Phase Stability and Physical Properties of Manganese Borides: A First-Principles Study

The thermodynamic and mechanical stabilities for the Mn–B system are investigated using the first-principles calculations method with density functional theory. The negative formation enthalpies of Mn 2 B–Al 2 Cu (Mn 2 B–Al 2 Cu represents Mn 2 B in the Al 2 Cu structure type, the same hereinafter), MnB–CrB, MnB–FeB, MnB 2 –ReB 2, MnB 2 –AlB 2, MnB 3 –TcP 3, and MnB 4 indicate that they are thermodynamically stable at zero pressure. It is found that MnB 2 –ReB 2 is more energetically favorable than synthetic MnB 2 –AlB 2, which indicates that experimental synthesized MnB 2 is a metastable phase. Among these studied compounds, monoclinic MnB 4 has the largest shear modulus, the largest Young’s modulus, and the smallest Poisson’s ratio. The results of density of states and Mulliken overlap population reveal the strong covalent bonding, which results in the high bulk and shear moduli as well as small Poisson’s ratio of MnB 2 –ReB 2 and MnB 4 . An analysis of the elastic constants, elastic moduli, formation enthalpy, electronic structure, and theoretical hardness shows that MnB 2 –ReB 2 and MnB 4 are potential superhard materials.