High Pressure Study of Lithium Azide from Density-Functional Calculations

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-axis which is also consistent with the experiment. The computed elastic constants clearly shows that LiN$_3$ is a mechanically stable system and the calculated elastic constants follows the order C$_{33}$ $>$ C$_{11}$ $>$ C$_{22}$ implies that the LiN$_3$ lattice is stiffer along c-axis and relatively weaker along 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 semi conductor at high pressures. The optical properties such as refractive index, absorption spectra and photo conductivity 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 increase in pressure. The observed peaks in the absorption and photo conductivity spectra are found to shift towards the higher energy region as pressure increases which imply that in LiN$_3$ decomposition is favored under pressure with the action of light.