Simultaneous high-pressure and -temperature volume measurements of H2O ice and the phase transition of ice VII

Details Simultaneous high-pressure and -temperature volume measurements of H2O ice and the phase transition of ice VII Simultaneous high-pressure and -temperature volume measurements of H2O ice and the phase transition of ice VII

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21b1219s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P21B-1219

Physics

[3919] Mineral Physics / Equations Of State, [3924] Mineral Physics / High-Pressure Behavior, [3949] Mineral Physics / Thermal Expansivity

Scientific paper

A recent 300 K compression study on H2O ice by Sugimura et al. (2008, Phys. Rev. B) showed that ice changes its compressibility at 40 and 60 GPa, possibly due to phase transitions of ice VII to dynamically-disordered ice VII, and subsequently to dynamically-disordered ice X. The intermediate phase, dynamically-disordered ice VII, is highly compressible due to quantum effects of protons, while dynamically-disordered ice X shows much smaller compressibility. It is important to investigate whether such an anomalous volume compression also occurs at higher temperatures or not, because it has a significant effect on density structures of H2O-rich icy planets and satellites. Here, we conducted simultaneous high-pressure (P) and -temperature (T) volume (V) measurements of H2O ice using angle-dispersive X-ray diffraction technique at the beamline BL10XU, SPring-8. High-P-T conditions were generated in an externally-heated diamond anvil cell. Isothermal volume compression was examined by collecting P-V data at 33-79 GPa with a constant temperature of 867-880 K (~873 K). Non-isothermal experiments at 430-740 K and 19-37 GPa were also conducted to construct a P-V-T equation of state of ice VII. In addition, volume data of ice up to 1013 K were collected at almost constant pressure of 77-80 GPa. The isothermal compression at 873 K shows that volumes of ice above 50 GPa are smaller than that of ice VII. Details are as follows. At 50-53 GPa, ice is highly compressible, and its volume eventually becomes 1.2% smaller than that of ice VII at the corresponding P-T. This highly compressible regime is most likely to be related to stability of dynamically-disordered ice VII. Combined with the result of previous experiment at 300K, our study indicates a positive P-T slope of ice VII - dynamically-disordered ice VII boundary, which contradicts with the work of a previous IR spectroscopic study suggesting a negative slope. At pressures >53 GPa (after the anomalous contraction), ice is compressed rather smoothly, and its volume maintains to be ~1% smaller than that of ice VII at each corresponding P-T; at the same time, the volume of ice is larger than that of dynamically-disordered ice X at 300 K, by, e.g., 4.5% at 55 GPa. If the volume after the anomalous contraction is assumed to be that of dynamically-disordered ice X, the 4.5%-volume increase is due to thermal expansion. However, a thermal expansion to this extent is unreasonably large at such a high pressure. It is plausible that at 873 K, dynamically-disordered ice VII does not undergo a pressure-induced transformation to dynamically-disordered ice X, but possibly to a new high-T phase.

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