1. Laboratory of Crystallography, Department of Materials, ETH Zurich, HCI G 515, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland
2. Computational Science, Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 13, CH-6900 Lugano, Switzerland

Correspondence to: Artem R. Oganov¹ Correspondence and requests for materials should be addressed to A.R.O. (Email: a.oganov@mat.ethz.ch).

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, 2, 3, 4, 5, 6} numerous geophysical observations associated with this layer—for example, the D" discontinuity⁷, its topography⁸ and seismic anisotropy within the layer⁹. 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 post-perovskite (in contrast to the previously expected {010} slip planes^{1, 2, 3, 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.

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