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Rare-earth zirconate pyrochlores, which exhibit various interesting properties, especially low thermal conductivity, are of great interest. Nonstoichiometry is an effective strategy involving defect engineering to reduce the thermal conductivity of materials. In this work, a long series of solid solutions, εSmO1.5·(1−ε)ZrO2 (ε = 0.200, 0.250, 0.350, 0.400, 0.450, 0.500, 0.525, 0.550, 0.600, and 0.650), were designed and synthesized to investigate the effects of nonstoichiometry on the structural evolution, defect chemistry, and thermal conductivity of SmO1.5–ZrO2 system across the entire pyrochlore phase field and phase transition regions. The similarity of all the Raman spectra provides an incredible opportunity to confirm the structural details and evolution of pyrochlore and defective fluorite, as well as the emergence of the expected point defects. A new solution mechanism concerning nonstoichiometry was proposed and confirmed by analyzing the variations in the experimental density and Raman spectra. The thermal conductivities of all the samples were measured, and investigated in terms of the phonon-scattering theory. The results show that the nonstoichiometry with SmO1.5 excess is more effective in reducing the thermal conductivity to an extremely low value of approximately 1 W/(m·K), which can be attributed to the dramatic decrease in the average acoustic velocity in addition to the strong phonon scattering of Zr ion vacancies and oxygen vacancies. On the other hand, the nonstoichiometry with ZrO2 excess significantly reduces the thermal conductivity at room temperature, but conversely leads to a slight increase of it above 200 °C. The former variation can be attributed to the phonon scattering of the 8a interstitial oxygen ions, which has a similar effectiveness to the defects on the SmO1.5-rich side. The latter variation may be attributed to the Umklapp scattering of phonons.
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