第一性原理研究Li2NH的晶格动力学和热力学性质

First-principles study of lattice dynamical and thermodynamic properties of Li2NH

采用第一性原理的赝势平面波方法系统地研究了Li2NH的电子结构、晶格动力学和热力学性质.计算得到的晶格常数与先前的理论和实验结果符合得很好.运用线性响应理论计算了整个布里渊区高对称方向上的声子色散曲线和相应的声子态密度,发现Li2NH(Pnma)声子色散曲线没有虚频,动力学性能相对最稳定,计算结果和先前实验及理论数据符合得很好.最后,利用得到的声子态密度进一步预测了Li2NH的热力学性质,包括晶格振动对Helmholtz自由能、内能、熵和热容的贡献,计算结果在一定程度上可为Li-N-H储氢体系的应用提供理论指导.

One of the key issues for scale applications of hydrogen energy is the availability of safe,efficient and ecnomicical hydrogen storage technologies.In the past few years,light metal hydrides have attracted considerable attention due to their high hydrogen capacity.With a hydrogen capacity up to^6.5 wt%,Li2NH is regarded as one of the most promising hydrogen storage materials.Although the hydrogen physical and thermodynamic properties of Li2NH have been studied,the electronic structure,phonon vibration mode and thermodynamic properties of Li2NH have not yet been resolved.In this paper,by using the first principles based on the density functional theory(DFT),we investigate the electronic structure,lattice dynamical and thermodynamic properties of Li2NH in detail.Firstly,the structure of Li2NH is optimized and the lattice parameters and total energy of the crystals are calculated.As shown by the calculation results,the lattice parameters are in good agreement with previous theoretical and experimental results.Our lowest-energy structure of Li2NH has orthorhombic Pnma symmetry at T=0 K for all of the proposed structures.Secondly,the electronic band-structure studies reveal that Li2NH has a small band gap of about 2.0 eV.The analysis of total and partial density of states of Li2NH show that the bonding between the N and H has a covalent character.Thirdly,the lattice dynamical properties of Li2NH are investgated at the corresponding equilibrium states.These results show that only the phonon dispersion curves of Li2NH(Pnma)without negative frequencies are calculated along the high-symmetry points.The optical modes of phonon frequencies at G point are assigned as Raman and Infrared-active modes.Based on the calculated phonon density of states,the thermodynamic properties are computed,such as the Helmholtz free energy,internal energy,entropy and the constant-volume specific heat versus temperature.The calculation results may explore the applications in areas of hydrogen storage for Li-N-H,which is of great importance forusing hydrogen in the future.