High-Pressure Study of Lithium Azide from Density-Functional Calculations

Abstract

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.

Pressure-induced planar N6 rings in potassium azide

“…Alkali azides are one class of compound among metal azides. Pressure-induced phase transitions in alkali azides have been reported by both experimental67891011 and theoretical investigations1213141516. Previous study reported that sodium azide undergoes a set of phase transitions68, and the anions in NaN 3 transform to polymeric nitrogen net above 120 GPa6.…”

Layered polymeric nitrogen in RbN3 at high pressures

“…5 , the four phase transitions of RbN 3 are accompanied by insulator-metal-metal-metal transitions. At ambient conditions, the I 4/ mcm structure is an insulator characterized by a large energy gap of 4.3 eV that is similar to the atmospheric pressure phases of other alkali metal azides 7 10 12 . However, the P -1, P 6/ mmm and C 2/m structures exhibit clear metallic behaviors by evidence of cross of band structures and the finite electronic DOS at the Fermi level.…”

“…In the process of nitrogen structural transition, alkali metal atoms in azides act as electronic donors to change the connection between nitrogen atoms and electronic properties of compounds. A lot of experimental and theoretical work has been done to study the high-pressure behaviors of nitrogen in LiN 3 4 8 11 12 13 14 15 , NaN 3 3 5 16 , KN 3 7 9 14 17 18 19 20 21 , and CsN 3 10 22 . Therefore, a study of the high-pressure behavior of RbN 3 would provide more insights into the mechanism of pressure-induced structural evolution of anions.…”

Metal azides under pressure: An emerging class of high energy density materials

“…12 From theoretical perspective, recently we have studied the high pressure behaviour of LiN 3 using density functional theory without including the van der Waals (vdW) interactions. 13 The calculated equilibrium volumes are slightly overestimated compared with the experiment, which can be attributed to the lack of inclusion of vdW interactions in the calculations. The study also revealed that the monoclinic LiN 3 is stable up to the studied pressure range of 60 GPa, which is in good agreement with experiment.…”