The free energy of binding between a cation vacancy and charge-balancing tetravalent cation in MgO is calculated from molecular dynamics simulations, at temperatures of 1200–2400 K and pressures of 0–25 GPa. The entropy of binding, obtained from the temperature dependence of the binding free energy, is found to be independent of pressure and to have a value of 13(5) J/mol K (2σ). The binding volume, obtained from the pressure dependence of the binding energy, is independent of temperature and has a value of 1.7(3) cm3/mol (2σ). These results are in excellent agreement with experimental estimates for several different trivalent cations, showing that the binding entropy and volume are not strongly dependent on the identity or charge of the cation. The binding energy is predicted to increase with depth along a mantle adiabat, leading to lower concentrations of unbound vacancies, and correspondingly slower diffusion rates for unbound cations.

中文翻译：

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方镁石空位-溶质结合自由能的分子动力学模拟

在 1200-2400 K 的温度和 0-25 GPa 的压力下，MgO 中阳离子空位和电荷平衡四价阳离子之间的结合自由能通过分子动力学模拟计算得出。从结合自由能的温度依赖性获得的结合熵被发现与压力无关并且具有 13(5) J/mol K (2σ) 的值。从结合能的压力依赖性获得的结合体积与温度无关，值为 1.7(3) cm3/mol (2σ)。这些结果与几种不同三价阳离子的实验估计非常一致，表明结合熵和体积并不强烈依赖于阳离子的身份或电荷。预计结合能会随着地幔绝热层的深度增加，