In the present study, the systematic evaluation of thermal properties was performed for CeO2–Gd2O3 (Gd2O3 = 0–15 wt%) samples. Gd2O3 doping into CeO2 reduced the grain size of CeO2–Gd2O3 solid solution and caused the expansion of the lattice parameter. The thermal conductivity of CeO2–Gd2O3 solid solutions decreased with increasing Gd2O3 content and decreased with temperature up to nearly 900 K, though the decreasing rates became smaller at a higher temperature. The values of the thermal conductivity were mostly attributed to phonon mean free path which decreased due to the Umklapp processes at high temperatures and reduced by phonon scattering due to Gd dopants and oxygen vacancies at low temperatures. The phonon scattering caused by grain boundaries, and chemical impurities can be neglected, and the isotope effect is considered to be small.
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