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

The Verwey transition in Fe 3 O 4 , 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 Fe 3 O 4 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 Cs 4 O 6 , a phase change from a cubic structure with a single crystallographic site for the molecular O 2 x − building units to a tetragonal structure with ordered superoxide O 2 − and peroxide O 2 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 Cs 4 O 6 as a model system for disentangling the complex interplay of charge, lattice, orbital, and spin degrees of freedom in Verwey-type CO processes.