Diverse ruthenium nitrides stabilized under pressure: a theoretical prediction

First-principles calculations were performed to understand the structural stability, synthesis routes, mechanical and electronic properties of diverse ruthenium nitrides. RuN with a new I-4m2 symmetry stabilized by pressure is found to be energetically preferred over the experimental NaCl-type and ZnS-type ones. The Pnnm-RuN(2) is found to be stable above 1.1 GPa, in agreement with the experimental results. Specifically, new stoichiometries like RuN(3) and RuN(4) are proposed firstly to be thermodynamically stable, and the dynamical and mechanical stabilities of the newly predicted structures have been verified by checking their phonon spectra and elastic constants. A phase transition from P4/mmm-RuN(4) to C2/c-RuN(4) is also uncovered at 23.0 GPa. Drawn from bonding and band structure analysis, P4/mmm-RuN(4) exhibits semi-metal-like behavior and becomes a semiconductor for the high-pressure C2/c-RuN(4) phase. Meanwhile the P2(1)/c-RuN(3) shows metallic feature. Highly directional covalent N-N and Ru-N bonds are formed and dominating in N-enriched Ru nitrides, making them promising hard materials.