A universal trend of structural, mechanical and electronic properties in transition metal (M = V, Nb, and Ta) borides: First-principle calculations

First-principle total energy calculations are employed to provide a fundamental understanding of the structural, mechanical, and electronic properties of transition metal (M = V, Nb, and Ta) borides with different boron concentrations (M2B, M3B2, MB, M5B6, M3B4, M2B3, and MB2). The results show that a universal trend in thermodynamic stability, elastic properties (bulk modulus and shear modulus) and electronic structure exists for V, Nb, and Ta borides. With the increase of boron concentrations, thermodynamic stability and bulk modulus of V, Nb, and Ta borides present an increasing trend, whereas shear modulus reaches the maximum at M2B3 and then decreases to MB2. The variation of electronic structure and Mulliken overlap population explain the origin of the general changes of these borides and their superior mechanical properties. Also, due to the relatively higher mechanical properties, M5B6, M3B4, and M2B3 are demonstrated to be possible candidates for superhard films.