Abstract
The effect of pressure on ionic diffusion in orthorhombic MgSiO3 perovskite has been investigated using density functional theory. An intensive investigation of possible silicon pathways revealed new positions of the saddle-points and an enthalpy of migration at 26.2 GPa of 4.7 eV that is in fair agreement with the experimental values of about 3.5 eV at 25 GPa. This is much lower than found in previous studies (~9 eV) and removes the need to explain silicon diffusion by a complicated process involving coupled oxygen vacancies, as has been previously proposed. Our migration enthalpies for oxygen and magnesium are in excellent agreement with experiments. We find that oxygen diffusion occurs via a chain of several inequivalent jumps along the octahedron edges, and that magnesium occurs via two inequivalent [110] jumps and one [001] jump. We also present activation volumes for all three species at 25 and 135 GPa.
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Acknowledgments
This work was funded by the European Commission through the Marie Curie Research Training Network “c2c” Contract No. MRTN-CT-2006-035957. This work made use of the facilities of HECToR, the UK’s national high-performance computing service, which is provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc and NAG Ltd, and funded by the Office of Science and Technology through EPSRC’s High End Computing Programme.
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Ammann, M.W., Brodholt, J.P. & Dobson, D.P. DFT study of migration enthalpies in MgSiO3 perovskite. Phys Chem Minerals 36, 151–158 (2009). https://doi.org/10.1007/s00269-008-0265-z
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DOI: https://doi.org/10.1007/s00269-008-0265-z