THERMODYNAMIC PROPERTIES OF FERROMAGNESIUM SILICATE PEROVSKITES FROM VIBRATIONAL SPECTROSCOPY

Infrared (IR) reflectance and absorbance spectra obtained from polycry stalline and single-crystal perovskites of MgSiO3 and (Mg0.9Fe0.1)SiO3 compositions yielded virtually all the expected fundamental modes. Ne ar-IR absorbance spectra indicate that the polycrystals were anhydrous , whereas the single crystal contained about 0.06 wt % H2O. A Kieffer- type model was used to calculate heat capacity, entropy, and Debye tem perature for the orthorhombic perovskite phase. The calculated values at 298 K are Cp = 82+/-2 J/mol K and S = 56+/-2 J/mol K. Our calculate d Cp agrees with calorimetric data within 5%. Above 500 K, the Debye t emperature is nearly constant and equals 960+/-30 K. The bulk modulus is calculated from vibrational frequencies to be 265+/-13 GPa which li es within 2% of recent experiments. The Clapeyron slope for the phase transition between ilmenite and orthorhombic perovskite is calculated to be 1.1+/-0.6 MPa/K which is close to the experimental determination .