The phase diagrams of beryllium and magnesium oxide at megabar pressures
The phase diagrams of beryllium and magnesium oxide at megabar pressures
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DOI：
10.1088/1361-648x/ac4b2a
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发表时间：
2022-01
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影响因子：
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通讯作者：
Jizhou Wu;Felipe González-Cataldo;F. Soubiran;B. Militzer
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作者：
Jizhou Wu;Felipe González-Cataldo;F. Soubiran;B. Militzer
我们在兆巴压力下对铍 (Be) 和氧化镁 (MgO) 进行从头开始模拟，并比较它们的结构和热力学性质。我们使用热力学积分技术对最近得出的 Be (Wu et al 2021 Phys. Rev. B 104 014103) 和 MgO (Soubiran and Militzer 2020 Phys. Rev. Lett. 125 175701) 的两个相图进行了详细比较，因为它们在形状方面表现出惊人的相似性。我们通过计算高压下的德拜温度来探讨林德曼准则是否可以解释这些材料的熔化温度。根据我们的自由能计算，我们发现这两种材料的熔线可以很好地用 Simon-Glazel 拟合 T m(P) = T 0(1 + P/a)1/c 表示，其中 Be 的 T 0 = 1564 K、a = 15.8037 GPa 和 c = 2.4154，而 T 0 = 3010 K、a = 10.5797 GPa 和 c = B1中的MgO为2.8683。对于 B2 相，我们使用值 a = 26.1163 GPa 和 c = 2.2426。两种材料在两个固相之间的边界上都表现出负克拉佩龙斜率，该斜率受到非谐效应的强烈影响，这也会影响固-固-液三相点的位置。我们发现准调和近似低估了低压相的稳定范围，即 Be 的 hcp 和 MgO 的 B1。我们还计算了这些材料每个相在低压和高压下的声子色散关系，并探索了声子态密度如何随温度变化。最后，除了两种材料的主休格尼奥曲线之外，我们还推导了二次冲击休格尼奥曲线，并研究了它们在固体和液体分支之间的压力偏移。
We perform ab initio simulations of beryllium (Be) and magnesium oxide (MgO) at megabar pressures and compare their structural and thermodynamic properties. We make a detailed comparison of our two recently derived phase diagrams of Be (Wu et al 2021 Phys. Rev. B 104 014103) and MgO (Soubiran and Militzer 2020 Phys. Rev. Lett. 125 175701) using the thermodynamic integration technique, as they exhibit striking similarities regarding their shape. We explore whether the Lindemann criterion can explain the melting temperatures of these materials through the calculation of the Debye temperature at high pressure. From our free energy calculations, we find that the melting line of both materials is well represented by the Simon–Glazel fit T m(P) = T 0(1 + P/a)1/c , where T 0 = 1564 K, a = 15.8037 GPa and c = 2.4154 for Be, while T 0 = 3010 K, a = 10.5797 GPa and c = 2.8683 for the MgO in the B1. For the B2 phase, we use the values a = 26.1163 GPa and c = 2.2426. Both materials exhibit negative Clapeyron slopes on the boundaries between the two solid phases that are strongly affected by anharmonic effects, which also influence the location of the solid–solid–liquid triple point. We find that the quasi-harmonic approximation underestimates the stability range of the low-pressure phases, namely hcp for Be and B1 for MgO. We also compute the phonon dispersion relations at low and high pressure for each of the phases of these materials, and also explore how the phonon density of states is modified by temperature. Finally, we derive secondary shock Hugoniot curves in addition to the principal Hugoniot curve for both materials, and study their offsets in pressure between solid and liquid branches.