Computational study of structural, electronic and optical properties of crystalline NH$_4$N$_3$

A systematic computational study on the structural, electronic, bonding, and optical properties of orthorhombic ammonium azide (NH$_4$N$_3$) has been performed using planewave pseudopotential (PW-PP) method based on density functional theory (DFT). Semiempirical dispersion correction schemes have been used to account for non-bonded interactions in molecular crystalline NH$_4$N$_3$. The ground state lattice parameters and fractional co-ordinates obtained using the dispersion correction schemes are in excellent agreement with experimental results. We calculated the single crystal elastic constants of NH$_4$N$_3$ and its sensitivity is interpreted through the observed ordering of the elastic constants (C$_{33}$ $>$ C$_{11}$ $>$ C$_{22}$). The electronic structure and optical properties were calculated using full potential linearized augmented plane wave (FP-LAPW) approach with recently developed functional of Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. The TB-mBJ electronic structure shows that NH$_4$N$_3$ is a direct band gap insulator with a band gap of 5.08 eV, while the calculated band gap with standard generalized gradient approximation is found to be 4.10 eV. The optical anisotropy is analyzed through the calculated optical constants namely dielectric function and refractive index along three different crystallographic axes. The absorption spectra reveal that NH$_4$N$_3$ is sensitive to ultraviolet (UV) light. Further, we also analyzed the detonation characteristics of the NH$_4$N$_3$ using the reported heat of formation and calculated density. NH$_4$N$_3$ is found to have a detonation velocity of 6.45 km/s and a detonation pressure about 15.16 GPa computed by Kamlet-Jacobs empirical equations.