Thermally and Optically Activated Migration of Charge Carriers in Alkali Metal Sesquioxides

The alkali metal sesquioxides A₄ O₆ (A=K, Rb, Cs) are mixed-valent with respect to oxygen and display several degrees of electronic and structural freedom, which give rise to diverse transport and ordering processes. We report on analyses of the respective underlying excitations by diffuse reflectance spectroscopy and thermally activated electron transport. Backed by DFT based band structure calculations we identify three possible mechanisms, inter valence charge transfer from peroxide to superoxide, excitation across the Jahn-Teller gap of tilted superoxide anions, and polaron migration. The activation energies as found by the three different approaches are in a rather narrow range of 0.62-0.89 eV for Rb₄ O₆ and 0.49-0.65 eV for Cs₄ O₆ , confirming opacity in the full range of visible light. The effect of the phase transition from cubic to tetragonal as demonstrated for the caesium representative corresponds to a marginal shift to higher activation energy.