Extremely low thermal conductivity of b-pyrochlore oxides with cage-like crystal structure

Abstract

The thermal conductivity, κ, of metal oxides ATaWO₆ (A = K, Rb, Cs) with β-pyrochlore structure was investigated in terms of the "rattling" motion of the smaller A cations, which is expected to scatter phonons effectively. Interestingly, the κ values are lower for the oxides containing the lighter A cations, obviously contradicting the conventional prediction of the phonon thermal conductivity. Moreover, the κ values observed for KTaWO₆ were virtually the same as the theoretically predicted minimum thermal conductivity, κ_min. These results strongly suggest that the phonon scattering in the oxide would really be maximized by the rattling motion of K+ in the cage.