Pressure-Induced Charge Disorder–Order Transition\nin the Cs₄O₆ Sesquioxide

Abstract

Cs₄O₆ adopts two distinct crystal structures\nat ambient pressure. At temperatures below ∼200 K, its ground\nstate structure is tetragonal, incorporating two symmetry-distinct\ndioxygen anions, diamagnetic peroxide, O₂^2–, and paramagnetic superoxide, O₂^–,\nunits in a 1:2 ratio, consistent with the presence of charge and orbital\norder. At high temperatures, its ground state structure is cubic,\ncomprising symmetry-equivalent dioxygen units with an average oxidation\nstate of −⁴/₃, consistent with the adoption\nof a charge-disordered state. The pressure dependence of the structure\nof solid Cs₄O₆ at 300 K and at 13.4 K was followed\nup to ∼12 GPa by synchrotron X-ray powder diffraction. When\na pressure of ∼2 GPa is reached at ambient temperature, an\nincomplete phase transition that is accompanied by a significant volume\nreduction (∼2%) to a more densely packed highly anisotropic\ntetragonal structure, isostructural with the low-temperature ambient-pressure\nphase of Cs₄O₆, is encountered. A complete transformation\nof the cubic (charge-disordered) to the tetragonal (charge-ordered)\nphase of Cs₄O₆ is achieved when the hydrostatic\npressure exceeds 6 GPa. In contrast, the pressure response of the\nCs₄O₆ cubic/tetragonal phase assemblage at 13.4\nK is distinctly different with the cubic and tetragonal phases coexisting\nover the entire pressure range (to ∼12 GPa) accessed in the\npresent experiments and with only a small fraction of the cubic phase\nconverting to tetragonal. Pressure turns out to be an inefficient\nstimulus to drive the charge disorder–order transition in Cs₄O₆ at cryogenic temperatures, presumably due to\nthe high activation barriers (much larger than the thermal energy\nat 13.4 K) associated with the severe steric hindrance for a rotation\nof the molecular oxygen units necessitated in the course of the structural\ntransformation.