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

The Verwey transition in FeO, 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 FeO turned out to be complex, and the mechanism of the Verwey transition remains controversial. We demonstrate an archetypical Verwey-type transition in an open -shell anionic mixed-valence compound using complementary diffraction and spectroscopic techniques. In CsO, a phase change from a cubic structure with a single crystallographic site for the molecular O building units to a tetragonal structure with ordered superoxide O and peroxide O 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 CsO as a model system for disentangling the complex interplay of charge, lattice, orbital, and spin degrees of freedom in Verwey-type CO processes.