Thermal expansivity of lower mantle phases MgO and MgSiO3 perovskite at high pressure derived from vibrational spectroscopy

New spectroscopic results for MgSiO3 perovskite to 65 GPa how Raman activity above 47 GPa, indicating that the structure of the perovskite is not cubic. However, at 40 GPa the vibrational modes show an average of 40% decrease in the pressure-frequency slope, indicating a profound change in compressional mechanism at this pressure. Thermal expansivity of MgO and MeSiO(3) perovskite were estimated using the thermodynamic Maxwell's relation (partial derivative S/partial derivative P)(Upsilon) = -(partial derivative V/partial derivative T)(P) and the entropies at high pressures derived using statistical thermodynamics and spectroscopic data. Previous spectral data of MgO to 37 GPa and the new spectral data of MgSiO3 perovskite were used to constrain the pressure dependence of the entropy. For MgO, the thermal expansivities, Gruneisen parameter gamma(th), and Anderson-Gruneisen parameter delta(Upsilon) were in excellent agreement with previous studies by other methods over the pressure and temperature range of these calculations. For MgSiO3 perovskite, thermal expansivity at 1 atm was found to be 18(1). 10(-6) K-1 at 300 K increasing to 27(1). 10(-6) K-1 at 1000 K, in agreement with previous large volume press measurements, but disagreeing with previous diamond cell measurements. The Gruneisen parameter gamma(th) was estimated as 1.43(7) and the Anderson-Gruneisen parameter delta(T) at room temperature as 3.4(2). At 40 Gpa, the changes in the pressure dependence of the Raman frequencies correspond to a large abrupt decrease in thermal expansivity at this pressure. delta(T) drops to 2.2 above this pressure but an average delta(T) of 4.8 can be used to describe the thermal expansivity to core-mantle boundary pressures.