Structural distortion and band gap opening of hard MnB4 in comparison with CrB4 and FeB4

Abstract MnB 4 was newly synthesized to crystallize in a monoclinic P 2 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 4 , as well as orthorhombic CrB 4 and FeB 4 . The results demonstrate that the presence of distorted rhomboidal-B 4 units and one-dimensional Mn chains in the monoclinic MnB 4 breaks the structural symmetry and lowers the total energy in comparison to the orthorhombic phase. The opening of band gap in MnB 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 4 explain the relatively higher incompressibility and hardness.