Thermodynamic properties of solid and liquid silicates and oxides in the TPa range from ab initio calculations

通过洛夫数k2对太阳系外超级地球的内部结构及其可观察到的引力响应进行改进描述-这是研究单位的中心目标之一-需要详细了解高达1 TPa和10,000 K的压力和温度下的材料特性。在这里，我们提出对硅酸盐和氧化物固体和液体的结构、稳定性和热力学性质进行从头计算。对于液体，将进行从头算分子动力学模拟，目的是提供能量和压力，以适应熔体的自一致热力学模型——以有限应变和降低温度的亥姆霍兹能量的泰勒展开形式表述——我们之前已经成功地应用于液体铁。在500gpa附近，MgO经历了从B1到B2结构的相变，实验和模拟结果基本一致。实验表明，FeO在高温下也经历了同样的转变，但这种转变在目前的模拟中仍然难以捉摸。本文提出利用随机自洽谐波晶格动力学（SSCHA）方法来研究非调和声子对这些相在高温下稳定性的影响。根据结果，我们将进一步尝试用簇扩展方法沿着MgO-FeO固溶体探索这种转变。从第一个资助期开始，在超级地球内部条件下硅酸盐固体中声子色散的先前结果将根据剪切模量进一步探索，剪切模量为行星引力响应的Love数k2提供了重要约束。

An improved description of the internal structure of extrasolar super-Earths and their observable gravitational response through the Love number k2 -- which is one of the central goals of the Research Unit -- requires a detailed knowledge of material properties at pressures and temperatures up to 1 TPa and 10,000 K. Here we propose to perform ab initio computations on the structure, stability and thermodynamic properties of silicate and oxide solids and liquids. For the liquids, ab initio molecular dynamics simulations will be performed with the goal to provide energies and pressures to fit a self-consistent thermodynamic model for the melts -- formulated as a Taylor expansion of Helmholtz energy in terms of finite strain and a reduced temperature -- that we previously have successfully applied to liquid Fe. In the vicinity of 500 GPa, MgO undergoes a phase transition from the B1 to the B2 structure, in general agreement between experiments and simulations. Experiments indicate that FeO undergoes the same transition at high temperature, but this transition has remained elusive in simulations to this point. Here we propose to explore the influence of anharmonic phonon contributions on the stability of these phases at high temperature, using the stochastic self-consistent harmonic lattice dynamics (SSCHA) approach. Depending on the results, we will further attempt to explore this transition along the MgO-FeO solid solution with the cluster expansion approach.Previous results for the phonon dispersion in silicate solids at conditions of super-Earth interiors from the first funding period will be further explored in terms of the shear modulus, which provides an important constraint on the Love number k2 for a planet's gravitational response.