High-Pressure Study of Lithium Azide from Density-Functional Calculations
High-Pressure Study of Lithium Azide from Density-Functional Calculations
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DOI：
10.1021/jp200907q
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发表时间：
2011-05-05
影响因子：
2.9
通讯作者：
Vaitheeswaran, G.
中科院分区：
文献类型：
--
作者：
Babu, K. Ramesh;Lingam, Ch. Bheema;Vaitheeswaran, G.
在交换和关联泛函的广义梯度近似下，利用平面波赝势研究了LiN 3在高压下的结构、电子、光学和振动性质.晶格参数的计算值与实验值符合得很好。计算的体积模量值为23.23 GPa，与实验值20.5 GPa吻合较好。我们的计算再现以及结构参数的高压行为的趋势。结果表明，LiN 3晶体的可压缩性是各向异性的，晶体学b轴比a轴和c轴更易压缩，这也与实验结果一致.弹性常数的预测，这仍然有待实验证实。计算的弹性常数清楚地表明，LiN 3是一个机械稳定的系统和计算的弹性常数遵循的顺序C-33 > C-11 > C-22，这意味着LiN 3晶格是刚性的沿着c轴和相对较弱的沿着b轴。在压力作用下，电子带隙值的大小减小，表明该系统在高压下有半导体化的趋势。在常压和高压下计算了晶体的折射率、吸收光谱和光电导沿着三个晶向。计算的折射率表明，该系统是光学各向异性和各向异性增加的压力。在吸收和光电导谱中观察到的峰被发现向更高的能量区域移动，随着压力的增加，这意味着在LiN 3分解是有利的压力下与光的作用。在环境条件下，以及在高压下的内部和晶格模式的LiN 3的振动频率进行计算，我们预测，晶格模式对压力的响应是相对较高的叠氮离子的内部模式相比。
The structural, electronic, optical, and vibrational properties of LiN3 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 LiN3 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 LiN3 is a mechanically stable system and the calculated elastic constants follow the order C-33 > C-11 > C-22, implying that the LiN3 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 LiN3 decomposition is favored under pressure with the action of light. The vibrational frequencies for the internal and lattice modes of LiN3 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.