First-principles investigation of hydrous post-perovskite

A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg_1-xSiH_2xO₃) has been determined by first-principles calculations of the vibrational and elastic properties up to 150GPa. 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 (K₀) and shear (G₀) moduli by 5% and 8%, respectively, for a structure containing ~1wt.% 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 120GPa. In contrast, the reduction of certain anisotropic elastic constants (C_ij) 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 ~3500cm^-1 that shift with pressure to lower wavenumber by about 2cm^-1/GPa. At 120GPa the hydrogen bonds in hy-ppv are still asymmetric. The stability of a hy-ppv structure containing 1-2wt.% H₂O at D^″ pressures implies that post-perovskite may be a host for recycled or primordial hydrogen near the Earth's core-mantle boundary.