  High-pressure can transform the structures and compositions of materials
either by changing the relative strengths of bonds or by altering the oxidation
states of atoms. Both effects cause unconventional compositions in novel
compounds that have been synthesized or predicted in large numbers in the past
decade. What naturally follows is a question: what if pressure imposes strong
effects on both chemical bonds and atomic orbitals in the same material. A
systematic DFT and crystal structure search study of Cs polyoxides under high
pressure shows a striking transition of chemistry due to the activation of the
Cs 5p core electrons. Opposing to the general trend of polyoxides, the O-O
bonds disappear in Cs polyoxides and Cs and O atoms form molecules and
monolayers with strong Cs-O covalent bonds. Especially, the abnormal transition
of structure and chemical bonds happens to CsO, a solid peroxide that is stable
under ambient pressure, at 221 GPa, which can be accessed by current
high-pressure experiments.
