Anisotropy of Earth’s D” layer and stacking faults in MgSiO3 post-perovskite

The post-perovskite phase of (Mg,Fe)SiO 3 (PPv) is believed to be the main mineral phase of the Earth's D " layer (2700-2890 km depths). Its properties explain 1-6 numerous geophysical anomalies associated with this layer: e.g., the D " discontinuity 7 , its topography 8 and seismic anisotropy 9. Here, using a novel simulation technique, first-principles metadynamics, we identify a family of low-energy polytypic stacking-fault structures intermediate between perovskite (Pv) and PPv. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the Pv-PPv phase transition, and as a likely mechanism for plastic deformation of Pv and PPv. In particular, the predicted slip planes are {010} for Pv (consistent with experiment 10,11) and {110} for PPv (in contrast to the previously expected {010} slip planes 1-4). Dominant slip planes define the lattice preferred orientation and elastic anisotropy of the texture. With {110} slip planes in PPv, we obtain a new interpretation of the shear-wave anisotropy in the D " layer, requiring a much smaller degree of lattice preferred orientation and more consistent with geophysical observations.