氮化锗多形体的四方、单斜和正交畸变的理论研究

Theoretical Studies on Tetragonal,Monoclinic and Orthorhombic Distortions of Germanium Nitride Polymorphs

采用量子化学从头算方法,对Ge3N4的四方、单斜和正交结构同质异相体的微结构、态密度和声子谱进行了研究.形成焓为负值、弹性常数满足Born稳定性准则和声子谱无虚频等结果证实在0～20 GPa范围内3种相都能保持结构稳定.温度变化影响到晶胞体积,从而使体模量发生改变.3种Ge3N4都属于半导体,Ge原子和N原子之间存在明显的s-p杂化现象.当压强增大时诱发了离域电子,从而使体系的带隙减小.本文还采用准谐近似对Ge3N4的热力学性质进行了研究,结果表明,温度和压强对热膨胀系数、熵、热容、德拜温度和格林爱森参数产生了明显影响.m-Ge3N4和t-Ge3N4的热膨胀系数分别为o-Ge3N4的3倍和2倍.t-Ge3N4和o-Ge3N4的晶格谐振频率基本不受温度的影响.

Applying the ab initio pseudo-potential technique,we had predicted the lattice structures,density of states,phonon dispersion curves of the recently-discovered tetragonal,monoclinic and orthorhombic phases of Ge3N4. The negative formation enthalpy,the satisfactory of Born ＇s stability criteria and no imaginary frequency can be seen in the phonon dispersion curves proof that the three Ge3N4 polymorphs can retain their stabilities in the pressure range of 0 ― 20 GPa. The temperature affects the cell volume,thereby decreasing the bulk modulus. The band gaps show that Ge3N4 are semiconductors,while obvious s-p hybridizations can be seen in the density of states. The band gaps decrease with applied pressure,which is due mainly to the generation of non-local electrons. Then,the quasi-harmonic approximation is used to study the thermodynamic properties of Ge3N4. The results show that the thermal expansion coefficient,entropy,heat capacity,Debye temperature and Grüneisen parameter are significantly affected by both temperature and pressure. The thermal expansions of m-Ge3N4 and t-Ge3N4 are three and two times greater than that of o-Ge3N4,respectively. The lattice vibration frequency of o-Ge3N4 keeps unchanged at different temperatures. Our results are concordant with the experimental data and the previous results. Therefore, the present results indicate that the combination of ab initio calculations and quasi-harmonic approximation is an efficient method to simulate the high-temperature behaviors of different Ge3N4 polymorphs. Generally speaking,the results listed in this work are all predictions,which need to be verified by experiments in the near future.