CRYSTAL STRUCTURE OF VACANCY ORDERED ZnGa₂Se_4. O. Nittono, T. Hanada, and Y. Nakamura, Department Metallurgical Engineering, Tokyo Institute of Technology, Oh-Okayama, Meguro-ku Tokyo 152, Japan

ZnGa₂Se₄ is a representative of II-III-VI compound semiconductors and the basic structure is chalcopyrite. In the ZnGa₂Se₄ compound, Se sublattice is perfect fcc but the cation sublattice is ternary: Zn, Ga and structure vacancy. The structure of ZnGa₂Se₄ is reported to be either defect stannite or defect chalcopyrite whose space group is I 2m or I respectively. In each structure, metal atoms and the structure vacancy are arranged in order. They are different in the configuration of metal atoms and structure vacancy even though the projected structures on the fundamental planes are identical. In the present work, convergent electron beam diffraction (CBED) are applied to determine the space group of ZnGa₂Se₄, and then crystal structure is refined by X-ray Rietveld method.

The specimens of ZnGa₂Se₄ are synthesized by melting of Ga₂Se₃ (3N) and ZnSe (5N) at 1673K, and then annealed at 1273K for 60 days. Sharp reflection spots are observed in the selected area electron beam diffraction (SAED) patterns, which indicates the good crystallinity of the specimens. TEM observation shows that domains grow up well and the size is as large as 5um. A CBED whole pattern taken along the [001] direction clearly exhibits 4_Rmm_R symmetry, so that it is found that ZnGa₂Se₄ belongs to the space group I m2. Based on the result, X-ray Rietveld refinement (F. Izumi, 1993) has been done and that converged at a=5.5093Å, b=10.9450Å, R_wp=18.49%, R_F=5.60%, S=1.7065.

F. Izumi, "The Rietveld Method," ed. by R. A. Young, Oxford University Press, Oxford (1993), Chap. 13