Extremely low thermal conductivity in oxides with cage-like crystal structure

Although successful examples of thermoelectric materials with the "rattler-in-a-cage" structure include filled skutterudites and clathrate compounds, there has been no report on "rattling" oxides. Here we report the thermal properties of defect pyrochlore (β-pyrochlore) oxides with smaller cations in an oversized cage-like crystal structure. It is clearly shown that the thermal diffusivity, α and the thermal conductivity, κ, are lower for the oxides containing lighter A-site cations, contradicting the conventional prediction of the phonon thermal conductivity, in which lighter comprising atoms must give higher α and hence κ. These results strongly suggest that the smaller A-site cations with larger size mismatch to the surrounding cage structure enhance the phonon scattering and thereby reduce α and κ. These findings directly evidence the "rattling" effect of the loosely bound cations on the thermal transport properties of oxide materials.