INFRARED-SPECTROSCOPY OF CAGEO3 PEROVSKITE TO 24-GPA AND THERMODYNAMIC IMPLICATIONS

Far-infrared absorbance spectra were collected from CaGeO3 with a meta stable orthorhombic perovskite structure from 0 to 24.4 GPa. The absor bance data are compatible with a reflectance spectrum which was collec ted at ambient conditions from a polished, densely compacted polycryst al. The reflectance spectrum shows 18 IR modes from 155 to 786 cm-1. A detailed model for the density of states constructed from these new d ata results in accurate calculation of heat capacity and new data on e ntropy. Peak positions increase linearly with pressure. Mode Gruneisen parameters (ranging from 0.72-1.56) decrease almost linearly with inc reasing mode frequency which is consistent with deformations of the ox ygen sublattice dominating the lattice vibrations. Neither discontinuo us changes in the number of modes nor in these frequencies nor in band widths are observed at pressures up to 24.4 GPa. Thus, conversion to the tetragonal phase at approximately 12 GPa is not indicated.