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

Thermodynamic stability as well as structural, electronic, and elastic properties of boron-deficient AlB2-type tantalum diborides, which is designated as α− TaB 2−x , due to the presence of vacancies at its boron sublattice are studied via first-principles calculations. The results reveal that α− TaB 2−x , where 0.167 ≲x≲ 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 α− TaB 2−x are predicted to be superior as compared to those of α− TaB2. The changes in the relative stability and also the elastic properties of α− TaB 2−x with respect to those of α− TaB2 can be explained by the competitive effect between the decrease in the number of electrons filling in the antibonding states of α− 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 α− TaB 2−x and the experimentally measured data of as-synthesized AlB2-type tantalum diborides with the claimed composition of TaB ∼2 , 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 ∼1.833 to TaB ∼1.75 . Furthermore, the substitution of vacancies for boron atoms in α− 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.