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

MnB4 was newly synthesized to crystallize in a monoclinic P2(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 MnB4, as well as orthorhombic CrB4 and FeB4. The results demonstrate that the presence of distorted rhomboidal-B-4 units and one-dimensional Mn chains in the monoclinic MnB4 breaks the structural symmetry and lowers the total energy in comparison to the orthorhombic phase. The opening of band gap in MnB4 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 MnB4 explain the relatively higher incompressibility and hardness. (C) 2014 Elsevier Inc. All rights reserved.