Strain-invariant near-zero Poisson's ratio emerging in 2D (CuX) ₂ P ₈ Se ₃ (X = Br, I) hybrid structures

Zero Poisson's ratio (ZPR) materials with exceptional mechanical properties offer promising opportunities for applications in aerospace, sensing, and flexible electronics. Here, we theoretically predict two monolayer hybrid compounds, (CuBr)₂P₈Se₃ and (CuI)₂P₈Se₃, that exhibit unconventional Poisson's ratio behavior based on first-principles calculations. Both are indirect bandgap semiconductors with calculated bandgaps of 2.44 eV and 2.32 eV, respectively, which can be effectively tuned by external strain. These monolayers display remarkable mechanical flexibility and strong in-plane anisotropy. Notably, both maintain nearly constant Poisson's ratios (<0.1) within a specific range of uniaxial strain, demonstrating strain-invariant mechanical responses. Structural and bonding analyses reveal that the hybrid configuration of covalently bonded P₈Se₃ clusters and ionically bonded (CuX)₂ (X = Br, I) units underlies their near-zero in-plane Poisson's ratio over a wide strain range. These findings identify (CuBr)₂P₈Se₃ and (CuI)₂P₈Se₃ as rare 2D materials with strain-insensitive near ZPR, in sharp contrast to most known materials that exhibit strong strain dependence, highlighting their potential for next-generation flexible and mechanically resilient devices.