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

Abstract

Two stable high-pressure phases (C2/m-MnN₄ and P-MnN₄) and four metastable phases (P4/mmm-MnN₄, P-MnN₅, C2/m-MnN₆ and P-MnN₈) are proposed by using ab initio evolutionary simulations. Besides the reported quasi-diatomic molecule N₂, the armchair chain and S-like chain, the N₄ ring and N₂₂ ring are firstly reported in the P4/mmm-MnN₄ and P-MnN₅ 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 P-MnN₄ and P-MnN₅ can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P-MnN₄ (2000 K) and P-MnN₅ (3000 K). Interestingly, P4/mmm-MnN₄ with the N₄ 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.