Electron Microscope Study of Cation-Deficient Chalcogenide Spinels
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ELECTRON MICROSCOPE STUDY OF CATION-DEFICIENT CHALCOGENIDE SPINELS
Y. Komura and S. Takeda Department of Materials Science, Faculty of Science, Higashi-senda-machi, Naka-ku, Hiroshima 730, Japan
Hiroshima University,
ABSTRACT High-resolution electron microscope observations were carried out on the cation-deficient spinel compounds Ga 2/3Cr2S4 and InCr 5/3S4 X-ray analysis of both compounds indicates that there exist 1/3 vacancy per formula unit in 8a positions. It was also found that the vacancy ordering occurs in Ga 2/3Cr 2S4 below about lIO0'C, as well as in InCr 5/3S4.
Lattice images of vacancy ordered and disordered
structures were obtained. In some electron micrographs of ordered InCr 5 / 3 S4 , several domains of disordered region were observed in between the matrix of the ordered structure. In a [1103 image, characteristic lattice defects were observed on the (110) plane.
INTRODUCTION The crystal structures of the chromium chalcogenide spinels Ga 2/3Cr 2S4 and lnCr /3S4
were recently studied by X-ray diffraction method [1].
X-ray
analysis of both compounds indicates that there exist 1/3 vacancy per formula unit in 8a positions and all chromium atoms in 16d positions, with the rest of the metal positions occupied by gallium or indium atoms. It was also found that the vacancy ordering occurs in Ga 2/3Cr 2S4 below about 11000C as well as in InCr 5/3S42 The purpose of this paper is to observe the ordering scheme in specimens by high-resolution electron microscopy technique.
the
VACANCY-ORDERED AND DISORDERED SPINEL STRUCTURES Figure l(a) shows the (001) projection of the disordered spinel structure belonging to the space group Fd3m, where only the 8a positions are drawn. There exist 1/3 vacancy in each 8a position, so that the circles in Fig. l(a) are filled by 2/3. The lattice parameter a is equal to about 10A 2 and the unit cell in the (001) projection is a square of a/2/2 by a/2V ,
(2oo&) FIG. 1. The (001) projections of (a) the vacancy-disordered and (b) the
vacancy-ordered spinel structures. 0; no vacancy, b; 1/3 vacancy, (; 2/3 vacancy per one site.
1OA-)'b (a (a) Mat.
Res.
Soc.
Symp. Proc. Vol.
(b) 21 (1984) (
Elsevier Science Publishing Co.,
Inc.
144
which is equal to d220. The vacancy-ordered spinel belongs to F43m, where the 8a positions in 111 Fd3m, 000; 1 1; F.C., split into two fourfold positions 4a, 000; F.C., and 111 4c, (4a)+ 4 4 4; F.C. It was reported [1] that no vacancy is found in the 4a and 2/3 vacancy in the 4c positions, as shown in Fig. l(b). The unit cell in the (001) projection is a square of a/2 by a/2. The hkO (h;k even) reflections with h+ki4n (e.g. 200), which are forThe spacing a/2 in bidden in Fd3m, are permitted in the ordered structures. Fig. l(b) corresponds to d200. EXPERIMENTAL PROCEDURES All of the samples were kindly given by Dr. Nakatani of the National Research Institute for metals. They were prepared by firing stoichiometric mixtures of the elements, which were in a powder of Cr(99.9%); grains of Ga The detailed procedure (99.999%) or In(99.999%);
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