Bulk modulus of H2O across the ice VII - ice X transition measured by time-resolved X-ray diffraction in dynamic Diamond Anvil Cell experiments
Abstract
H2O ice is present in a large variety of planetary bodies, including the ice giants, Uranus and Neptune [1], mini-Neptune exoplanets and icy satellites, having a fundamental impact on the heat and chemical exchange in their interiors. The polymorphs ice VII and ice X are the stable forms of H2O ice that exist at pressures larger than 2 GPa, and in the transition between them, a hydrogen-bond symmetrization takes place that is characterized by a sequence of proton order-disorder processes. These processes are continuous and are not evident in the x-ray diffraction signature of ice. Previous results based on Brillouin spectroscopy reported a softening of the bulk modulus of ice at pressures of 40-60 GPa, likely associated with the onset of the ice VII - ice X transition sequence [2]. However, the elastic behavior of ice beyond 60 GPa remains elusive. Here, we have collected quasi-continuous X-ray diffraction data across the ice VII - ice X transition and up to P=180 GPa at room temperature employing a dynamic Diamond Anvil Cell (dDAC, [3]) in combination with time-resolved x-ray diffraction at the Extreme Conditions Beamline (P02.2) at PETRA III. The setup allows for monitoring the effect of pressure on the unit cell volume of ice with quasi-continuous pressure-resolution. The uniquely dense sampling of pressure and volume in our data allows us to directly determine changes in the bulk modulus of ice by local differentiation without an assumption on the equation-of-state formulation. Our data document three changes in compressive behavior in the ranges of 35-45, 50-55 and 90-110 GPa, likely corresponding to the formation of pre-transition dynamically disordered ice VII and ice X, and static ice X, respectively. Our results further confirm computational predictions of a distinctly higher bulk modulus for ice X than for ice VII [4,5].
References [1] Redmer et al. (2011). Icarus, 211, 798-803. [2] Li et al. (2019). Am. Mineral., 104, 1307-1314. [3] Jenei et al. (2019). Rev. Sci. Instrum., 90(6), 065114. [4] French & Redmer (2015). Phys. Rev. B, 91, 014308. [5] Journaux et al. (2014). Phys. Earth Planet. In., 236, 10-15.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMMR025..03S
- Keywords:
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- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3934 Optical;
- infrared;
- and Raman spectroscopy;
- MINERAL PHYSICS;
- 3954 X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- MINERAL PHYSICS;
- 3994 Instruments and techniques;
- MINERAL PHYSICS