Atomistic simulation of the effect of temperature and pressure on point defect formation in MgSiO3 perovskite and the stability of CaSiO3 perovskite

We present an atomistic simulation method for calculating the defect formation free energy and defect volume using lattice dynamics. The periodic simulation methods have been extended to allow charged systems with subtraction of defect-defect interactions to be studied routinely. This allows constant pressure minimization to be used rather than the more traditional constant volume method, allowing direct calculation of defect formation volumes and defect free energies or enthalpies rather than lattice energy. We have applied this method to the calculation of enthalpies and volumes of vacancy and Schottky formation as a function of pressure and to the solution of Ca in the lower mantle mineral MgSiO3 perovskite. The results indicate that as the pressure increases the defect volume of vacancy formation decreases and above approximately 50 GPa this decrease actually leads to a reduction in the enthalpy of formation rather than the expected increase as the pressure is increased further. The total Schottky enthalpy however continues to increase as a function of pressure although the rate of change of energy decreases with increasing pressure. The solution of Ca with MgSiO3 perovskite in shown to be very unfavourable and indicates that Ca will form its own CaSiO3 perovskite phase within the conditions expected within the lower mantle. This result is important when considering the amount and location of trace elements such as Al within the mantle that have been shown to be preferentially located within CaSiO3 perovskite rather than MgSiO3 perovskite.