If you squeeze an atom hard enough, its 'forbidden' inner core starts forming chemical bonds that shouldn't even be possible.
April 13, 2026
Original Paper
Activation of Inner-Shell 4p-Orbital Electrons of Rubidium Driven by Asymmetric Coordination at High Pressure
arXiv · 2604.08901
The Takeaway
Normally, only the outer electrons of an atom do the work of bonding, but crushing Rubidium forces its inner shell to participate. This effectively rewrites the basic chemical rules for how this element behaves.
From the 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 orbita