高压下BeP2N4结构相变和电子结构的第一性原理计算

 基于密度泛函理论的第一性原理方法，计算了硅铍石型和尖晶石型结构BeP₂N₄的总能量随体积的变化关系。利用Brich-Murnaghan状态方程，通过能量和体积拟合，得到了2种结构的体变模量及其对压强的一阶导数。在压力作用下，BeP₂N₄的相变是从硅铍石型结构（空间群R-3，No.148）转变到尖晶石型结构（空间群Fd-3m，No.227），计算出的相变点与其它理论值符合得非常好。同时计算了BeP₂N₄的相对晶格常数a/a₀和相对体积V/V₀的压缩率，在低压下发现，尖晶石结构BeP₂N₄的压缩率接近金刚石，进一步计算了不同压力下的体弹模量B_H、剪切模量G_H、B_H/G_H和杨氏模量E。此外，对两种结构的BeP₂N₄的电子态密度和带隙随压强的变化关系进行了计算和分析。结果表明：在压力作用下，上价带顶向费米能级移动，并有一定的展宽。Be—N、P—N键缩短，电子转移增加，导致电荷发生重新分布。

First-Principles Investigations on Structural Transformation and Electronic Properties of BeP2N4 under High Pressure

Total energy of BeP₂N₄ as a function of unit cell volume was calculated for phenakite and spinel-type structures using the density function theory (DFT). According to the Brich-Murnaghan's equation of state, the bulk modulus B₀ and B′₀=dB/dp for these two structures were obtained. The calculated results are all in good agreement with other theoretical data available in the literature, which indicate that BeP₂N₄ will transform from phenakite to spinel-type with increasing pressure. The results are in good agreement with experimental and other theoretical results. The energy gap slightly increases with pressure in the phenakite and spinel-type phases. The compression ratio of a/a₀ and V/V₀ were calculated. We find the the compression ratio of a/a₀ and V/V₀ of spinel structure BeP₂N₄ is the same that of diamond at lower 5 GPa. The B_H, G_H, B_H/G_H and E were also calculated at high pressure. By analyzing the changes of the total electronic density of states (TDOS) and the band gap under pressure, it is found that the energy band width and charge transfer for the two structures increase with increasing pressure. Moreover, the Be—N, P—N bond length is shortened and the electric charges are redistributed.