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

Two stable high-pressure phases (2/-MnN and 1̄-MnN) and four metastable phases (4/-MnN, 1̄-MnN, 2/-MnN and 1̄-MnN) are proposed by using evolutionary simulations. Besides the reported quasi-diatomic molecule N, the armchair chain and S-like chain, the N ring and N ring are firstly reported in the 4/-MnN and 1̄-MnN phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. molecular dynamics simulations show that 1̄-MnN and 1̄-MnN can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of 1̄-MnN (2000 K) and 1̄-MnN (3000 K). Interestingly, 4/-MnN with the N 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.