Abstract
A thermal equation of state of phase D with a Mg1.02Si1.73H3.03O6 composition has been derived from in situ X-ray diffraction experiments using synchrotron radiation and multianvil apparatus with sintered diamond anvils at pressures up to 46 GPa and temperatures up to 1300 K. The refined unit-cell parameters at ambient conditions are: a=4.7643(7) Å, c=4.346(2) Å, and V 0=85.43(4)Å3 and the zero-pressure density of ρ0=3.357(2) g/cm3 (Z=1; formula weight=172.70 g/mol). At room temperature, the unit-cell parameters show anisotropic compression behavior up to ∼35 GPa, where the c-axis is more compressible than the a-axis. Above 35 GPa, however, the c-axis becomes less compressible than the a-axis. This change of the axial compression behavior tends to start at higher pressure with increasing temperature. The change of the linear compressibilities may be related to the hydrogen-bond symmetrization in phase D, as suggested by recent first-principles calculations.
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