Activation of Inner-Shell 4p-Orbital Electrons of Rubidium Driven by Asymmetric Coordination at High Pressure

Abstract

While the high oxidation states in heavy alkali fluorides (Cs, Ba, Ra) have been attributed to a pressure-driven upshift of energy level of inner p states, this route is largely ineffective for Rb because its smaller ionic radius suppresses the required level rise even under strong compression. Here, we predict a high-pressure layered ternary phase, RbBF5, in which 12-fold truncated-cube-like F coordination around Rb breaks local symmetry and activates the Rb 4p inner shell. The resulting orbital splitting selectively elevates the in-plane Rb 4px,y levels toward the F 2p manifold, enabling inner-shell participation and stabilizing Rb-F bonding under compression. More broadly, this symmetry-lowering coordination motif may provide a general mechanism for activating inner-shell p states in other alkali metals (e.g., K and Cs inner p states). These findings extend inner-shell chemistry to lighter main-group elements and establish a design principle for accessing unconventional bonding and oxidation states at high pressure.