Crystal Structure and Thermoelectric Properties of β-Pyrochlore-Type Alkali Iron Tungsten Oxides with Cage-Like Structure

We report the electrical and thermal properties of β-pyrochlore (defect pyrochlore) oxides AFe_0.33W_1.67O₆ (A = K, Rb, Cs) with a crystal structure having a small cation surrounded by oversized cage-like framework. The thermal conductivity, κ, of CsFe_0.33W_1.67O₆ and RbFe_0.33W_1.67O₆ showed extremely low values as oxides (below 1.0 W/mK) similar to those of ATaWO₆ (A = K, Rb, Cs) which we have already reported. These low κ values are ascribed to a “rattling” motion of the A cations, evidenced by their crystal structure refinement and the Raman spectra. Their electrical conductivity, σ, was in the order of 10⁻³ S/cm, and the Seebeck coefficient, S, was −500 to −600 μV/K. The electrical conductivity of AFe_0.33W_1.67O₆ (A = Rb, Cs) was much higher than those of ATaWO₆ (A = Rb, Cs), suggesting that an appropriate selection of the framework composition enables us to have better thermoelectric performance.