Boron vacancy-driven thermodynamic stabilization and improved mechanical properties of AlB2-type tantalum diborides as revealed by first-principles calculations

Abstract

Thermodynamic stability as well as structural, electronic, and elastic properties of boron-deficient AlB2-type tantalum diborides, which is designated as TaB, due to the presence of vacancies at its boron sublattice are studied via first-principles calculations. The results reveal that TaB, where 0.167 0.25, is thermodynamically stable even at absolute zero. On the other hand, the shear and Young’s moduli as well as the hardness of stable TaB are predicted to be superior as compared to those of TaB2. The changes in the relative stability and also the elastic properties of TaB with respect to those of TaB2 can be explained by the competitive effect between the decrease in the number of electrons filling in the antibonding states of TaB2 and the increase in the number of broken bonds around the vacancies, both induced by the increase in the concentration of boron vacancies. A good agreement between our calculated lattice parameters, elastic moduli and hardness of TaB and the experimentally measured data of as-synthesized AlB2-type tantalum diborides with the claimed composition of TaB, available in the literature, suggests that, instead of being a line compound with a stoichiometric composition of TaB2, AlB2-type tantalum diboride is readily boron-deficient, and its stable composition in equilibrium may be ranging at least from TaB to TaB. Furthermore, the substitution of vacancies for boron atoms in TaB2 is responsible for destabilization of WB2-type tantalum diboride and orthorhombic Ta2B3, predicted in the previous theoretical studies to be thermodynamically stable in the Ta−B system, and it thus enables the interpretation of why the two compounds have never been realized in actual experiments.