LATTICE-DYNAMICS OF SPINEL-TYPE CHLORIDES, OXIDES, SULFIDES AND SELENIDES

Lattice dynamical calculations of spinel-type ZnCr(2)X(4) (X=O, S, Se) and Li2ZnCl4 were performed using rigid ion and polarizable ion model s. The main results obtained are that (i) the eigenvectors and potenti al energy distributions of the /k/ = 0 phonon modes differ greatly wit h respect to the masses of the atoms involved and the different ionici ties of the isostructural compounds under investigation, (ii) the pote ntial energy is mainly controlled by short-range tetrahedral Zn-X and octahedral II Cr(Li)-X force constants which correlate with the respec tive bond lengths, (iii) the ionicities, which are revealed from both 111 the effective dynamical charges and the Coulomb force contribution s to the potential energies, range as Li2ZnCl4 > ZnCr2O4 >> ZnCr2S4 > ZnCr2Se4 and (iv) the effective dynamical charges of Zn are much small er (or nearly zero for X=S and Se) than those in the CrX(6) (and LiX(6 )) octahedra, revealing that the bonding in the ZnX(4) tetrahedra is m ainly covalent.