Mechanical and optical properties of polymeric nitrogen achieved by compression: DFT study

Abstract

The recent successful characterization of black phosphorus alike N8 compound at 140 GPa has spurred interest in solid nitrogen and its potential industrial applications. In this work, the previously unexplored mechanical and optical properties of nitrogen at 140 GPa has been systematically studied using first-principles calculations and various physically motivated empirical approaches. Experimentally proven orthorhombic-N8 structure was correctly reproduced. Body-centered cubic-N4 phase of much lower formation enthalpy was also uncovered under similar pressure. Both the superdense, wide bandgap compounds have sufficient mechanical and dynamical stabilities. While their Vickers hardness of 40.29 and 39.45 GPa indicates a softer degree than diamond under similar pressure, analyses of their elastic parameters and linear optical dielectric function reveal strong covalent bonding, anomalously high elastic moduli, fracture toughness (5.86 and 6.26 MPa m1/2) that bear close comparison with those of diamond, large refractive index (> 2.2) over broad energy spectrum and high absorption coefficient (104-105 cm−1) in ultraviolet range. Moreover, cubic-N4 with a charge energy density of 79.93 J cm−3 could be potential material for energy storage devices.