1. Laboratory of Crystallography, Department of Materials, ETH Zurich, Wolfgang Pauli Strasse 10, CH-8093 Zurich, Switzerland
2. Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan

Correspondence to: Artem R. Oganov¹ Email: a.oganov@mat.ethz.ch

Abstract

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO₃ perovskite, with lesser amounts of (Mg,Fe)O and CaSiO₃ (ref. 1). But it has not been possible to explain many unusual properties of the lowermost 150 km of the mantle (the D" layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D" layer, MgSiO₃ transforms from perovskite into a layered CaIrO₃-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D" layer: its seismic anisotropy², the strongly undulating shear-wave discontinuity at its top^{3, 4, 5, 6} and possibly the anticorrelation between shear and bulk sound velocities^{7, 8}.

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