First-principles investigation of hydrous post-perovskite

Joshua P. Townsend*, Jun Tsuchiya, Craig R. Bina, Steven D. Jacobsen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

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 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 (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 ~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.

Original languageEnglish (US)
Pages (from-to)42-48
Number of pages7
JournalPhysics of the Earth and Planetary Interiors
Volume244
DOIs
StatePublished - Jul 1 2015

Keywords

  • Elasticity
  • Hydrogen
  • Lower mantle
  • Post-perovskite

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Geophysics
  • Physics and Astronomy (miscellaneous)
  • Space and Planetary Science

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