First-principles investigation of hydrous post-perovskite

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Abstract 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 ( K 0 ) and shear ( G 0 ) 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.

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