Elastic hardening upon approaching hydrogen-bond symmetrization in high-pressure ice polymorphs: experiments and calculations

Abstract:

Hydrogen and water are widely present in a large variety of planetary environments in the solar system and play important roles in the dynamics and evolution of planetary interiors. The hydrogen-bond behavior in the structure of high-pressure H₂O ices has been the focus of interest in the past decades. Especially the symmetrization of hydrogen bond leading to the transition into symmetric ice (ice X) has been one of the major subjects of debate in chemistry and physics for over a half century and is still poorly understood. In this study, we synthesized large ice-VII single crystals in an externally-heated diamond anvil cell at high pressures and temperatures and performed Brillouin scattering and X-ray diffraction measurements of the single-crystal ices. We measured both V_P and V_S of the single-crystal ice by Brillouin scattering and determined its orientation and structure by synchrotron X-ray diffraction up to 70 GPa. Our experimentally derived elastic constants show the Cauchy relation C₁₂ ≈ C₄₄ for ice VII at 2-30 GPa, whereas C₁₂ becomes increasingly larger than C₄₄ at 30-55 GPa. At pressures higher than 55 GPa, all the three elastic constants (C₁₁, C₁₂, and C₄₄) become drastically hardened or higher. Our first-principles calculations corroborate the observed elastic hardening of the high-pressure ice starting from ~55 GPa and predict a continuing quick increase of the elastic constants until ~110 GPa at which the transition into the symmetric ice X would occur. The concerted experimental and computational study allows a better understanding of the nature of the transition from ice-VII to ice X and the hydrogen-bond symmetrization, which could significantly affect physical and chemical properties of the ice polymorphs.