Pressure-Driven Activation of Semi-core p-Orbitals in Rb and Cs via Crystal-Field Splitting
High-pressure unconventional oxidation states are attributed to exceptional charge transfer, which is itself induced by the differential pressure response of atomic orbital energy levels. The high oxidation states observed in heavy alkali metals (Cs, Ba, Ra) exemplify this behavior, as their semi-core p-states surpass the valence orbital levels of oxidants, e.g., the 2p-orbital of F, at high pressure. However, semi-core p-electron activation in lighter alkali metals, such as Rb, is hindered because the relative orbitals lie at intrinsically lower energies due to their smaller radii, making pressure-induced upshift challenging. We demonstrate that crystal-field splitting of alkali-metal semi-core p-orbitals selectively elevates a portion of these states, substantially lowering the pressure threshold for their chemical activation. Here, our computations reveal that the semi-core 4p-electrons of Rb form weak metallic bonds with F in $P2_1/c$-RbBF5 at around 250 GPa, a pressure accessible in modern high-pressure experiments. Extending to Cs-B-F systems, the crystal-field splitting drives 5p participation in Cs-F bond formation in CsBF5. These findings unveil a new paradigm for semi-core or inner-shell chemistry, providing a unifying framework for semi-core electron activation and bonding. Crucially, they establish that semi-core electrons of light main-group elements, including Rb, are accessible at experimentally feasible pressures.