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 AlB _2 -type tantalum diborides, which is designated as $\alpha-$ TaB $_{2-x}$ , due to the presence of vacancies at its boron sublattice are studied via first-principles calculations. The results reveal that $\alpha-$ TaB $_{2-x}$ , where 0.167 $\lesssim\,x\,\lesssim$ 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 $\alpha-$ TaB $_{2-x}$ are predicted to be superior as compared to those of $\alpha-$ TaB _2 . The changes in the relative stability and also the elastic properties of $\alpha-$ TaB $_{2-x}$ with respect to those of $\alpha-$ TaB _2 can be explained by the competitive effect between the decrease in the number of electrons filling in the antibonding states of $\alpha-$ TaB _2 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 $\alpha-$ TaB $_{2-x}$ and the experimentally measured data of as-synthesized AlB _2 -type tantalum diborides with the claimed composition of TaB $_{\thicksim2}$ , available in the literature, suggests that, instead of being a line compound with a stoichiometric composition of TaB _2 , AlB _2 -type tantalum diboride is readily boron-deficient, and its stable composition in equilibrium may be ranging at least from TaB $_{\thicksim1.833}$ to TaB $_{\thicksim1.75}$ . Furthermore, the substitution of vacancies for boron atoms in $\alpha-$ TaB _2 is responsible for destabilization of WB _2 -type tantalum diboride and orthorhombic Ta _2 B _3 , 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.

Keywords

• transition-metal diborides
• first-principles cluster expansion
• thermodynamic stability
• mechanical properties
• boron-deficient tantalum diboride