Glass-like thermal conductivity in ytterbium-doped lanthanum zirconate pyrochlore

Glass-like thermal conductivity is observed in (La_1-xYb _x)₂Zr₂O₇ (1/6 ≤ x ≤ 1/3), which exhibits great potential as a high-temperature thermal barrier coating material. In the pyrochlore-type La₂Zr₂O₇, the large 16c-site La³⁺ is weakly bonded by its surrounding 48f-site oxygen ions, and substitution of La³⁺ with smaller and heavier Yb³⁺ gives rise to a large atomic displacement parameter (ADP) of Yb³⁺ which behaves as a "rattler", as evidenced by the X-ray diffraction refinement. The localized "rattling" of Yb ³⁺ in the cation sublattice significantly scatters the heat-carrying phonons and lowers the thermal conductivity close to the amorphous limit in combination with the intrinsic oxygen vacancies in the anion sublattice. In contrast, substituting Yb³⁺ with the larger La³⁺ in Yb₂Zr₂O₇ does not result in as remarkable a decrease in the thermal conductivity as in Yb-doped La₂Zr ₂O₇ due to the absence of rattling atoms. In this study, a resonant phonon scattering is proposed as a new approach to reduce the thermal conductivity of oxides which are important in various thermal engineering applications.