Strain-independent negative Poisson’s ratio in layered transition metal borides

Two-dimensional transition metal borides (MBenes), with orthorhombic crystals and wrinkled B-TM-B bonding networks, emerge as promising candidates for negative Poisson’s ratio (NPR) materials. However, the electronic-structural synergy governing NPR in these metallic systems remains elusive. Here, using density functional theory calculations, we uncover an intrinsic strain-independent NPR in Ti₂B₂ MBene (ν = −0.10 at θ = 45◦), maintaining across 0–30% strain—surpassing black phosphorus (ν = −0.027) and graphene (straindependent NPR?> 18%). A mechanistic descriptor is established, quantifying NPR dependence on transition metal’s electron affinity, electronegativity, and valence electron count. The NPR originates from synergistic wrinkle B-Ti-B bond-angle expansion and strong p-d orbital hybridization. Remarkably, Cu doping further enhances the NPR of Ti2B2 to −0.25 via size effect and electronic modulation, enabling its potential applications for mechanically adaptive electronics and cyclic deformation-resistant Li-air battery electrodes.