Particle Swarm Predictions of a SrB₈ Monolayer with 12-Fold Metal Coordination

Materials containing planar hypercoordinate motifs greatly enriched the fundamental understanding of chemical bonding. Herein, by means of first-principles calculations combined with global minimum search, we discovered the two-dimensional (2D) SrB₈ monolayer, which has the highest planar coordination number (12) reported so far in extended periodic materials. In the SrB₈ monolayer, bridged B₈ units are forming the boron monolayer consisting of B₁₂ rings, and the Sr atoms are embedded at the center of these B₁₂ rings, leading to the Sr@B₁₂ motifs. The SrB₈ monolayer has good thermodynamic, kinetic, and thermal stabilities, which is attributed to the geometry fit between the size of the Sr atom and cavity of the B₁₂ rings, as well as the electron transfer from Sr atoms to electron-deficient boron network. Placing the SrB₈ monolayer on the Ag(001) surface shows good commensurability of the lattices and small vertical structure undulations, suggesting the feasibility of its experimental realization by epitaxial growth. Potential applications of the SrB₈ monolayer on metal ions storage (for Li, Na, and K) are explored.