High-pressure study on novel structure, mechanical properties and high energy density of RuN₄

Abstract

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 P$\overline{1}$-RuN₄ phase at 37 GPa, which transforms into the tetragonal P4/mbm-RuN₄ structure at 62 GPa. Interestingly, the P$\overline{1}$-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 P$\overline{1}$-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.

The P4/mbm-RuN₄ consists of RuN₈ rectangles and N₂ polymers. In the RuN₈ rectangular unit, the ruthenium atoms occupy the centre of the tetragonal lattice and the upper and lower lateral prisms, and each ruthenium atom is surrounded by eight equally spaced nitrogen atoms.

GRAPHICAL ABSTRACT

KEYWORDS:

• Polymeric nitrogen
• first principles calculations
• high pressure phase transition
• mechanical properties
• high-energy density

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This project was supported by the National Natural Science Foundation of China [grand number: 11804031, 11904297], the Fundamental Research Funds for the Central Universities [grant number: SWU-KT22049], Chongqing Outstanding Young Talents [grant number: 202005007] and the Innovation and Entrepreneurship Program for College Students of Yangtze University [grant number: Yz2020313].