Structural distortion and band gap opening of hard MnB{sub 4} in comparison with CrB{sub 4} and FeB{sub 4}

MnB{sub 4} was newly synthesized to crystallize in a monoclinic P2{sub 1}/c structure, different from previous experimental and theoretical reports. Here, based on first-principles calculations, we perform a comparative study of geometric and energetic features, mechanical behaviors, electronic property and chemical bonding of the experimentally identified monoclinic MnB{sub 4}, as well as orthorhombic CrB{sub 4} and FeB{sub 4}. The results demonstrate that the presence of distorted rhomboidal-B{sub 4} units and one-dimensional Mn chains in the monoclinic MnB{sub 4} breaks the structural symmetry and lowers the total energy in comparison to the orthorhombic phase. The opening of band gap in MnB{sub 4} is induced by Peierls-paired Mn atoms, differing from the metallic behaviors of recently studied tetraborides. Specifically, the preservations of covalent bonding in distorted boron-rhomboids in MnB{sub 4} explain the relatively higher incompressibility and hardness. - Graphical abstract: P2{sub 1}/c-type structure for MnB{sub 4} characterizes rhomboid-B{sub 4} units and alternately short and long Mn–Mn chains. - Highlights: • The novel P2{sub 1}/c-type MnB{sub 4} compound is studied by first-principles calculations. • P2{sub 1}/c-type MnB{sub 4} has lower total energy and relatively higher stability. • An energy gap opening is found for P2{sub 1}/c-type MnB{sub 4} induced by Mn atom pairs.more » • P2{sub 1}/c-type MnB{sub 4} exhibits excellent mechanical properties. • The mechanical properties of TcB{sub 4} and ReB{sub 4} in P2{sub 1}/c structure are also studied.« less