Verwey-type charge ordering transition in an open-shell p-electron compound

The Verwey transition in Fe₃O₄, a complex structural phase transition concomitant with a jump in electrical conductivity by two orders of magnitude, has been a benchmark for charge ordering (CO) phenomena in mixed-valence transition metal materials. CO is of central importance, because it frequently competes with functional properties such as superconductivity or metallic ferromagnetism. However, the CO state in Fe₃O₄ turned out to be complex, and the mechanism of the Verwey transition remains controversial. We demonstrate an archetypical Verwey-type transition in an open p-shell anionic mixed-valence compound using complementary diffraction and spectroscopic techniques. In Cs₄O₆, a phase change from a cubic structure with a single crystallographic site for the molecular O₂ building units to a tetragonal structure with ordered superoxide O₂^- and peroxide O₂^2- entities is accompanied by a drastic drop in electronic conductivity and molecular charge fluctuation rates. The simple CO pattern of molecular units and the lack of magnetic order suggest Cs₄O₆ as a model system for disentangling the complex interplay of charge, lattice, orbital, and spin degrees of freedom in Verwey-type CO processes.