The quest for a bidirectional auxetic, elastic, and enhanced fracture toughness material: Revisiting the mechanical properties of the BeH monolayers.

Herein we show a density functional theory-based study performed on two recently predicted polymorphs of the BeH monolayer, α-BeH and β-BeH . The α-BeH phase possesses an in-plane negative Poisson's ratio (NPR), introducing it into the unique group of auxetic materials. Our assessment delves into the linear-elastic and finite-strain regimes to understand both polymorphs' structural and mechanical responses to deformation. We find that the in-plane NPR is shown to be only parallel to the bonds in α-BeH and remains along the uniaxial tensile path. Concomitantly, an out-of-plane transition toward auxetic is also revealed in regions exhibiting conventional Poisson's ratios, making α-BeH a bidirectionally auxetic material. While phase transitions in β-BeH are triggered at very short strains, α-BeH displays excellent elasticity against tension, superior to that of most currently known 2D materials.