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 chal cides CoCr2S4, ZnCr2S4, (CdCr2S4), ZnCr2Se4, CdCr2Se4, and HgCr2Se4 we re performed using short-range (SRM), rigid-ion (RIM) and polarizable- ion models (PIM) with structure data, symmetry coordinates, IR and Ram an frequencies, permittivities, and the masses of the atoms involved a s input parameters. The mean deviations between calculated and observe d phonon energies are in the range 1-5 cm(-1) (PIM). The results obtai ned 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), dy namical effective ionic charges, eigenvectors, and potential energy di stributions within the compounds studied, the potential models used, a nd the reliability of the structural parameters available, respectivel y. Thus, the force constants due to the tetrahedral A-X bonds are in t he order CoCr2S4 < ACr(2)S(4) < ACr(2)Se(4) (A = Zn, Cd, Hg). Those du e to the octahedral Cr-X bonds increase on going from SRM to PIM calcu lations, 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.