To address the effects of lattice anharmonicity across the perovskite to postperovskite transition in $\mathrm{MgSi}{\mathrm{O}}_3$, we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on $abinitio$ molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit $\left(N\to \infty \right)$ 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 Grüneisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope $\left(dP/dT\right)$ 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.

中文翻译：

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MgSiO3 中钙钛矿后转变的非谐热力学性质和相界

为了解决钙钛矿到后钙钛矿转变的晶格非谐性的影响$\mathrm{镁硅}{\mathrm{○}}_3$，我们使用声子准粒子（PHQ）方法进行计算。PHQ 是基于$\mathrm{一个}b\mathrm{一世}n\mathrm{一世}吨\mathrm{一世}○$分子动力学，原则上，捕获完全的非谐性。热力学极限中的自由能$\left(ñ\to \infty \right)$使用声子气体模型中与温度相关的准粒子分散来计算。报告了非谐热力学性质和相界的系统结果。执行局部密度近似和广义梯度近似计算以提供对这些属性的置信约束。通过将结果与使用准谐波近似 (QHA) 获得的结果进行比较来证明非谐波效应。QHA 的不足之处在于其对热膨胀率和热力学 Grüneisen 参数的高估以及其在高温极限下的收敛等容热容。PHQ 相界具有 Clapeyron 斜率$\left(d磷/d吨\right)$随温度升高。该结果与 QHA 相界几乎为零的曲率形成对比。非谐性使相界在高压下弯曲到较低温度。讨论了地幔地热对相界的双重交叉的影响。