CAJ | 학술논문

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NbSi2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NbSi2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NbSi2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NbSi2两种相结构的脆性和延展性,发现压力会导致六角相NbSi2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NbSi2两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。
채용기우입자군우화산법적결구예측정서CALYPSO,병결합제일성원리적VASP정서,재175 GPa발현NbSi2적기이립방고압상。재차결구중, Nb원자형성금강석결구,이Si원자칙형성정사면체양감재금강석결구중。성자보계산결과표명해결구시동역학은정적。전자결구분석표명,륙각상화립방상NbSi2균위금속,대금속성공헌교대적시Nb원자,이차Nb화Si원자지간존재명현적p-d잡화현상,전하경다지취집재Si사면체중。이용“응력응변”방법,계산료NbSi2적탄성상수,분석료기체적모량、전절모량、양씨모량화덕배온도등열동역학성질수압력적변화병진행료상세적토론。근거전절모량화체적모량적비치분석료NbSi2량충상결구적취성화연전성,발현압력회도치륙각상NbSi2적연전성증가,단대립방상결구적연전성영향교소；채용경험산법계산료NbSi2량충상결구경도변화정황,결합저일비치진행료상세적분석。탄성각향이성계산결과표명,수착압력증가,륙각결구적각향이성증강,이립방결구적각향이성감소。
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. NbSi2 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 AG and the universal anisotropic index AU (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.