几种BC_2N超硬材料力学性能与电子结构的理论研究

Theoretical study of mechanical properties and electronic structures of several BC_2N superhard materials

本文用基于密度泛函理论(DFT)结合平面波方法的第一性原理研究了四种不同相的B-C-N的晶体结构、力学性能和电子结构.结果表明:c-BC2N,d-BC2N和t-BC2N的体弹模量大于400GPa,而cp-BC2N的体弹模量最小～380GPa.其原因在cp-BC2N相中仅有B—C键和C—N键,而不存在另外三种结构中都有的C—C和B—N键.拉伸时各个方向上都出现最大应力,则在剪切时至少有一个方向上不会出现最大应力,反之亦然.能带分析表明四种BC2N均具有直接跃迁型结构,Mulliken布居分析表明,C—N,C—B,B—N间存在电荷转移.

Based on the first-principles density functional theory with plane wave psuedopotential methods, the crystal structure, mechanical and electronic properties of four BC2N superhard material are theoretically studied. It is found that the bulk modulus of c-BC2N, d-BCzN and t-BC2N is greater than 400 GPa, whereas that of cp-BC2N is about 380 GPa. The mechanism is that there are only B--C and C--N bonds in cp-BC2N, but not C--C and B--N bonds which exist in other three crystals. If the tension in each direction appear on maximum stress, in the shear at least one direction will not appear on the maximum stress, and vice versa. An analysis of energy bands and state of electrons shows that the four are direct transition and semi-conductors. From Mulliken populations it can be seen that charge shifts between C and N, C and B, B and N.