Particle Swarm Predictions of a SrB 8 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) SrB 8 monolayer, which has the highest planar coordination number (12) reported so far in extended periodic materials. In the SrB 8 monolayer, bridged B 8 units are forming the boron monolayer consisting of B 12 rings, and the Sr atoms are embedded at the center of these B 12 rings, leading to the Sr@B 12 motifs. The SrB 8 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 B 12 rings, as well as the electron transfer from Sr atoms to electron-deficient boron network. Placing the SrB 8 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 SrB 8 monolayer on metal ions storage (for Li, Na, and K) are explored.