NORMAL COORDINATE ANALYSES AND LATTICE DYNAMIC CALCULATIONS OF SPINEL-TYPE ZNCR2O4

Lattice dynamical calculations of spinel-type ZnCr2O4 have been performed using various potential models (short-range, rigid-ion, polarizable-ion). The potential energy is mainly controlled by Cr-O (octahedron) and Zn-O (tetrahedron) interactions as well as Cr-Cr repulsive forces with force constants of 2.1, 1.8, and 0.3 N cm-1, respectively, whereas O-O interactions are less important. The tetrahedral ZnO4 units are less ionic than the octahedral CrO6 groupings (effective dynamical charges: Zn: 0.54 e; Cr: 1.49 e). The vibrational modes (eigenvectors) and potential energy distributions of the zone-centre phonons partly differ from those of MCr2S4. There are significant differences between corresponding transversal (TO) and longitudinal (LO) phonons in the case of the two high-energy F1u modes owing to the large macroscopic field in this highly ionic compound.