难熔金属钽、钼、钨的高温高压状态方程及声速的第一性原理研究

难熔金属的高压熔化曲线在动-静高压实验之间存在巨大争议,而在发生冲击熔化之前是否存在固-固相变是目前的研究热点问题.本文以3种典型难熔金属钽、钼、钨为例,通过第一性原理晶格动力学方法,计算了钽、钼、钨的声子色散曲线.采用准谐近似的方法,获得了Hugoniot状态方程以及Hugoniot声速.对于钽和钨的声速计算表明,其基态体心立方结构在高压下一直保持其稳定性;而钼的晶格动力学计算表明其基态结构的稳定性在高压下消失,而钼的Hugoniot声速在175–275GPa区域内发生了拐折,这一结果证实了冲击波实验中对于钼的声速测量的结果:在210GPa压力附近声速发生间断.

Equation of state and acoustic velocities of Ta, Mo and W under high pressure and high temperature

The high pressure phase transition of transition metal before shock melting is an intriguing issue. We investigate the equation of state and acoustic velocities of bcc Ta, bcc Mo and bcc W under high pressure and high temperature by using density functional theory. The phonon dispersion curves of Ta, Mo and W agree well with the experiments. Under high pressure, partial phonon frequencies of Mo soften to imaginary frequencies, which indicate a structural instability. The Hugoniot equation of state and elastic constants of Ta, Mo and W are also obtained successfully. From the elastic constant, we calculated the Hugoniot acoustic velocities. The calculated results show the bcc Ta and bcc W is a stable phase under high pressure and high temperature. The break in acoustic velocities of Mo locates around 175-275 GPa, which indicates the phase transition.