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, εSmO_1.5·(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 SmO_1.5–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 SmO_1.5 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 SmO_1.5-rich side. The latter variation may be attributed to the Umklapp scattering of phonons.

Due to the complex products and irradiation-induced defects, it is hard to understand and even predict the thermal conductivity variation of materials under fast neutron irradiation, such as the abrupt degradation of thermal conductivity of boron carbide (B₄C) at the very beginning of the irradiation process. In this work, the contributions of various irradiation-induced defects in B₄C primarily consisting of the substitutional defects, Frenkel defect pairs, and helium bubbles were re-evaluated separately and quantitatively in terms of the phonon scattering theory. A theoretical model with an overall consideration of the contributions of all these irradiation-induced defects was proposed without any adjustable parameters, and validated to predict the thermal conductivity variation under irradiation based on the experimental data of the unirradiated, irradiated, and annealed B₄C samples. The predicted thermal conductivities by this model show a good agreement with the experimental data after irradiation. The calculation results and theoretical analysis in light of the experimental data demonstrate that the substitutional defects of boron atoms by lithium atoms, and the Frenkel defect pairs due to the collisions with the fast neutrons, rather than the helium bubbles with strain fields surrounding them, play determining roles in the abrupt degradation of thermal conductivity with burnup.