 1999; Vol. 1, The Elements and Native Oxides. (44) Lam, D. J.; Veal, B. W.; Ellis, D. E. Phys. Rev. B 1980, 22, 5730. (45) Dubois, J.; Bisiaux, M.; Mimoun, H.; Cameron, C. J. Chem. Lett. 1990, 19, 967. (46) Kim, K. S.; Davis, R. E. J. Electron Spectrosc. Relat. Phenom. 1972-1973, 1, 251.

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Table 7. XPS Results for the Surface of the LNF and LNF-Red ratio

LNF

LNF-red

[La]/[Fe] [La]/[O] [O]/([La] þ [Fe])

1.19 0.13 4.00

3.19 0.25 3.02

LNF-red decreases compared to the parent LNF (Table 7). The binding energy of lattice oxygen in La2NiO4þδ is about EB ∼ 529.1 eV,47 which is higher compared to that observed in the present study for both LNF and LNF-red. In addition, in the case of a formation of La2NiO4þδ on the surface the [O]/[La] ratio should increase compared to that for the La0.95Ni0.6Fe0.4O3-δ. Therefore, a formation of La2NiO4þδ can be ruled out. Similar tendencies observed for La3d5/2 and Fe2p3/2 XPS spectra allow assuming a formation of LaFeO3 phase at the surface of the LNF-red. Notice that LaFeO3 was revealed in the XRD pattern of the LNF after 0.2 h of exposure to strong reducing conditions (H2-Ar atmosphere).34 Phase transformations taking place at the surface of the LNF-red during exposure to moderate reducing conditions could be described by the following reaction, indicating surface heterogeneity: LaNi0:579 Fe0:421 O3 -δ þ 5 mol%NiO f LaFeO3 þ n mol % La2 O3 þ ð5 þ nÞ mol % NiO þ mO2 v ð2Þ This is consistent with the results of the BET analysis. The initial A-site deficient LNF composition calcined in air at 1350 C for 5 h has low surface area (SBET = 0.266 ( 0.019 m2/g). The BET surface area of the LNFred, which was exposed to argon at 800 C for 12 h, equals to 0.279 ( 0.001 m2/g. After further exposure the LNFred powder under argon at 800 C for 12 h (24 h total time), the BET surface area of the LNF-red powder increased up to 0.516 ( 0.006 m2/g. However, XRD analysis carried out for these samples did not show any difference. This result could indicate that the length of (47) Choisnet, J.; Abadzhieva, N.; Stefanov, P.; Klissurskif, D.; Bassat, J. M.; Rives, V.; Minchev, L. J. Chem. Soc., Faraday Trans. 1994, 90, 1987.

grain boundary and its chemical composition would change substantially during long-term exposure of the La0.95Ni0.6Fe0.4O3-δ under moderate reducing conditions, thereby increasing the grain boundary contribution and affecting transport properties. 4. Conclusions The neutron powder diffraction study demonstrates evidently that at room temperature the A-site deficient nominal composition La0.95Ni0.6Fe0.4O3-δ can be represented as a mixture of a stoichiometric perovskite with rhombohedral structure and NiO as a secondary phase (∼5 mol %). The concentration of the secondary phase in the reduced form of La0.95Ni0.6Fe0.4O3-δ slightly increases up to 7 mol %, which seems to be caused by the surface decomposition of the perovskite phase and results in surface heterogeneity. The perovskite phase in the A-site deficient La0.95Ni0.6Fe0.4O3-δ composition undergoes rhombohedral (R3c) to cubic (Pm3m) phase transition over 550 C as observed by in situ high-temperature neutron powder diffraction. The change in the linear thermal expansion was observed in nearly the same temperature range (550-600 C) and was related to the phase transition. The perovskite phase in the reduced form of the La0.95Ni0.6Fe0.4O3-δ possesses slightly higher deviation from the oxygen stoichiometry, which increases with the temperature variation. The fraction of nickel cations, which are Ni2þ situated on the B sites of the perovskite phase, increases in the reduced form of La0.95Ni0.6Fe0.4O3-δ. The rhombohedral structure allows the presence of Ni2þ cations on the B sublattice and deviation from oxygen stoichiometry, both of which seem to influence the temperature of the R3c f Pm3m phase transition. Acknowledgment. The authors thank EPSRC for funding. We also gratefully acknowledge Mr. L. Gendrin (ILL Grenoble) for help with neutron diffraction measurements, Dr. S. M. Francis for XPS measurements analysis, Mrs. S. Williamson for BET analysis, and Dr. Yu. Andreev for his helpful discussions.

Effect of Oxygen Non Stoichiometry and Oxidation State of Transition

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