NbSi_2奇异高压相及其热力学性质的第一性原理研究  

Pressure-induced structural transition and thermodynamic properties of NbSi_2 from first-principles calculations

作　　者：濮春英[1] 王丽[1] 吕林霞[1] 于荣梅[1] 何朝政[1] 卢志文[1] 周大伟[1] 

机构地区：[1]南阳师范学院物理与电子工程学院,南阳473061

出　　处：《物理学报》2015年第8期284-290,共7页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:11247222;51374132);国家自然科学基金-河南人才培养联合基金(批准号:U1304612;U1404608;U1404216);南阳师范学院科学基金(批准号:ZX2013017)资助的课题~~

摘　　要：采用基于粒子群优化算法的结构预测程序CALYPSO,并结合第一性原理的VASP程序,在175 GPa发现Nb Si2的奇异立方高压相.在此结构中,Nb原子形成金刚石结构,而Si原子则形成正四面体镶嵌在金刚石结构中.声子谱计算结果表明该结构是动力学稳定的.电子结构分析表明,六角相和立方相Nb Si2均为金属,对金属性贡献较大的是Nb原子,而且Nb和Si原子之间存在明显的p-d杂化现象,电荷更多地聚集在Si四面体中.利用"应力应变"方法,计算了Nb Si2的弹性常数,分析了其体积模量、剪切模量、杨氏模量和德拜温度等热动力学性质随压力的变化并进行了详细的讨论.根据剪切模量和体积模量的比值分析了Nb Si2两种相结构的脆性和延展性,发现压力会导致六角相Nb Si2的延展性增加,但对立方相结构的延展性影响较小;采用经验算法计算了Nb Si2两种相结构硬度变化情况,结合这一比值进行了详细的分析.弹性各向异性计算结果表明,随着压力增加,六角结构的各向异性增强,而立方结构的各向异性减小.Applying the particle swarm optimization algorithm to the crystal structure prediction, we first predict a novel high pressure phase of NbSi2 with Si tetrahedron embedded diamond structure of Nb. NbSi~ alloy undergoes a first-order phase transition from hexagonal phase to cubic phase at about 175 GPa with a volume collapse of 4.1%, indicating the first-order transition. New predicted NbSi2 phase is dynamically stable in the absence of any imaginary phonon frequency in the whole Brillouin zone of phonon spectrum. The calculations of total and partial density of states indicate that the NbSi2 is in hexagonal phase at 0 GPa and it is in cubic structure at 180 GPa, both of which exhibit metal behaviors, which is dominated by Nb atom. There exists obviously the p-d hybridization between Nb and Si, and more charges accumulate in Si tetrahedron. Based on the ＂stress-strain＂ method, elastic constants, bulk modulus, shear modulus, Young＇s modulus, and Debye temperature of NbSi2 in two phases under pressure are systematically investigated using first principles calculations combined with the quasi-harmonic Debye model. To evaluate the ductile and brittle characteristics of NbSi2 alloy, pressure dependence of G/B ratio is investigated. Furthermore, the values of hardness and percent anisotropy AB and Ac and the universal anisotropic index Av （inset） for NbSi2 alloy in hexagonal and cubic structures are also calculated. Our results show that external pressure has different effects on the values of ductility and hardness and anisotropy of the two phases. External pressure can improve the ductility of hexagonal phase, while it has a negligible effect on that of cubic phase. The hardness values of two phases of NbSi2 are analyzed in detail by using the G/B ratio. As pressure increases, the elastic anisotropy of hexagonal phase increases rapidly, while that of cubic phase remains unchanged.

关 键 词：第一性原理 结构相变 热力学性质 

分 类 号：O521.23[理学—高压高温物理]