Thermal equation of state of MgSiO4H2 phase H determined by in situ X-ray diffraction and a multianvil apparatus

Phase H (MgSiO₄H₂) is the high-pressure form of dense hydrous silicate that could deliver surface water into the lower mantle. In this study, we determined the thermal equations of the state of phase H using in situ X-ray diffraction measurements, under conditions ranging from 34 to 62 GPa and 300 and 1300 K, using a multianvil apparatus. Analysis of the data, based on the Mie-Grüneisen-Debye model using third-order Burch-Murnaghan equations at a reference pressure of 35 GPa, yielded the following results V_ref = 49.61 ± 0.01 ų, K_ref = 344.6±4.1 GPa, Kref' = 3.05 ± 0.32, θ_ref = 974 ± 146 K, γ_ref = 1.8 ± 0.1, and q = 1.79 ± 0.55. The compressibility of phase H observed in this study agrees well with that derived from theoretical calculations in pressure regions where hydrogen bond symmetrization is predicted. It was also found that the volume and compressibility of phase H and δ-AlOOH were similar.