Elastic properties of phase D (MgSi2O6H2) under pressure: Ab initio investigation

Recently, the hydrogen bond of phase D (MgSi2O6H2) has been predicted to transform from the asymmetric to the symmetric one at ∼40 GPa. We have investigated the effects of hydrogen bond symmetrization on the elastic properties of phase D by first-principles calculation. We found some of the elastic constants of phase D with the symmetric hydrogen bond ordering are significantly larger than those with the asymmetric one. Among them, diagonal C33, which corresponds to the uniaxial compression along the hydrogen bond in phase D, anomalously increases across the hydrogen bond symmetrization, while shear C44, C55 and C66 smoothly increase with pressure showing no anomalous behavior. Accordingly, the bulk modulus, VP and Vϕ irregularly increase and the shear modulus and VS do not across the hydrogen bond symmetrization. Since such elastic hardenings have not been reported in other structural model of phase D which does not show hydrogen bond symmetrization under high pressure, these elastic behavior may be a characteristic aspect of the hydrogen bond symmetrization.