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