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Thermal pressure in MgO and MgSiO₃ perovskite at lower mantle conditions

Department of Chemical Engineering and Materials Science, and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.

Correspondence: ^* E-mail: karki{at}cems.umn.edu

The behavior of the thermal pressure, P_TH, in MgO and MgSiO₃ perovskite is investigated at lower mantle conditions using first-principles lattice dynamical calculations based on density functional perturbation theory. In both cases, P_TH is a slowly varying function of volume: it decreases linearly with compression at low temperatures but shows a slight non monotonical trend at high temperatures. Because P_TH strongly increases with increasing temperature, its volume-induced variations at high temperatures (above 1000 K) can be neglected to a good approximation. It is possible to estimate the thermal equations of state of these minerals at typical lower mantle temperatures only with the knowledge of the measured P_TH vs. temperature data at ambient pressure and isothermal compression data at ambient temperature. The coefficient of thermal expansion for MgO at high pressures and temperatures obtained in this manner agrees well with the first-principles predictions and available experimental data.