Computational mineral physics and the physical properties of perovskite

The inherent uncertainties in modern first-principles calculations are reviewed using geophysically relevant examples. The elastic constants of perovskite at lower-mantle temperatures and pressures are calculated using ab initio molecular dynamics. These are used in conjunction with seismic tomographic models to estimate that the lateral temperature contrasts in the Earth's lower mantle are 800 K at a depth of 1000 km, and 1500 K at a depth of 2000 km. The effect of Al3+ on the compressibility of MgSiO3 perovskite is calculated using three different pseudopotentials. The results confirm earlier work and show that the compressibility of perovskites with Al3+ substituted for both Si4+ and Mg2+ is very similar to the compressibility of Al3+-free perovskite. Even when 100% of the Si4+ and Mg2+ ions are replaced with Al3+, the bulk modulus is only 7% less than that for Al3+-free perovskite. In contrast, perovskites where Al3+ substitutes for Si4+ only and that are charge balanced by oxygen vacancies do show higher compressibilities. When corrected to similar concentrations of Al3+, the calculated compressibilities of the oxygen-vacancy-rich perovskites are in agreement with experimental results.