Thermodynamic properties of ferromagnesium silicate perovskites from vibrational spectroscopy

Infrared (IR) reflectance and absorbance spectra obtained from polycrystalline and single‐crystal perovskites of MgSiO 3 and (Mg 0.9 Fe 0.1 )SiO 3 compositions yielded virtually all the expected fundamental modes. Near‐IR absorbance spectra indicate that the polycrystals were anhydrous, whereas the single crystal contained about 0.06 wt % H 2 O. A Kieffer‐type model was used to calculate heat capacity, entropy, and Debye temperature for the orthorhombic perovskite phase. The calculated values at 298 K are C p = 82 ± 2 J/mol K and S = 56 ± 2 J/mol K. Our calculated C p agrees with calorimetric data within 5%. Above 500 K, the Debye temperature is nearly constant and equals 960±30 K. The bulk modulus is calculated from vibrational frequencies to be 265±13 GPa which lies within 2% of recent experiments. The Clapeyron slope for the phase transition between ilmenite and orthorhombic perovskite is calculated to be 1.110.6 MPa/K which is close to the experimental determination.