Unusual rigidity and ideal strength of CrB4 and MnB4

By means of first-principles calculations, we report superior rigidity, ideal tensile, and shear strength for orthorhombic CrB4 and MnB4. The analysis of microscopic deformation mechanism reveals that the unique rectangular boron units in CrB4 and MnB4 are responsible for the consolidation of the directionality of boron-boron covalent bonds and the exceptional rigidity and ideal strength. The unusual mechanical properties of the orthorhombic tetraborides highlight their potential applications as intrinsically superhard materials. The unique rectangular boron unit also implies a criterion for designing and synthesizing transition metal boride based-materials with ultimate hardness and strength.