Pressure-Induced Charge Disorder-Order Transition in the CsO Sesquioxide.

CsO adopts two distinct crystal structures at ambient pressure. At temperatures below ∼200 K, its ground state structure is tetragonal, incorporating two symmetry-distinct dioxygen anions, diamagnetic peroxide, O, and paramagnetic superoxide, O, units in a 1:2 ratio, consistent with the presence of charge and orbital order. At high temperatures, its ground state structure is cubic, comprising symmetry-equivalent dioxygen units with an average oxidation state of -/, consistent with the adoption of a charge-disordered state.

Elusive Valence Transition in Mixed-Valence Sesquioxide CsO.

CsO is a mixed-valence molecular oxide with a cubic structure, comprising valency-delocalized O 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 O and O anions.

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.