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 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

Phase stabilities and vibrational analysis of hydrogenated diamondized bilayer graphenes: A first principles investigation

The phase stabilities as well as some intrinsic properties of hydrogenated diamondized bilayer graphenes, 2-dimensional materials adopting the crystal structure of diamond and of lonsdaleite, are investigated using a first-principles approach. Our simulations demonstrate that hydrogenated diamondized bilayer graphenes are thermodynamically stable with respect to bilayer graphene and hydrogen molecule even at 0 GPa, and additionally they are found to withstand the physical change in structure up to at least 1000 K, ensuring their dynamical and thermal stabilities. The studied hydrogenated diamondized bilayer graphenes are predicted not only to behave as direct and wide band gap semiconductors, but also to have a remarkably high resistance to in-plane plastic deformation induced by indentation as implied by their high in-plane elastic constants comparable to those of diamond and of lonsdaleite. The mechanical stability of the materials is confirmed though the fulfilment of the Born stability criteria. Detailed analysis of phonon vibrational frequencies of hydrogenated diamondized bilayer graphenes reveals possible Raman active and IR active modes, which are found to be distinctly different from those of hydrogenated diamond-like amorphous carbon and defective graphene and thus could be used as a fingerprint for future experimental characterization of the materials. © 2019 Elsevier Lt