Anharmonic thermodynamic properties and phase boundary across the postperovskite transition in MgSiO3

Article

关键词: EQUATION-OF-STATE;    POST-PEROVSKITE PHASE;    LOWER MANTLE;    THERMOELASTIC PROPERTIES;    (MG,FE)SIO3 PEROVSKITE;    CRYSTAL-CHEMISTRY;    LATTICE-DYNAMICS;    TEMPERATURE;    PRESSURE;    DENSITY;   

DOI  :  10.1103/PhysRevB.106.054103

来源: SCIE

【 摘 要 】

To address the effects of lattice anharmonicity across the perovskite to postperovskite transition in MgSiO3, we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on ab initio molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit (N -> infinity) are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation and the generalized gradient approximation calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Gruneisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope (dP/dT) that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

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