Anisotropy of Earth's D layer and stacking faults in the MgSiO3 post-perovskite phase

The post-perovskite phase of (Mg, Fe) SiO3 is believed to be the main mineral phase of the Earth's lowermost mantle (the D '' layer). Its properties explain(1-6) numerous geophysical observations associated with this layer - for example, the D '' discontinuity(7), its topography(8) and seismic anisotropy within the layer(9). Here we use a novel simulation technique, first-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and post-perovskite. In particular, the predicted slip planes are {010} for perovskite ( consistent with experiment(10,11)) and {110} for postperovskite ( 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. The {110} slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D '' layer.