The postperovskite transition in MgSiO ₃ at conditions similar to those expected at the D″ discontinuity of Earth's lower mantle offers a paradigm for interpreting the properties of this region. Despite consistent experimental and theoretical predictions of this phase transformation, the complexity of the D″ region raises questions about its geophysical significance. Here we report the thermoelastic properties of Cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle. These are ( i ) the jumps in shear and longitudinal velocities similar to those observed in certain places of the D″ discontinuity and ( ii ) the anticorrelation between shear and bulk velocity anomalies as detected within the D″ region. In addition, the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle.

Ultrahigh-pressure phase relationship of SiO ₂ silica in multimegabar pressure condition is still quite unclear. Here, we report a theoretical prediction on a previously uncharacterized stable structure of silica with an unexpected hexagonal Fe ₂ P-type form. This phase, more stable than the cotunnite-type structure, a previously postulated postpyrite phase, was discovered to stabilize at 640 GPa through a careful structure search by means of ab initio density functional computations over various structure models. This is the first evidential result of the pressure-induced phase transition to the Fe ₂ P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO ₉ tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO ₂ on the stability of MgSiO ₃ and CaSiO ₃ at multimegabar pressures. A postperovskite phase of MgSiO ₃ breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe ₂ P-type SiO ₂ and CsCl (B2)-type MgO. CaSiO ₃ perovskite, on the other hand, directly dissociates into SiO ₂ and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO ₂ , MgSiO ₃ , and CaSiO ₃ indicate that the Fe ₂ P-type SiO ₂ could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.