Crystal chemistry of rare-earth chalcogenides

Crystal structures of Ln-X, Ln-Ln'-X, Ln-M-X, and Ln-X-Z compounds (Ln and Ln' are rare-earth elements; M is subgroup I-VIIIa or II-Vb cation and Z is subgroup V-VIIb, anion; X is S, Se, or Te) were systematized and analyzed using data obtained by us and data from the literature. The structures of binary Ln-X chalcogenides can be divided into 4 groups: derivatives of NaCl-type structures, structures with one small parameter b approximate to 4 Angstrom, structures similar to those of oxides and pniktides, and derivatives of PbFCl-type structures. Triple Ln-Ln'-X chalcogenides have structures similar to those of the binary chalcogenides of the first, second, and third groups. It was shown that the crystal structure of Ln-M-X depends on the size of cation M, its formal charge, electronegativity value, and electronic structure. The crystal structures of the phases in the Ln-M-X and Ln-X-Z compounds were compared with binary chalcogenide structures. Different variants of structure characterization and their potentialities are considered. Special attention is given to the defect-containing and ordered structures and to elucidating the role of the cation and anion vacancies in the phase structures.