极端条件下过渡金属钼固相结构及声速的理论预测

The ultrahigh pressure properties of Molybdenum (Mo), a calibration standard, plays a very important role in the implosion dynamics. What is the most stable phase of Mo under ultrahigh pressure? Experimentally, the shock wave acoustic velocity measurements showed that there was a sharp break on the Hugoniot curve at high pressure, which indicated that a solid-solid phase transition occurred prior to melting. But the static compression experiments found that the bcc Mo was stable at high pressure. Theoretically, there are phase transitions under different pressures and temperatures. But the phase diagram remains inconclusive up to now. In this work, we will investigate the phase diagram and equation of state of Mo under high pressure and high temperature from calculations. First, the novel package CALYPSO will be used to predict the high pressure phase of Mo directly under high pressure. And then, the high pressure sound velocity of new high pressure phase of Mo will be calculated. It can help us to understand the scientific disputation on the high pressure phase transition of Mo and the high pressure melting of transition metal.

标准材料钼的超高压物性在内爆动力学模拟研究中起至关重要的作用。关于钼在高压下是否存在固-固相变，国内外在动、静高压实验及理论计算三者之间存在很大的争议，因此，本项目拟采用理论计算研究钼的高压相变问题。采用最新发展的基于粒子群优化算法的第一性原理方法直接预测钼的高压结构；基于预测的新相结构，研究高压声速。本项目的研究对于澄清“钼在高压下是否会发生固-固相变”这一科学争议有重要的指导意义，同时也能为今后解决过渡金属熔化问题提供有益参考。

在本项目中，我们利用基于粒子群优化算法的晶体结构预测程序CALYPSO结合VASP预测了钼在高压下的晶体结构，通过分析比较认为高压下最有可能的相结构是dhcp结构，其次为fcc结构；基于上一步预测的高压相结构，通过声子色散曲线计算，我们进行晶格动力学稳定性分析，dhcp和fcc 结构在高压下都处于动力学稳定状态，并且dhcp结构先于fcc结构稳定；进而计算了钼的多相物态方程及不同压力下的弹性常数，并分析得到了高压下的声速数据，能够有效弥补目前实验未能覆盖的压力区间的数据。在此基础上，我们对钼的熔化曲线做了一些研究，为下一步深入研究难熔过渡金属熔化问题提供有益参考。