Phase transition and compression behavior of phase D up to 46 GPa using multi-anvil apparatus with sintered diamond anvils

Abstract

A thermal equation of state of phase D with a Mg_1.02Si_1.73H_3.03O₆ 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)ų 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.

Keywords:

• DHMS
• phase D
• in situ X-ray diffraction
• high-temperature equation of state

Acknowledgements

We are grateful to Y. Wang, H. Kagi, H. Ohfuji and T. Inoue for valuable comments and discussions to improve this paper. We also thank T. Kunimoto, A. Yamada and Y. Tange for the help with synchrotron radiation experiments. The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2005B0357-ND2b-np).