高温高压下MgSiO_3熔体结构的第一性原理分子动力学研究

First-Principles Molecular Dynamics Study of the Structure of MgSiO_3 Melt at High Temperatures and High Pressures

基于第一性原理分子动力学方法,计算了MgSiO3熔体在0～144GPa、2 000～6 000K的微观结构及其随压力、温度的变化特征。计算的近零压2 000K下O—Si、O—Mg和O—O对分布函数的第一峰值位置分别为0.163 5、0.197 0和0.269 5nm,与实验结果吻合很好。随着压力和温度的变化,MgSiO3熔体结构发生了显著变化,尤其是随着压力增加,结构变得更致密;当密度为4.59g/cm3时,原子间的平均键长随温度（小于5 000K）增加而减小,在常压和更高的压力下,原子间的平均键长随温度变化不明显。在133GPa、4 000K条件下,MgSiO3熔体的O—Si、O—Mg和O—O平均键长分别为0.161 0、0.183 5和0.230 0nm;从地表常压到核幔边界压力,平均Si—O配位数从4变到6,桥氧数目比例由31.3%增高到72.9%。MgSiO3熔体微观结构的认识对了解地幔内硅酸盐流体性质及其对地幔动力学的影响有重要意义。

The microstructures of MgSiO3 melt and their variation with temperature and pressure were investigated based on first-principles molecular dynamic simulations at high pressures（0-144GPa）and high temperatures（2 000-6 000K）.The calculated first peak positions of the pair correlation function of O—Si,O—Mg and O—O under the condition of 0GPa and 2 000 Kare 0.163 5,0.1 970 and 0.269 5nm,respectively,which are consistent with the previous experimental values.As the pressure and temperature change,the structure of MgSiO3 melt undergoes a significant change.Especially when the pressure increases,the structure becomes denser.When the temperature is below 5 000 K,the average bond lengths between two atoms decrease with the increasing temperature with density 4.59g/cm3.While under nomal or higher pressure,the average bond length change with the increasing temperature is not obvious.At 133 GPa and 4 000 K,the average bond lengths of O—Si,O—Mg and O—O are 0.161 0,0.183 5and 0.230 0nm,respectively;the average Si—O coordination number increases from 4to 6,and the number of bridging oxygen ratio increases from 31.3%to 72.9%,from atmospheric pressure to the core-mantle boundary.The knowledge of MgSiO3 melt microstructure is important to understand the mantle silicate fluid nature of mantle dynamics.