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

Abstract

Cs4O6 adopts two distinct crystal structures at ambient pressure. At temperatures below similar to 200 K, its ground state structure is tetragonal, incorporating two symmetry-distinct dioxygen anions, diamagnetic peroxide, O-2(2-), and paramagnetic superoxide, O-2(-), 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 -4/3, consistent with the adoption of a charge-disordered state. The pressure dependence of the structure of solid Cs4O6 at 300 K and at 13.4 K was followed up to similar to 12 GPa by synchrotron X-ray powder diffraction. When a pressure of similar to 2 GPa is reached at ambient temperature, an incomplete phase transition that is accompanied by a significant volume reduction (similar to 2%) to a more densely packed highly anisotropic tetragonal structure, isostructural with the low-temperature ambient-pressure phase of Cs4O6, is encountered. A complete transformation of the cubic (charge-disordered) to the tetragonal (charge-ordered) phase of Cs4O6 is achieved when the hydrostatic pressure exceeds 6 GPa. In contrast, the pressure response of the Cs4O6 cubic/tetragonal phase assemblage at 13.4 K is distinctly different with the cubic and tetragonal phases coexisting over the entire pressure range (to similar to 12 GPa) accessed in the present experiments and with only a small fraction of the cubic phase converting to tetragonal. Pressure turns out to be an inefficient stimulus to drive the charge disorder-order transition in Cs4O6 at cryogenic temperatures, presumably due to the high activation barriers (much larger than the thermal energy at 13.4 K) associated with the severe steric hindrance for a rotation of the molecular oxygen units necessitated in the course of the structural transformation.