Two-dimensional Be 2 Al and Be 2 Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity.

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be 2 M (M = Al and Ga) 2D monolayer with P 6/ mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be 2 Al and Be 2 Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature ( T c ) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their T c . The high thermochemical and kinetic stability of Be 2 M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be 2 M on the electride Ca 2 N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be 2 M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.