压力下CaN2结构稳定性和电子结构的第一性原理研究
A first-principles study on the structure stabilities and electronic structures of CaN2 under pressure

通过运用基于密度泛函理论的第一性原理计算方法结合广义梯度近似对压力下CaN2的结构稳定性和电子结构进行了理论研究。对结构稳定性的研究表明，ZnCl2型结构是CaN2在环境压力下最稳定的结构，而实验上观察到的CaC2-I型结构是CaN2高压下（>8.7 GPa）的稳定性结构。在50 GPa的压力范围内，CaN2将发生从ZnCl2型结构到ThC2型结构再到CaC2-I型结构的两次压致结构相变，其相变压力分别为0.81 GPa和8.77 GPa。而对电子结构的研究表明ZnCl2型、 ThC2型和CaC2-I型三种结构的CaN2都表现出了金属特征，三种结构CaN2当中Ca-N键的离子-共价性特征和N原子间的N=N双键特征得到了确认。
The structural stabilities and electronic properties of CaN2 under pressure have been investigated by using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation (GGA). The obtained results show that the ZnCl2-type structure is the ground-state of CaN2 at ambient pressure. The CaC2-I-type structure observed in experiment, however, becomes the thermodynamically preferred phase at high pressures (>8.77 GPa). Two pressure-induced phase transitions are found at 0.81 and 8.77 GPa, from the initial ZnCl2-type structure (Space group Pna21) through a ThC2-type structure (Space group C2/c) to a CaC2-I-type structure (Space group I4/mmm). Moreover, electronic structure calculations reveal that CaN2 in all three structures are metallic. The covalent-ionic nature of Ca-N bonding and the covalent N=N double bonding in all three CaN2 structures are confirmed.