Elusive Valence Transition in Mixed-Valence Sesquioxide Cs₄O₆

Cs₄O₆ is a mixed-valence molecular oxide with a cubic structure, comprising valency-delocalized O₂^4/3- units and with properties highly sensitive to cooling protocols. Here we use neutron powder diffraction to authenticate that, while upon deep quenching the cubic phase is kinetically arrested down to cryogenic temperatures, ultraslow cooling results in an incomplete structural transition to a contracted tetragonal phase. Two dioxygen anions in a 1:2 ratio are identified, providing evidence that the transition is accompanied by charge and orbital order and stabilizes a Robin-Day Class II mixed-valence state, comprising O₂^2- and O₂^- anions. The phenomenology of the phase change is consistent with that of a martensitic transition. The response of the low-temperature phase assemblage to heating is complex, involving a series of successive interconversions between the coexisting phases. Notably, a broad interconversion plateau is present near 260 K, signifying reentrant kinetic arrest of the tetragonal phase upon heating because of the combined effects of increased steric hindrance for molecular rotation and melting of charge and orbital order. The geometrically frustrated pyrochlore lattice adopted by the paramagnetic S = ¹/₂ O₂^- units provides an intimate link between the crystal and magnetic properties of charge-ordered Cs₄O₆, naturally accounting for the absence of magnetic order.