高压下金属Ba的结构稳定性以及热动力学的第一原理研究

摘要

采用第一原理方法计算了高压下金属Ba的三个高压相Ba-Ⅰ,Ba-Ⅱ和Ba-V的稳定性及热动力学性质.结果表明,Ba的三个高压相在0 K时在其压力范围内都是动力学和力学稳定的;但随压力增加,Ba-Ⅰ和Ba-Ⅱ的声子谱频率出现异常＂软化＂,而Ba-V则出现＂硬化＂.虽然Ba-Ⅱ和Ba-V同为六方密堆（hcp）结构,计算表明它们在高压下表现出了不同的弹性各向异性.计算同时发现Ba-Ⅱ在更高的压力下仍满足力学稳定条件,但声子谱有虚频存在,表明动力学失稳是Ba-Ⅱ在压力下向Ba-Ⅳ相转变的原因.计算和比较了同为六方密堆（hcp）结构的Ba-Ⅱ和Ba-V在高压下的声速、德拜温度、体模量、剪切模量等力学和热学性质,展现了金属Ba在压力下的稳定机制和热动力学性质.%The stabilities and the thermal dynamical properties of the three high-pressure phases of Ba（Ba-Ⅰ,Ba-Ⅱand Ba-Ⅴ） are investigated systemically by first principles method.Our results show that all the three phase meet the criteria of mechanical and dynamical stabilities at 0 K.With pressure increasing,the phonon frequencies in Ba-Ⅰand Ba-Ⅱphases become softened,whereas the Ba-V phase exhibits the hardening of phonon frequencies.Although both Ba-Ⅰand Ba-Ⅴphase are the same hcp structures,they show different elastic anisotropies under high pressure.We also find that Ba-I phase at higher pressure still meets the mechanical stability criterion,but does not meet the dynamical stability criterion.The absence of dynamical stability may be the reason for the transition from Ba-Ⅰphase to Ba-Ⅳ phase.We also calculate and make comparisons of sound velocity,Debye temperature,bulk modulus and shear modulus etc between Ba-I and Ba-Ⅴphases,showing that Ba possesses the thermal dynamical properties under high pressure.