Defect chemistry of Cr-B binary and Cr-Al-B MAB phases: Effects of covalently bonded B networks

Abstract

Transition metal borides, which are three-dimensional (3D) layered materials containing covalently bonded B networks, have shown a number of excellent properties, such as radiation resistance and the ability to act as a diffusion barrier in integrated circuits. However, defect behavior, which controls many of the materials' properties, has remained unknown in these materials. Here, we investigate the effects of the B networks on the defect chemistry in both binary borides (CrB, ${\mathrm{Cr}}_{3}{\mathrm{B}}_{4}$, ${\mathrm{Cr}}_{2}{\mathrm{B}}_{3}$) and ternary MAB phases (${\mathrm{Cr}}_{2}\mathrm{Al}{\mathrm{B}}_{2}$, ${\mathrm{Cr}}_{3}\mathrm{Al}{\mathrm{B}}_{4}$, ${\mathrm{Cr}}_{4}\mathrm{Al}{\mathrm{B}}_{6}$) using first-principles calculations. We find that increasing the number of B rings in the structure leads to lower formation energies and higher concentrations of Frenkel pairs. The results can be explained by the fact that the strongest Cr-B bond is weakened when borides have more B rings, leading to a reduction in the formation energy of Cr and B vacancies. Also, the bonds associated with Cr atoms bonded within B rings are softer in structures containing more B rings, which allows Cr interstitials to form with a lower energy cost and contributes to an increase in the concentration of Cr interstitials.