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 temperature (up to 1570 K at room pressure) on the Raman spectrum of CaTiO3 is presented. No significant changes, which could be attributed to a major structural change, are observed in the spectra up to 22 GPa. The pressure shifts of the Raman modes can be related to a significant compression of the Ti - 0 bond. Discontinuous changes in the spectra upon heating may be related to phase changes observed by calorimetry and X-ray diffraction. The important temperature shifts of some low-frequency modes can be related to an increase in the Ti - 0 - Ti angle in agreement with the X-ray data showing a decrease of the structural distortion with increasing temperature. These data are compared to those available for MgSiO3-perovskite and show that CaTiO3 is a good structural analogue 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 in the 0-1300 K temperature range. A similarly defined model is proposed 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 agreement with the values inferred from experimental equilibrium data. Finally we briefly discuss the values of the Gruneisen parameters of both perovskites inferred from macroscopic and microscopic data.