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Abstract: The effects and determining factors of substitutional defects on thermal conductivity (k) have been studied by introducing Hf4+ (96% heavier than, but a similar size to, Zr4+) and Ce4+ (21% larger and 50% heavier), to replace Zr4+ on the B site of La2Zr2O7 pyrochlores. La2(Zr1− x Ce x )2O7, exhibits a flatter k–T curve and a dramatically reduced high-temperature plateau k min. This is because the size rather than the mass difference between guest and host ions determines the flatness of k–T curves, whereas the absolute mass and size values of a dopant – particularly the latter – determine the plateau k min, as k min is proportional to the square root of the ratio of the elastic modulus E to the average atomic mass. Further, larger guest ion tends to effectively weaken a bond that shows ionic character and thus reduces E. The insights gained into the high-temperature plateau regime provide guidelines for the design and selection of a low-k material at high temperatures. [Copyright &y& Elsevier]