Mechanical properties and hardness of boron pnicogens BX (X = N, P, As)

Abstract

Cubic boron compounds (c-BN, c-BP, and c-BAs) are emerging semiconductor materials with extraordinary chemical and physical properties, e.g., record-high thermal conductivity. Because of these, they have attracted increased interest for applications in heat management and electronics. However, many fundamental properties especially for c-BP and c-BAs remain poorly understood. Herein, we report a systematic first-principles study of the important physical properties of boron compounds, including elastic constants, mechanical properties, and deformation behavior under tensile and shearing (pure shear and Vickers indentation shear) loads. The stress-strain relations show isotropic elastic behavior at a small strain and strong anisotropic responses with varied peak stresses along different tensile direction and shear system at a larger strain. In particular, we observe a large disparity between the tensile and shear strengths for c-BP and c-BAs due to shearing load-induced metallization and phonon softening. We examine the deformation process in terms of bond-breaking to understand the microscopic origin and impact on the strength of the materials. We show that both c-BP and c-BAs are not superhard materials but they offer a balance between hardness, synthesis, and sustainability that has been an issue for both c-BN and diamond for applications.