  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
numerous geophysical observations associated with this layer - for example, the
D'' discontinuity, its topography and seismic anisotropy within the layer. 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 postperovskite. In particular, the predicted slip
planes are (010) for perovskite (consistent with experiment) and (110) for
postperovskite (in contrast to the previously expected (010) slip planes).
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.
