Hydrogen bond symmetrization and equation of state of phase D

Abstract

We have synthesized phase D at 24 GPa and at temperatures of 1250-1100°C in a multianvil press under conditions of high silica activity. The compressibility of this high-silica-activity phase D (Mg 1.0Si1.7H3.0O6) has been measured up to 55.8 GPa at ambient temperature by powder X-ray diffraction. The volume (V) decreases smoothly with increasing pressure up to ∼40 GPa, consistent with the results reported in earlier studies. However, a kink is observed in the trend of V versus pressure above ∼40 GPa, reflecting a change in the compression behavior. The data to 30 GPa fit well to a third-order Birch-Murnaghan equation of state (EoS), yielding Vo = 85.1 ± 0.2 Å3; Ko = 167.9 ± 8.6 GPa; and K′o = 4.3 ± 0.5, similar to results for Fe-Al-free phase D reported by Frost and Fei (1999). However, these parameters are larger than those reported for Fe-Al-bearing phase D and for Fe-Al-free phase D. The abnormal volume change in this study may be attributed to the reported hydrogen bond symmetrization in phase D. Fitting a third-order Birch-Murnaghan EoS to the data below 30 GPa yields a bulk modulus Ko = 173 (2) GPa for the hydrogen-off-centered (HOC) phase and Ko = 212 (15) GPa for the data above 40 GPa for the hydrogen-centered (HC) phase, assuming Ko is 4. The calculated bulk modulus Ko of the HC phase is 18% larger than the bulk modulus Ko of the HOC phase. Copyright 2011 by the American Geophysical Union.