First principles high pressure studies on group-14 element pernitrides

Abstract

The search of ultra hard materials is inevitable in high pressure device applications. Nitrides of group-14 elements have been foreseen as potential candidates in replacing existing hard materials. In a recent experiment, pyrite structures of SiN2, GeN2 and SnN2 have been synthesized at high pressure and are shown to have high bulk modulus. Though their existence and bulk modulus are known, limited studies have been devoted to SiN2, GeN2 and SnN2. In the present work, we have performed the first principles calculations to investigate structural, electronic, mechanical and vibrational properties at ambient as well as high pressure condition. The physical properties of SnN2 and high pressure lattice dynamical properties of SiN2 and GeN2 are explored for the first time. SiN2 has higher bulk modulus among all MN2 (where M = Si, Sn and Ge), and increases further with increase in pressure. The increase in elastic moduli of MN2 have been related to the shortening of M-N bond length at high pressure. Electronic properties of these pyrites suggest that the bandgap increases with pressure. We further characterize these MN2 at high pressure using theoretically calculated Raman spectra. Frequency of N–N stretching Ag and Tg modes confirms the single bond character of nitrogen dimer in all studied MN2 compounds.