COMPUTER-SIMULATION OF THE INFRARED AND RAMAN ACTIVITY OF PYROPE GARNET, AND ASSIGNMENT OF CALCULATED MODES TO SPECIFIC ATOMIC MOTIONS

The lattice dynamics computer code PARAPOCS was successfully used to c alculate the 240 vibrational frequencies of pyrope garnet, Mg3Al2Si3O1 2, at ambient conditions. The atomic displacement vectors (eigenvector s) for each frequency were also calculated and their symmetry relation s analyzed with the aid of factor group analysis (FCA), to determine t he symmetry species of each vibrational mode. Comparison with the expe rimental IR and Raman data shows excellent agreement, but no LO-TO rev ersals were identified. Calculation of the frequency shifts due to the isotopic substitution of Mg-26 and Si-30, together with a more detail ed analysis of the calculated eigenvectors, enabled identification of the dominant site or cation motion contributing to each vibrational mo de. Previous assignments of the high-frequency vibrations to pure SiO4 internal modes and the lower-frequency vibrations to mixed cation mod es are supported. We conclude that the specific number of site/atom mo tions predicted by site group analysis (SGA) is not adhered to due to substantial mode mixing, and that FCA and SCA, in which the SiO4 tetra hedra are treated as isolated units, are only applicable at high frequ encies. The agreement observed between the calculated and experimental data leads us to conclude that the method of computer modeling used a nd the interatomic potentials employed in the simulations provide a go od description of the lattice dynamical behavior of pyrope garnet.