The Strucutures and physical properties of ultrahigh-pressure silicate phases at high temperatures and high pressures in the lower mantle

na22o - cao - mgo - al_2o_3 - sio_2 （NCMAS）体系中的晶体和液体是地幔的重要组成部分。一个可转移的原子间势模型已被开发用于计算机模拟晶体和液体在这个重要的地球物理系统。系统的势能取成对加性库仑、范德华和排斥相互作用的和。为了保证NCMAS体系中不同组成相之间的可转移性，将Na、Ca、Mg、Al、Si和O离子的净电荷约束为2q(Na)=q(Ca)=q(Mg)=2/3q(Al)=1/2q(Si)=-q(O)。通过分子动力学（MD）模拟验证了电位模型的可靠性和适用性，结果表明：(1)NCMAS体系中29种不同晶体的观察结构和测量体积模量；(2)5种硅酸盐液体的观察温度-压力-体积方程参数：顽火辉石（MgSiO_3）、硅灰石（CaSiO_3）、透辉石（CaMgSi_2O_6）、钙长石（CaAl_2Si_2O_8）和钠长石（NaAlSi_3O_8）。利用MD技术成功地研究了(1)高温下镁橄榄石（Mg_2SiO_4）液态的状态方程参数，(2)高温高压下透辉石晶体与液态之间可能发生的密度反转，(3)下地幔普遍存在的超高压高温下MgSiO_3钙钛矿的熔融温度。

Cryatals and liquids in the system Na_2O-CaO-MgO-Al_2O_3-SiO_2 (NCMAS) are important constituents of the Earth's mantle. A transferable interatomic potential model has been developed for use in computer simulation of both crystals and liquids in this geophysically important system. The potential energy of the system is taken as the sum of pairwise additive Coulomb, van der Waals, and repulsive interactions. The net charges on the Na, Ca, Mg, Al, Si, and O ions are constrained to be 2q(Na)=q(Ca)=q(Mg)=2/3q(Al)=1/2q(Si)=-q(O) in order to preserve the requirement of transferability between phases with different composition in the NCMAS system. The reliability and applicability of the potential model has been tested using the molecular dynamics (MD) simulation, with the results that the simulation has succeeded in reproducing accurately (1) the observed structures and measured bulk moduli of a wide variety of 29 crystals in the NCMAS system, and (2) the observed temperature-pressure-volume equation-of-state parameters of the five silicate liquids with the composition, enstatite (MgSiO_3), wollastonite (CaSiO_3), diopside (CaMgSi_2O_6), anorthite (CaAl_2Si_2O_8) and albite (NaAlSi_3O_8). The MD technique is then successfully used to investigate (1) the equation-of state parameters of forsterite (Mg_2SiO_4) liquid at high temperatures, (2) the possible occurrence of the density inversion between diopside crystal and liquid at high temperatures and high pressures, and (3) the melting temperature of MgSiO_3 perovskite at ultrahigh pressures and high temoperatures prevailing in the lower mantle.