Mechanical and electronic properties of transition metal hexa-nitrides in hexagonal structure from density functional theory calculations

Abstract

We investigated structural, vibrational, mechanical, electronic, and magnetic properties of all 29 nitrogen-rich transition metal nitrides (TMNs) with a stoichiometric 1:6 (M:N) ratio in the hexagonal phase (space group: R3¯m with Wyckoff positions M (3a) and N (18 h)) by utilizing density functional theory-based calculations. Only 13 of these compounds are found to be both mechanically and vibrationally stable. Among them, FeN6 is predicted to be the hardest compound with a Vickers hardness of 22 GPa. Compounds from group 8 elements which have a valence electron concentration of 38 (FeN6, RuN6, and OsN6) are predicted to be the hardest and they also possess the highest densities. In general, hardness increases with decreasing M−M or M−N bond lengths. The calculation of local density of states and electronic band structures further suggests that all 13 stable MN6 compounds have metallic bonding character. Three compounds from group 4 elements are semiconductors, however, they are only mechanically stable. The bonding analysis reveals that hardness is primarily attributed to M−N and M−M interactions. Eight of the investigated MN6 compounds have a magnetic moment.