J. MATER. CHEM., 1994, 4( 11, 19-22 Preparation and Characterization of Sr,-,La,FeO,(O <x 61) Yasuo Takeda,*” Kaori Imayoshi: Nobuyuki Imanishi,” Osamu Yamamoto” and Mikio Takanob a Department of Chemistry, Faculty of Engineering, Mie University, Tsu Mie-ken 514, Japan Institute for Chemical Research, Kyoto University, Uji Kyoto-fu 611, Japan Solid-solution Sr,-,La,FeO, crystallizing in the K,NiF, structure has been obtained for 0 bx b1. In the Sr-rich region, the K2NiF, structure was stable only at low temperature. Heat treatment under an oxygen pressure of 100-500 atm was carried out to form oxygen stoichiometric samples. The structure was refined by the Rietveld method assuming space group /4/mmm. The FeO, octahedron in Sr,FeO, showed almost cubic symmetry, the La substitution causing an increase in the Fe-0 (apical) length without modifying the a-axis length.All the samples showed semiconducting behaviour and positive Seebeck coefficients. An antiferromagnetic transition at 60 K was observed for Sr,FeO,, becoming obscure with increasing La content. The room-temperature Mossbauer spectra showed a symmetric singlet due to Fe4+ and a doublet due to Fe3+, except for Sr,FeO, which showed a quadrupole doublet. Of several oxides containing iron in the unusual valence state Fe4+, perovskite oxides such as SrFeO, and CaFeO, have been extensively studied. ‘9, The octahedrally coordinated Fe4+ ions in SrFe0, are in a high-spin tZg3a*l configuration, where the orbital doubly degenerate narrow a* band of e-orbital parentage is one-quarter filled.The behaviour of the a: electron in CaFeO, can be characterized by a dispro-portionation of the Fe4+ ion into Fe3+ and Fe5+, i.e.: 2Fe4+( t;,a*’) (high-temperature phase) Fe3+(t:get)+ Fe5+(t;,e;) (low-temperature phase) In the Sr-Fe4+-0 system, Sr2Fe04, having the K2NiF, structure, has beehnown to exist. However, only a limited number of studies have been reported on this oxide, because it is difficult to prepare the oxygen stoichiometric samples. Gallagher et al. prepared an oxygen-deficient sample Sr,FeO,,, and identified Fe3+ and Fe4+ ions in the Mossbauer ~pectrum.~Recently, Dann et a!. prepared oxygen stoichio- metric Sr,FeO, and examined the crystal structure by an X-ray Rietveld method., They reported that Fe4+ ions are in a slightly distorted octahedral environment, (4 x 1.93 A)+(2 x 1.95 A).In this study, we prepared almost stoichiometric Sr,-xLa,FeO,(Obx < 1) and refined the crystal structure by X-ray Rietveld analysis. The electrical conductivity, Seebeck effect, magnetic susceptibility, and Mossbauer effect were measured. Experimental Mixtures of Sr(N03)2, La203 and Fe metal powder were dissolved in nitric acid. Before use, La203 was dried at 1000“C for 1 h. The solutions containing these metallic ions at various ratios were dried until the nitrates completely decomposed. The solid thus formed was calcined at 600°C for 15 h and subsequently ground, pelletized, and heated at various tem- peratures depending upon the metallic composition in an O2 gas flow.To attain oxygen stoichiometry, these samples were annealed at 400-700°C under 100-500 atm of oxygen pressure. X-Ray diffraction (XRD) patterns of powdered samples were obtained on Rigaku RAD-RC (12KW) using monochro- mated Cu-Ka radiation. For Rietveld analysis, intensity data were collected at each 0.02” step for 2 or 3 s over a 20 range of 10-100”.5 The average oxidation state of iron was deter- mined by iodometry. The Seebeck coefficient was measured in the temperature range 300-700 K and a temperature differ- ence within a sample of 3-20 K. The magnetic susceptibility was measured by using a SQUID magnetometer (Quantum Design, MPMS,) in a field of 1000G. The Mossbauer effect of 57Fe was performed at room temperature.The velocity was calibrated by using pure iron metal as the standard material. Results and Discussion The phases identified by XRD measurements are summarized in Fig. 1 as a function of synthesis temperature and composi- tion for treatments under 1 atm of oxygen. In an Sr rich region, K2NiF, structure is stable only at low temperatures. For example, Sr,Fe04 -decomposes to Sr,Fe20, -z3-6 and SrO above 800°C. In air or a more reducing atmosphere the decomposition temperature decreases. However, the reaction rate is slow at these low temperatures, especially for A cO.4. The triangles in Fig. 1 show the coexistence of Sr, -,La ,FeO, and La203 or SrCO, after heating for more than 100 h with 15001 :0 00 0 1200 1 0 0’0 0 p 1100 ,,*o ,I‘ V V1000 L V I700tI , , , , , 0.0 0.2 0.4 0.6 0.8 1.0 Fig.1 Formation diagram for the Sr,-,La,FeO, system (0,gas flow): 0, Sr, -,La,FeO,; c7, Sr, -.La,FeO, and La,O,; 0, Sr, -,La,FeO, and Sr, -.La,Fe,O,; 0,Sr,Fe,O, and SrCO, J. MATER. CHEM., 1994, VOL. 4 a few intermittent grindings. Since the specimens thus obtained were oxygen deficient, especially in the Sr-rich region, annealing under more than 150atm of oxygen was carried out at 450 "C for 70 h. As Gallagher et al. also reported,, the as-prepared oxygen-deficient samples underwent decompo- sition in a few weeks even if they were isolated from moisture, while oxygen stoichiometric samples remained stable for more than a few months.In Table 1 are listed the synthesis conditions, lattice param- eters and oxygen contents for the specimen used for the Rietveld analysis. The samples prepared at low temperatures (e.g. 8OOOC) did not give any significant R-factor because of a non-homogeneous mixing of Sr and La ions in the structure. The sample for x =0.1 was finally annealed under 350 atm of oxygen at 1000°C for 6 h to have a well mixed state of Sr and La. Final reliability factors achieved were less than 13, 10, 3 and 5 for Rwp, R,, RE and RI, respectively, except for x=O.l-0.3, for which 20, 17, 3 and 8 were the minimum values obtained. The tetragonal lattice parameters a and c obtained by Rietveld analysis are plotted in Fig. 2 against the La content.The c-axis increases with increasing La content and shows a maximum at x=O.8, while the a axis shows no significant change, the tetragonality ratio c/a showing a maximum at x x0.8. Fe4+ in SrFeO, has an electronic configuration, t;, e:, which would tend to induce a Jahn-Teller distortion of the FeO, octahedron. However, the eg electron in SrFe0, is delocalized and SrFeO, exhibits metallic conductivity and cubic symmetry, with no sign of structural distortion down to 4.2 K. Sr,FeO,, however, has an anisotropic K,NiF, struc-ture which makes us expect the onset of a Jahn-Teller distortion. The structure of Sr,-,La,FeO, was refined by assuming space group I4/rnrnrn. The Sr, La, and O(2) ions are located at 4e sites with coordinates (O,O, z), the Fe atoms at (O,O, 0) in sites 2a, and the O(1) atoms at ( 3 ,0, 0), (0, 3 ,0) in sites 4c.Sr and La were assumed to be randomly mixed at 4e sites. Fig. 3 shows observed, calculated, and difference plots for Sr2-,LaXFeO4 (x=O.9). Table 2 lists the refinement results. The Fe-0 bond length is plotted us. x in Fig. 4. The coordination of Fe4+ in Sr,Fe04 is an almost regular octa- hedron, without any indication of the Jahn-Teller effect. This agrees with the result of Dann et a!., However, the FeO, 12.8 a. 12.7 0 0 05-12.6 02 f? CTI a a,.s 12.5 c -0 0 12.4 3.9 3.8 0.0 0.2 0.4 0.6 0.8 1.0 x in Sr2.&afe0, Fig. 2 Variation of the lattice parameters in Sr, -,La,FeO,: ,c-axis; 0,a-axis octahedron becomes elongated along the c-axis as x is increased.The degree of elongation, Fe--O(2)/Fe-O( l), increases from 1.00 (x=O) to 1.11 (x=l). Substituted La3+ ions which are smaller and more positively charged than Sr2+ would shorten the La(Sr)-0(2) distance along the c axis. Moreover, the excess electrons injected into the [FeO,] -x sheets would occupy the di orbit directed toward the O(2) ions. The Fe- O(2) distance is extended consequently without modifying the a-axis length. The room-temperature resistivity increases with La content from 1.2 x lo2ncm-' (x=O) to 4.5 x lo3i2 cm-' (x=0.4). Fig. 5 shows the Seebeck coefficient for x =0.0-0.6 from room temperature to 400°C, around which the oxygen content Table 1 Synthetic conditions, lattice parameters and oxygen contents in the system Sr,-,La,FeO, lattice parameter (hexagonal) synthetic conditions X a/A CIA oxygen content T/"C P(0Z )/atm time/h 0.0 3.8582(1) 12.3977(4) 3.997( 9) 750 1 120 450 450 70 0.1 3.8554(2) 12.4458( 6) 3.959( 5) 800 1 150 1000 350 6 0.2 3.8537( 2) 12.482(1) 4.01(1) 800 1 72 450 150 70 0.3 3.8512(3) 12.570(1) 4.002( 5) 850 1 60 450 150 70 0.4 3.8495(2) 12.6154(9) 4.005( 2) 900 1 60 450 150 70 0.5 3.8518(2) 12.6568(9) 4.OO2(6) 1000 1 60 450 150 70 0.6 3.8534(2) 12.6974( 6) 3.983(6) 1100 1 60 450 150 70 0.7 3.84981(7) 12.7394( 3) 4.002(6) 1200 1 40 450 150 70 0.8 0.9 1.o 3.85251(5) 3.86366(4) 3.87339(4) 12.7523( 2) 12.7385(2) 12.7171 (2) 4.02 1(4) 4.001(4) 3.996(7) 1200 1300 1300 1 1 1 40 40 40 J.MATER. CHEM., 1994, VOL. 4 21 The magnetic behaviour of compositions x =0.0, (1.1, 0.3, and 0.5 is shown in Fig. 6. Sr2Fe0, shows an antiferromagnetic49 I transition at ca. 60 K. At higher temperatures, the susceptibil-ity obeys the Curie-Weiss law. Calculation of the effective magnetic moment from the linear portion of 1/x-T gives a value of 10.OpB,which is much higher than the spin-only value of 4.9 for high-spin Fe4+.The substitution of only 5% of La in Sr weakens the peak of antiferromagnetic ordering, which finally disappears in Sr,,,Lao.2Fe04.Above 100 K, the susceptibility of Sr2-,La,Fe04(x =0.1,0.3 and 0.5) also obeys the Curie-Weiss law; however, the effective magnetic moments 20 30 40 50 60 70 80 90 100 are somewhat smaller than that of Sr2Fe0,, viz.5.9 pB!5.5 pB 28ldegrees and 5.2 pB,respectively, which reflects the mixed state of high-spin Fe4+ (d4) and Fe3+ (d5). Fig. 3 Observed, calculated and difference plots of Sr,-,La,FeO, The room-temperature Mossbauer spectra are shown in (X =0.9) Fig. 7. The spectrum of Sr,FeO, shows two lines of dmost equal intensity at -0.22 mm s-l and +0.22 mm s-'. Irz 1966, Gallagher et al. reported a Mossbauer spectrum of ouygen-begins to decrease. The positive values suggesting p-type deficient Sr2Fe0,.,3 showing a pair of peaks of different conduction increase with increasing x except for Sr2Fe04.The intensities at -0.24 mm s-l (weak) and +0.24 mm s-l electronic state of Sr2Fe04leading to the strong temperature (strong).Gallagher et al. attributed these peaks to tetravalent dependence of the Seebeck coefficient seems to be strongly (Fe4+) and trivalent (Fe3+) states, respectively. However, affected by La substitution. because our samples are almost oxygen stoichiometric with Table 2 Rietveld refinement results for Sr,-,La,FeO, ~~~~~~ fractional coordinates and thermal parameters Sr, La Fe x in Sr,-,La,FeO, Z B/A2 Z 0.0 0.3572(5) 0.5(2) 0.5(2) 0.5(2) 0.157(3) 0.5(2) 0.1 0.3561(7) 0.3(2) 0.3(5) 0.3(7) 0.161(4) 0.3(7) 0.2 0.356(1) 0.3(3) 0.3(7) 0.6( 11) 0.160(6) 0 6(11) 0.3 0.356(1) 0.1(3) 0.9(8) 0.4( 12) 0.155(7) 0 4( 12) 0.4 0.356( 1) 0.1(3) 0.7(7) 0.4( 11) 0.157(6) 04(11) 0.5 0.356(1) 0.1(3) 0.9(7) 0.4( 12) 0.160(6) 0.4(12) 0.6 0.3570(7) 0.3(2) 0.8(5) 0.5(8) 0.163(4) 0.5(8) 0.7 0.3576(4) 0.3(1) 0.3(1) 0.3( 1) 0.165( 3) 0 3(1) 0.8 0.3578 (4) 0.3(1) 0.3( 1) 0.3(1) 0.166(2) 0 3(1)0.9 0.3584(5) 0.1( 1) 0.1(1) 0.1(1) 0.166(3) 0 1(1) 1.o 0.3588(5) 0.1(1) 0.3(3) 0.7(7) 0.169(3) 0 7(7) 2.21 T 100 -I \\ -I80 I I \I52.1 -L 8 3 I t I Ia 1c II.-in .-60--0 u I I a ELL a I0 I -I I102.0 Y8 40-I a ..---..I a,a ", ... -..-..-.._.._.._.._..__._._..0.6I UI 20 --.....<1.9 0.4 1 ----_ 0.2 I I I -----10.0I1 I I I I 11 0 0.0 0.2 0.4 0.6 0.8 1.0 x in Sr,.&a>eO4 Fig.4 Variation of Fe-0 bond distances in the Sr2-,La,Fe0, Fig. 5 Temperature dependence of the Seebeck coefficient of the system: 0, Fe-O( l)//a-axis; 0,Fe-O(2)//c-axis Sr2-,La,Fe0, system, for different values of x as marked 3.0r 0.0 -----____0.1-.----.-0.3-.-..-,--..-.-0.5 I I I I400 0.00 100 200 300 T/K Fig. 6 Magnetic susceptibility for Sr2-,La,Fe04 for x=O.O (-), 0.1 (---), 0.3 (-.-) and 0.5 (--.--), at 1000 G Fig. 7 Mossbauer spectra for Sr,-,La,FeO, at room temperature for different values of x as marked oxygen deficiencies of < 1%, we have considered the double peaks at k0.22 mm s-' to be a quadrupole doublet: the Fe4+ ions in Sr,FeO, have an isomer shift of zero and a quadrupole J. MATER. CHEM., 1994, VOL.4 splitting of 0.44 mm s-'. The isomer shift is similar to those of SrFeO, (0.054mm s-')~and CaFeO, (0.073 mm s-').~The quadrupole interaction is essentially absent for x =0.2 and 0.4 in Fig. 7, as it is for SrFeO, and CaFeO,. The gap in electric and magnetic properties between x=O and x->O.l as seen in Fig. 4 and 5 seems to have the same root as the disappearance of the quadrupole interaction. One possibility is that the excess electrons injected on chemical reduction jump among several Fe ions located near the substituting La3+ ions. When La3+ content is increased to x >0.4, another quadrupole doublet due to Fe3+ ions becomes identifiable. Conclusions In summary, we have synthesized a solid solution of Sr,~,La,FeO,(O <x <1) and studied their structural, electrical and magnetic properties.A Rietveld analysis of the powder XRD data showed that the almost cubic Fe4+0, octahedron in Sr2Fe0, became elongated along the c axis with increasing x, especially for x>0.4. The measurements of Seebeck coefficient, magnetic susceptibility and Mossbauer effect sug-gested the existence of a gap in the electric states between Sr2Fe04 and the La-substituted ones. Further details are under study. This study was financially supported by 'Grant-in-Aid for Science of High T, Superconductivity' given by Ministry of Education, Science and Culture of Japan. All computations for structural analysis were carried out at the Mie University Information Processing Center.References J. B. MacChesney, R. C. Sherwood and J. F. Potter, J. Chem. Phys., 1965, 43, 1907; Y. Takeda, K. Kanno, T. Takada, 0. Yamamoto, M. Takano, N. Nakanishi and Y. Bando, J. 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Paper 3/04301K; Receitied 21st July, 1993