Elusive Valence Transition in Mixed-Valence Sesquioxide Cs₄O₆

Abstract

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.

Pressure-Induced Charge Disorder–Order Transition in the Cs4O6 Sesquioxide

“…A theoretical investigation of the response of Rb 4 O 6 to pressure has predicted a complex sequence of phase transitions from an insulating strongly correlated antiferromagnet to a ferromagnet at ∼5 GPa, then to a half-metallic ferromagnetic state near 75 GPa, and finally to a normal metallic state at ∼150 GPa . However, these results are again contradicted, at least at ambient pressure, by experiment, which shows the absence of long-range magnetic order due to frustration . Here we report on the structural properties of the heavier analogue, Cs 4 O 6 , as a function of pressure up to ∼12 GPa at both 300 and 13.4 K (i.e., temperatures at which at ambient pressure the cubic and tetragonal phases, respectively, are the ground states) by synchrotron X-ray powder diffraction.…”

Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb4O6

“…7(b), (c) and (d)], which are also not predicted for the model of isolated dimers. Finally, it is difficult to comprehend that isolated spin dimers would form by the observed small structural change of the otherwise high-temperature tetragonal phase with three-dimensional (3D) pyrochlore exchange network of O − 2 sites [15]. For these reasons we suggest that dimers of O − 2 spins are in fact weakly coupled.…”

“…In contrast to the pure superoxides CsO 2 and RbO 2 in which O − 2 spins ultimately undergo three-dimensional antiferromagnetic order near 10 K [17,18] and 15 K [21], respectively, there are no indications for long-range magnetic order from the neutron diffraction patterns of the Cs 4 O 6 [14,15] and Rb 4 O 6 [ Fig. 6(a)].…”

“…A structural transition from the cubic to the tetragonal phase [Fig. 1(b)] occurring in Cs 4 O 6 at 250 K is accompanied by charge ordering [14,15] as the fourth electron orders in such a way that a mixed-valence state of magnetic O − 2 and diamagnetic O 2− 2 anions is formed in the ratio of 2:1. As a result, the conductivity drops by several orders of magnitude [14], bearing striking similarities with the Verwey transition [5].…”

Nanoarchitectonics for Coordination Asymmetry and Related Chemistry

“…50 Prassides and coworkers investigated asymmetric mixedvalence sesquioxide Cs4O6 for their elusive valence transition. 51 Aratani, Yamada, and co-workers synthesized a cyclic 1,8pyrenylene tetramer capable of back-to-back binding to fullerene C60 molecules, giving asymmetric complex structures between C60 and 1,8-pyrenylene tetramer. 52 2.2 Metal-Organic Framework, Coordination Polymer, and Other Porous Materials.…”