PHASE-CHANGES AND THERMODYNAMIC PROPERTIES OF CATIO3 - SPECTROSCOPIC DATA, VIBRATIONAL MODELING AND SOME INSIGHTS ON THE PROPERTIES OF MGSIO3 PEROVSKITE

The effect of pressure (up to 21 GPa at room temperature) and temperat ure (up to 1570 K at room pressure) on the Raman spectrum of CaTiO3 is presented. No significant changes, which could be attributed to a maj or structural change, are observed in the spectra up to 22 GPa. The pr essure shifts of the Raman modes can be related to a significant compr ession of the Ti - 0 bond. Discontinuous changes in the spectra upon h eating may be related to phase changes observed by calorimetry and X-r ay diffraction. The important temperature shifts of some low-frequency modes can be related to an increase in the Ti - 0 - Ti angle in agree ment with the X-ray data showing a decrease of the structural distorti on with increasing temperature. These data are compared to those avail able for MgSiO3-perovskite and show that CaTiO3 is a good structural a nalogue for MgSiO3-perovskite. The present spectroscopic data are used to calculate the specific heat and entropy of CaTiO3. The role of the low frequency modes in the calculations is emphasized. Good agreement is observed between calculated and experimentally determined values i n the 0-1300 K temperature range. A similarly defined model is propose d for MgSiO3-perovskite. It is found that the entropy lies between 57 and 64 J/mol/K at 298 K and between 190 and 200 J/mol/K at 1000 K in a greement with the values inferred from experimental equilibrium data. Finally we briefly discuss the values of the Gruneisen parameters of b oth perovskites inferred from macroscopic and microscopic data.