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

Two stable high-pressure phases (C2/m-MnN₄ and P1̄-MnN₄) and four metastable phases (P4/mmm-MnN₄, P1̄-MnN₅, C2/m-MnN₆ and P1̄-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 P1̄-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 P1̄-MnN₄ and P1̄-MnN₅ can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P1̄-MnN₄ (2000 K) and P1̄-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.