Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad, Andhra Pradesh, Hyderabad, India.
J Phys Chem A. 2011 May 5;115(17):4521-9. doi: 10.1021/jp200907q. Epub 2011 Apr 12.
The structural, electronic, optical, and vibrational properties of LiN(3) under high pressure have been studied using plane wave pseudopotentials within the generalized gradient approximation for the exchange and correlation functional. The calculated lattice parameters agree quite well with experiments. The calculated bulk modulus value is found to be 23.23 GPa, which is in good agreement with the experimental value of 20.5 GPa. Our calculations reproduce well the trends in high-pressure behavior of the structural parameters. The present results show that the compressibility of LiN(3) crystal is anisotropic and the crystallographic b-axis is more compressible when compared to a- and c-axes, which is also consistent with experiment. Elastic constants are predicted, which still awaits experimental confirmation. The computed elastic constants clearly show that LiN(3) is a mechanically stable system and the calculated elastic constants follow the order C(33) > C(11) > C(22), implying that the LiN(3) lattice is stiffer along the c-axis and relatively weaker along the b-axis. Under the application of pressure the magnitude of the electronic band gap value decreases, indicating that the system has the tendency to become semiconductor at high pressures. The optical properties such as refractive index, absorption spectra, and photoconductivity along the three crystallographic directions have been calculated at ambient as well as at high pressures. The calculated refractive index shows that the system is optically anisotropic and the anisotropy increases with an increase in pressure. The observed peaks in the absorption and photoconductivity spectra are found to shift toward the higher energy region as pressure increases, which implies that in LiN(3) decomposition is favored under pressure with the action of light. The vibrational frequencies for the internal and lattice modes of LiN(3) at ambient conditions as well as at high pressures are calculated from which we predict that the response of the lattice modes toward pressure is relatively high when compared to the internal modes of the azide ion.
采用基于广义梯度近似的平面波赝势方法，研究了高压下 LiN(3) 的结构、电子、光学和振动性质。计算的晶格参数与实验结果相当吻合。计算得到的体弹性模量值为 23.23 GPa，与实验值 20.5 GPa 吻合较好。我们的计算很好地再现了结构参数在高压下的变化趋势。结果表明，LiN(3)晶体的压缩性是各向异性的，与 a-和 c-轴相比，晶轴 b 更具可压缩性，这与实验结果一致。预测了弹性常数，有待实验证实。计算得到的弹性常数清楚地表明，LiN(3)是一个力学稳定的体系，且计算得到的弹性常数满足 C(33) > C(11) > C(22)的顺序，表明 LiN(3)晶格沿 c 轴更硬，沿 b 轴相对较弱。在压力作用下，电子带隙值减小，表明该体系在高压下有半导体化的趋势。计算了沿三个晶轴的折射率、吸收光谱和光导率等光学性质，同时也计算了常压和高压下的光学性质。计算得到的折射率表明该体系具有各向异性，且各向异性随压力的增加而增加。吸收光谱和光导率谱中观察到的峰随着压力的增加向高能区移动，这意味着在 LiN(3)中，光的作用下，在高压下有利于分解。在常压和高压下计算了 LiN(3)内部和晶格模式的振动频率，据此我们预测，与叠氮离子的内部模式相比，晶格模式对压力的响应相对较高。
