First-Principles Prediction of Two-Dimensional Be Allotropes and Alkaline-Earth Analogues (Mg, Ca, Sr, and Ba): from Hexagonal 2D Beryllenes to IIA Xenes

The chemical space of two-dimensional (2D) elemental beryllium (beryllene) was systematically explored via an ab initio evolutionary algorithm, revealing a rich polymorphism. We identified eight distinct allotropes (α: P6/mmm, Z = 1; β: P3̅m1, Z = 2; γ: P6̅m2, Z = 3; δ: P3̅m1, Z = 3; ε: P2/m, Z = 8; ζ: Pmma, Z = 4; η: P4/mmm, Z = 3; θ: P2/m, Z = 5) exhibiting excellent thermodynamic, mechanical, dynamical, and thermal stability. The ε-, ζ-, η-, and θ-beryllene phases are newly predicted. Analysis of the electron localization function at Be3 centers in the β and γ phases proves to be consistent with 3-center-2-electron bonding rather than electride character. Additionally, thirty-two stable bulk-derived 2D slabs were identified, with the (001) slabs demonstrating superior experimental feasibility due to their low formation enthalpies (0.069–0.799 eV/atom) and cleavage energies (0.089–0.113 eV/Å2). Extending these eight beryllene allotropes to other Group IIA metals yielded 16 new 2D Xenes (magnesene, calcene, strontene, barene) within the γ, δ, ε, and ζ prototypes. This work underscores the polymorphism of 2D Group IIA metals and provides critical theoretical insights into their synthesis.