First-principles investigation of hydrous post-perovskite

• We have predicted a stable hydrogen defect structure in post-perovskite, stable to 150 GPa.
• The shear-wave velocity of ppv with 1.6 wt.% water is identical to that of dry bridgmanite.
• The bulk sound velocity is insensitive to hydration of ppv; slowing by 3.5 m/s per wt.% water.

A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg1−xSiH2xO3) has been determined by first-principles calculations of the vibrational and elastic properties up to 150 GPa. Among three potential hy-ppv structures analyzed, one was found to be stable at pressures relevant to the lower-mantle D″ region. Hydrogen has a pronounced effect on the elastic properties of post-perovskite due to magnesium defects associated with hydration, including a reduction of the zero-pressure bulk (K0K0) and shear (G0G0) moduli by 5% and 8%, respectively, for a structure containing ∼1 wt.% H2O. However, with increasing pressure the moduli of hy-ppv increase significantly relative to ppv, resulting in a structure that is only 1% slower in bulk compressional velocity and 2.5% slower in shear-wave velocity than ppv at 120 GPa. In contrast, the reduction of certain anisotropic elastic constants (Cij) in hy-ppv increases with pressure (notably, C55, C66, and C23), indicating that hydration generally increases elastic anisotropy in hy-ppv at D″ pressures. Calculated infrared absorption spectra show two O–H stretching bands at ∼3500 cm−1 that shift with pressure to lower wavenumber by about 2 cm−1/GPa. At 120 GPa the hydrogen bonds in hy-ppv are still asymmetric. The stability of a hy-ppv structure containing 1–2 wt.% H2O at D″ pressures implies that post-perovskite may be a host for recycled or primordial hydrogen near the Earth’s core-mantle boundary.