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

Abstract

Two stable high-pressure phases (<i>C</i>2/<i>m</i>-MnN₄ and <i>P</i>1̄-MnN₄) and four metastable phases (<i>P</i>4/<i>mmm</i>-MnN₄, <i>P</i>1̄-MnN₅, <i>C</i>2/<i>m</i>-MnN₆ and <i>P</i>1̄-MnN₈) are proposed by using <i>ab initio</i> 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 <i>P</i>4/<i>mmm</i>-MnN₄ and <i>P</i>1̄-MnN₅ phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. <i>Ab initio</i> molecular dynamics simulations show that <i>P</i>1̄-MnN₄ and <i>P</i>1̄-MnN₅ can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of <i>P</i>1̄-MnN₄ (2000 K) and <i>P</i>1̄-MnN₅ (3000 K). Interestingly, <i>P</i>4/<i>mmm</i>-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.