LATTICE VIBRATION-SPECTRA .84. LATTICE-DYNAMICS OF SPINEL-TYPE COCR2S4, ZNCR2S4, ZNCR2SE4, CDCR2SE4, AND HGCR2SE4

Lattice dynamical calculations of ACr(2)X(4) spinel-type chromium chalcides CoCr2S4, ZnCr2S4, (CdCr2S4), ZnCr2Se4, CdCr2Se4, and HgCr2Se4 were performed using short-range (SRM), rigid-ion (RIM) and polarizable-ion models (PIM) with structure data, symmetry coordinates, IR and Raman frequencies, permittivities, and the masses of the atoms involved as input parameters. The mean deviations between calculated and observed phonon energies are in the range 1-5 cm(-1) (PIM). The results obtained are discussed with respect to the change in the short-range force constants (A-X, Cr-X, Cr-Cr, and X-X valence and repulsive forces), dynamical effective ionic charges, eigenvectors, and potential energy distributions within the compounds studied, the potential models used, and the reliability of the structural parameters available, respectively. Thus, the force constants due to the tetrahedral A-X bonds are in the order CoCr2S4 < ACr(2)S(4) < ACr(2)Se(4) (A = Zn, Cd, Hg). Those due to the octahedral Cr-X bonds increase on going from SRM to PIM calculations, indicating (in addition to the respective effective charges) the more ionic nature of this bond compared to that of the A-X bonds. (C) 1995 Academic Press, Inc.