Pressure-stabilized polymerization of nitrogen in manganese nitrides at ambient and high pressures

Abstract

A systematic high pressure study is performed on Mn–N compounds by using the particle swarm optimization methodology.

Abstract

Two stable high-pressure phases ( C2/ m-MnN ₄ and P1̄-MnN ₄) and four metastable phases ( P4/ mmm-MnN ₄, P1̄-MnN ₅, C2/ m-MnN ₆ and P1̄-MnN ₈) are proposed by using ab initio evolutionary simulations. Besides the reported quasi-diatomic molecule N ₂, the armchair chain and S-like chain, the N ₄ ring and N ₂₂ ring are firstly reported in the P4/ mmm-MnN ₄ and P1̄-MnN ₅ phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. Ab initio molecular dynamics simulations show that P1̄-MnN ₄ and P1̄-MnN ₅ can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P1̄-MnN ₄ (2000 K) and P1̄-MnN ₅ (3000 K). Interestingly, P4/ mmm-MnN ₄ with the N ₄ ring exhibits outstanding mechanical properties, including high incompressibility, high hardness, uniform strength in the 2-D direction and excellent ductility. Strong N–N covalent bond and weak Mn–N ionic bond interactions are observed in the predicted Mn–N compounds, and the charge transfer between the Mn and N atoms provides an important contribution to the stabilization of polymeric N-structures. All the proposed structures are metallic phases. Our results provide a deep understanding of the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of these new manganese polynitrides in future.