The quest for a bidirectional auxetic, elastic, and enhanced fracture toughness material: Revisiting the mechanical properties of the BeH 2 monolayers.

Herein we show a density functional theory-based study performed on two recently predicted polymorphs of the BeH 2 monolayer, α-BeH 2 and β-BeH 2 . The α-BeH 2 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 2 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 2 a bidirectionally auxetic material. While phase transitions in β-BeH 2 are triggered at very short strains, α-BeH 2 displays excellent elasticity against tension, superior to that of most currently known 2D materials.