外压下VN相变和力学性质的第一性原理研究

运用基于赝势平面波基组的密度泛函程序VASP并结合Quantum ESPRESSO，Phonopy软件包对压力下VN的结构、力学性质、声子色散关系进行了第一性原理的研究.分别对NaCl型（B1），CsCl型（B2），WC型（Bh）三种构型的VN进行了计算，三种结构的体积能量曲线、焓压关系和声子谱表明在常压下六角WC结构与立方结构相比更稳定.随着压力增加VN由Bh结构到B1结构的相变点发生在30GPa左右，而B1结构到B2结构的相变点可能发生在150GPa左右.常压下三种结构的VN是力学稳定的，其弹性常数和弹性模量都有随压强的增大而增加的趋势，三者都是脆性材料.B1结构和B2结构坐标基矢方向上的杨氏模量数值与体对角线方向上的差距较大，体现出明显的各向异性.随压力的增加B1结构各向异性程度增大而B2结构各向异性程度减小

First principles study on phase transformation and mechanical properties of VN under external pressure

The structure, elastic properties and phonon dispersion relation of VN under high pressure are studied by using density functional program VASP, Quantum ESPRESSO and Phonopy package. Three configurations of NaCl (B1), CsCl (B2) and WC (Bh) phases of VN are investigated, respectively. The volume energy curve, enthal-py-pressure relation and phonon spectrum show that the hexagonal WC structure is more stable compared with the cubic structures at atmospheric pressure. With the increase in pressure we find that the phase transition of VN from Bh structure to B1 structure may occur at about 30GPa and that from B1 structure to B2 structure at about 150GPa. The three kinds of VN are mechanical stable at ambient pressure, the elastic constants and elastic moduli increase with the increasing pressure. Three kinds of VN are brittle materials. The Young's modulus values on the B1 and B2 structure coordinates structural basis vector directions differ significantly from those on the body diagonal direction and show obvious anisotropy. The anisotropy of B1 structure increases, while that of B2 structure decreases with the increase in pressure.