Peculiar-Bonding Quasi-Planar Beryllium Nitride Monolayer with Reversible Crystalline–Amorphous Phase Transition, Auxeticity, and Halide Ion Conduction Ability

Searching for new 2D materials with novel structure, bonding, and properties is attractive and challenging over time. Through comprehensive global structure searching, the most thermodynamically stable configuration of the beryllium nitride monolayer (α-2D-Be 3 N 2 ) is identified. α-2D-Be 3 N 2 monolayer has peculiar Be bonding behavior with multicenters-like and ionic bonding characters, including the strong Be–N interactions “few residual electrons gathered infinite centers bond” with high coordinated numbers of N and Be in its semiplanar structure. Structural analyses confirm the monolayer’s dynamic stability and outstanding thermal stability with a reversible crystalline–amorphous phase transition. Electronic structure calculations reveal an indirect bandgap of 5.09 eV, accompanied by intense deep-UV optical absorption. The material also exhibits anomalous mechanical properties with purely in-plane negative Poisson’s ratios from –0.080 to −0.193, and excellent halide ion conduction ability with low barrier energy (only 0.20 eV for Cl ions). Therefore, the α-2D-Be 3 N 2 monolayer is a promising multifunctional material for applications, such as insulators, deep-UV detectors, auxetic materials, reversible crystalline–amorphous materials, temperature-controlled information storage, iontronic devices, and solid halide ion conductors.