High-pressure study on novel structure, mechanical properties and high energy density of RuN 4

Novel structures with high energy density could be created with the addition of transition-metal elements into polymeric nitrogen. Based on ab initio evolutionary algorithm for crystal structure prediction and first-principles calculations, we have conducted a comprehensive search on the crystal structure of RuN4 under the pressure range of 0−300 GPa and uncovered its pressure-induced phase transition. The consequences indicate that RuN4 decomposes into Ru and N2 under ambient pressure. Subsequently, RuN4 can stabilise with the triclinic P1¯-RuN4 phase at 37 GPa, which transforms into the tetragonal P4/mbm-RuN4 structure at 62 GPa. Interestingly, the P1¯-RuN4 phase contains RuN6 distorted octahedron and extended chain (N∞), while the P4/mbm-RuN4 structure is composed of RuN8 rectangles and N2 units. Mechanical property analyses have revealed that the P1¯-RuN4 and P4/mbm-RuN4 crystals possess high hardness, excellent compression resistance and strong rigidity. Intriguingly, the Vickers hardness value for the P4/mbm RuN4 phase can reach 33.50 GPa. In addition, the electronic structure calculations demonstrate both phases are metallic. Moreover, strong Ru−N and N−N covalent bonds exist in both structures. At ambient conditions, the P4/mbm RuN4 phase has an energy density of 2.81 kJ/g and a volumetric energy density of 18.70 kJ/cm3, which is considered as an excellent high-energy density material. Hence, the P4/mbm RuN4 crystal is a newly multifunctional material that possesses both high hardness and high-energy density.