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 Mn2B-Al2Cu (Mn2B-Al2Cu represents Mn2B in the Al2Cu structure type, the same hereinafter), MnB-CrB, MnB-FeB, MnB2-ReB2, MnB2-AlB2, MnB3-TcP3, and MnB4 indicate that they are thermodynamically stable at zero pressure. It is found that MnB2-ReB2 is more energetically favorable than synthetic MnB2-AlB2, which indicates that experimental synthesized MnB2 is a metastable phase. Among these studied compounds, monoclinic MnB4 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 MnB2-ReB2 and MnB4. An analysis of the elastic constants, elastic moduli, formation enthalpy, electronic structure, and theoretical hardness shows that MnB2-ReB2 and MnB4 are potential superhard materials.