LATTICE VIBRATION-SPECTRA .88. LATTICE-DYNAMICS OF SUZUKI-TYPE LI(6)M(II)CL(8)(M(II)=FE,CO,NI)

Lattice dynamical calculations of Suzuki-type Li6FeCl8, Li6CoCl8, and Li6NiCl8 have been performed using various potential models (rigid-ion , central and angular force-constants). The potential energy is mainly controlled by Coulomb forces and short-range M(II)-Cl interactions (K -1 = 0.79 - 0.82, alpha(1) = 0.84 - 0.88 N cm(-1)). There are signific ant differences between the corresponding TO and LO phonon modes (spec ies F-1u), which are caused by the strong macroscopic field in these h ighly ionic compounds, The sequence of the Raman allowed modes taken f rom the literature has to be reversed as A(1g) > E(g) > F-2g(1) > F-2g (2), The higher wavenumbers of the A(1g) and E(g) phonons are due to t he involvement of the force-constant K-1 in the respective potential e nergies, FIR reflection spectra of the title compounds including oscil lator-fit calculations are presented. (C) 1996 Academic Press, Inc.