Structure of metastable strontium ferrite

Mat. Res. Bull., Vol. 27, pp. 919-924, 1992. Printed in the USA. 0025-5408/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.

STRUCTURE OF METASTABLE STRONTIUM FERRITE

P. Berthet 1, J. Berthon 1, G. Heger2 and A. Revcolevschi 1 1Laboratoire de Chimie des Solides, URA446-CNRS, Bat. 414, Universit6 Pads Sud, 91405 Orsay Cedex (France) 2Laboratoire L6on Brillouin, CEA-CNRS, CEN Saclay, 91191 Gif-sur-Yvette Cedex (France) (Received May 22, 1992; Communicated by P. Hagenmuller) ABSTRACT The metastable compound SrFe204 was prepared by crystallization at 630°C of a glass prepared by melt-spinning. It decomposes above 900°C. It has an orthorhombic unit cell at room temperature with a = 8.031(10)/k, b = 18.208(18)/k, c = 5.454(6)/k, Z = 8, space group Pbc21. MATERIALS INDEX : iron, strontium, f e r r i t e s , oxides

Introduction Whereas stable alkaline earth ferrites MFe204 exist with various M cations such as Mg, Ca or Ba, the strontium compound SrFe204 has not been observed in the phase diagram of the Fe203-SrO system (fig. 1). The structure adopted by the MFe204 compounds depends on the size of the M cation. Thus MgFe204 has a structure of the inverse spinel type whereas CaFe204 forms double rutile chains of FeO 6 octahedra enclosing 8-coordinated calcium ions (2) and BaFe204 crystallizes in a stuffed tridymite structure with all iron ions located on tetrahedral sites (3). The structures adopted by CaFe204 and BaFer204 are quite different, but it seems that none of them can provide convenient sites to accommodate strontium. However, the existence of metastable SrFe204 has already been suggested earlier (4). We have attempted to prepare it by ultra fast quenching from the melt of the corresponding composition 50 Fe203-50 SrO, and subsequently crystallizing the resulting glass (5) . The X-ray diffraction (XRD) analysis of the crystallized material has evidenced the existence of a phase the structure of which could be related to that of BaFe204. This relationship is also supported by an X-ray absorption spectroscopy study that we published previously (6).

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Exoerimental The starting material for the melt-spinning experiments was prepared at the composition 50 Fe.203 - 5 0 SrO (mol%) by pressing appropriate mixtures of strontium oxalate and iron sesquioxide Fe203 into rods which were fired at 950°C for four days. X-ray diffraction indicated the presence of two phases SrFel2Ol9 and Sr7FeloO22 in agreement with the phase diagram of the Fe203-SrO system (fig. 1).

1600

sis//fJi

T(*C)

!1

1400

!

i

1200 20

;rO

jl

Sr3Fe206

• 401

t,

60

Sr2Fe20S SrTFelo022

80

I

SrFel2019 Fe203

FIG.1. Phase diagram of the Fe203-SrO system (wt%), from ref.(1).

Ultra rapid quenching was achieved with a melt-spinning device which has been described in detail in a previous article (5). The starting material was melted in a RF-heated platinum crucible pierced at the base and ejected onto a stainless steel wheel rotating at a very high speed. The quenching rate was estimated to be higher than 106 K/s. The quenched material was composed of short pieces of ribbon about 2 cm long, 1 mm wide and 30 #m thick together with a large number of flakes. All the following experiments were carded out on the ribbon shaped material. A permanent magnet was used to eliminate material containing minute amounts of ferrimagnetic strontium hexaferrite SrFe12019. Characterization The DTA curve (fig.2) of the as-quenched material exhibits a strong exothermic peak at 610°C and a broader peak above 900°C. The X-ray diffraction patterns of the as-quenched material and of the samples annealed at temperatures lower than 600°C consist of broad diffuse halos together with a few weak lines which do not belong to the equilibrium phases. Samples annealed for a few hours at temperatures ranging from 630°C to 800°C all exhibit the

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same XRD pattern which consists of a large number of lines (Table 1). The weak lines observed for the as-quenched samples belong to this pattern. Samples annealed at temperatures higher than 900°C have an XRD pattern exhibiting only the lines of the equilibrium phases Sr7Fe10022 and SrFel2019. From these data we associate the first exothermic peak observed on the DTA curve with the crystallization of the glass produced by melt-spinning. The analysis of the XRD pattern of the crystallized material is discussed in the next section, and clearly indicates that this pattern can be attributed to a single phase of composition SrFe204. This phase undergoes an exothermic decomposition above 900°C ; it seems to be metastable since we were never able to prepare it by solid state reaction even after 45 days at 630°C.

I

--

I

400.

i

I

I

I

700.

800.

900.

I

IIXO

500.

600.

T (°C)

FIG.2. DTA curve for amorphous 50 Fe203- 50 SrO (tool%) samples (heating rate : 12°C/min, air atmosphere).

XRD Pattern Analysis The XRD pattern was taken with a Guinier camera with Co Ka 1 radiation using aluminium tetrametaphosphate A14(P4OI2)3 as an internal standard (cubic, a = 13.730(1)A at 25°C). A first examination of the XRD diagram of the crystallized phase suggested a structure related to that of ,7-BaFe204 (3). Consequently, we tried an indexation of the diagram based on an orthorhombic cell. Since the first line of the phase under study corresponds to an interplanar spacing (d = 6.002 A) significantly lower than that found for BaFe204 (d = 7.728 A), we supposed that the indexation of these lines should be different for the two compounds. On the other hand, we assumed that the strongest lines observed for SrFe204 and

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BaFeqO 4 with similar d spacings should have the same indexations. These assumptions made it possible to obtain lattice parameters enabling the following refinement procedure. A least-squares fit of the orthorhombic lattice parameters was performed using 14 low angle reflections with unambiguous indexation (the (231) reflection was excluded due to relatively large deviations, Table I) and yielded a = 8.031(10) A, b = 18.208(18) A, c = 5.454(6) A ; V = 797.6(1.0) A 3. All measured reflections could be indexed satisfactorily on the basis of the refined unit cell. A compilation of measured and calculated interplanar distances, dobs and dealt respectively, is listed in Table 1. The lattice parameters which are obtained are close to those reported for BaFe204 (3): a = 8.448/k, b = 19.05 A, c = 5.390 A. But the observed reflections of SrFe204 do not agree with the extinction rules of space group Cmc21-No.36 of the Ba compound (in ref.(3) a non-conventional setting according to b, e, a was used leading to the equivalent space group Bb21m). In particular, it is obvious from Table 1 that the C-centering (only reflections with h + k = 2n are possible) is no longer valid. Due to the close relationship between the two compounds, and in order to find a space group for SrFe204 which would agree with our observations, we studied the different possible maximal subgroups of the space group Cmc21 which belong to the same crystal class : mm2. In fig.3, the conditions limiting possible reflections are added for each of the listed space groups. The subgroups Pbn21 and Pbc21 have non-conventional settings. They can be transformed as indicated to Pna21-No.33 and Pca21-No.29.

Cmc21-No- 36 (hkl) : h + k = 2n ((0kl) : k = 2n) (h01) : 1 = 2n, (h = 2n, h + l = 2n) / k2

/ k2

\ k2

\ k2

Pmc21 -No. 26

Pmn2 i-No. 31

Pbn21 (b a ~. : Pna21-No.33)*

Pbc21 (b a g : Pca21-No.29)*

(h01) : 1 = 2n

(h01) : h + l = 2n

(h01) : h + l = 2n (0kl) : k = 2n

(h01) : 1 = 2n (0kl) : k = 2n

FIG.3. Maximal subgroups of the space group Cmc2]-No.36 which belong to the same crystal class : mm2. The symmetry reduction of order 2 is indicated by "k2" (k : klassengleich). The conditions limiting possible reflections are added for each space group in the same unit cell. The conventional notation of the space groups with their numbers is given according to the International Tables for X-ray Crystallography (7). * Standard setting of the above-mentioned group.

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TABLE 1

Comparison of Measured dobs and Calculated dcalc Values (in angstr6ms) for SrFe204 and Visual Estimation of Intensities.

h

k

1

dobs

dcalc

I

1 0 0 1 2 1 0 1 2 2 2 2 1 0 0 2 1 1 1 3 1 0

2 2 4 2 2 3 4 4 1 2 4 3 5 0 6 4 0 6 2 1 3 4

0 1 0 1 0 1 1 1 1 1 0 1 1 2 1 1 2 1 2 1 2 2

6.002 4.671 4.547 4.037 3.672 3.625 3.500 3.212 3.189 3.050 3.009 2.872 2.828 2.725 2.654 2.636 2.579 2.523 2.481 2.392 2.378 2.336

6.023 4.679 4.552 4.043 3.674 3.621 3.495 3.205 3.184 3.047 3.011 2.854 2.834 2.727 2.652 2.636 2.582 2.518 2.484 2.383 2.376 2.339

10 20 5 20 20 10 10 40 30 50 10 25 30 100 80 100 50 20 20 10 25 5

2.254 [-2.256 2.254

3o

3 3 I 2.230 2.234 2 6 1 2.225 2.213 2 7 0 2.183 2.183 3 5 0 2.170 2.157 3 4 1 2.125 2.125 2 7 1 2.026 2.027 2 4 2 2.021 3 4 0 0E0 322 . 0. 1 20 0 8 62.008

10 5 10 10 15 40

210 27,

h

k

1

dobs

dcalc

I

2 8 0 4 2 0 3 0 2 3 2 2 1 9 1 4 4 0 2 9 0 31 107 1 - ]0

1.975 1.956 1.911 1.870 1.847 1.837 1.804 1.771

70 20 20 40 40 10 10

4 5 3 8 1 3 1 4 2 1 2 2 1 11 40 3 6 2 3 2 10 5 2 4 3 5 3 5 1 4 4 5 40 0102~ 2 5 012 33 5 4

0 0 3 3 3 3 0 ~

1.756 1.733 1.702 1.650 1.648 1.630

3 1 0 2 0 1 2

1.597 1.587 1.578 1.563 1.553 1.535 1.524

1.980 1.961 1.910 1.870 1.846 1.837 1.807 ['-1"776 1.765 1.758 1.734 1.702 1.652 1.649 1.629 1.621 E1.617 1.617 1.598 1.587 1.582 1.562 1.553 1.535 1.524 1.515 E1.514 1.508 1.462 E1.460 1.459

1.618

1.513 1 ~31.460

15 15 10 20 40 40 40 40 50 20 15 40 40 20 10 30 40

80

A check of the d values listed in Table 1 shows that only two (h01) reflections with h + l = 2n+ 1 are observed, (102) and (302), which contradict the extinction rules of space groups Pmn21-No.31 and Pbn21. On the other hand, all the reflections with k = 0 and 1 = 2n+ 1 are absent systematically. In order to decide between the two remaining space groups P m c 2 r N o . 2 6 and Pbc21, we had to search for reflections with h = 0 and k = 2 n + l . As no reflections of this type were observed, our indexing scheme and the assumption of a close relationship between the crystal structures of BaFe204 and SrFe204 led directly to Pbc21 (Pca2rNo.29) as the probable space group for the strontium compound.

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In the single face-centered lattice of BaFe204 there are 8 formula units per unit cell : Z = 8 (3). In the conventional setting (Cmc21) Ba is located on two 4a sites (7) : 0, y, z; 0, y, 1A+z; + C which find their analogs in the general position (4a) of space group Pbc21 (Pca21-No.29). In SrFe204, the framework of FeO4 tetrahedra is distorted to accommodate the strontium ions which have a significantly smaller ionic radius than the barium ions (r(Sr2+) = 1.13 A, r(Ba2+) = 1.35 A) ; this deformation of the framework leads to the observed symmetry reduction. The two Fe sites (8b) of the BaFe204 compound split into a total of four different 4a sites in Pbc21 (Pca2FNo.29). Conclusion The possibility of indexing the XRD pattern of the material obtained by crystallization of a 50 Fe203 -50 SrO (mol%) glass clearly indicates that the preparation of the compound SrFe204 has been achieved. This phase is metastable ; its preparation from a glass is related with the four-fold coordination of iron atoms in both the amorphous and the crystalline phases. This four-fold coordination of iron, which was established in our previous study by EXAFS and XANES (6), is confirmed by the analysis of the XRD pattern of the crystallized material. Furthermore, the low symmetry of the unit cell of SrFe204 deduced from its XRD pattern, explains the high dispersion of the Sr-O and Fe-O distances indicated by the EXAFS data. The distortion of the framework of SrFe204 is likely to be too high for a stable compound.

References 1. P. Batti, Ann. Chim. (Roma) 52, 1941 (1962). 2. A.F. Wells, Structural Inorganic Chemistry, Oxford University Press, Oxford, England (1984). 3. H. Mitsuda, S. Mori and C. Okazaki, Acta Cryst. 1327, 1263 (1971). 4. J. Monteil, L. Padel and J.C. Bernier, J. Solid State Chem. 25, 1 (1978). 5. P. Berthet, J. Berthon and A. Revcolevschi, J. of Crystal Growth 79, 590 (1986). 6. P. Berthet, J. Berthon and A. Revcolevschi, "2nd Conference on Progress in X-ray Synchrotron Radiation Research". Conference Proceedings Vol.25, p.675. Sociefi Italiana di Fisica, Bologna (1990). 7. International Tables for X-ray Crystallography, Vol.I, Kynoch Press, Birmingham, England (1969).