A novel approach for the composition design of high-entropy fluorite oxides with low thermal conductivity

High-entropy fluorite oxides (HEFOs) show significant potential for thermal protection applications due to their advantageous combination of low thermal conductivity and high Yong’s modulus. However, the factors influencing its formation have not been well studied, and a systematic method for compositional design has not yet been established. In this paper, the effects of oxygen vacancy concentration (O_vac) and mean cation radius ( $\overline{r}$) on formability of HEFOs were investigated to develop a compositional design approach. The results indicate that an appropriate $\overline{r}$ and O_vac is crucial for promoting the formability of single-phase (Ca_xCe_y1Zr_y2Hf_zSn_zTi_z)O_2−δ HEFOs. High mass/size disorder and an appropriate O_vac (10%) result in (Ca_0.2Ce_0.14Zr_0.12Hf_0.18Sn_0.18Ti_0.18)O_2−δ exhibiting the lowest thermal conductivity of 1.24 W·m⁻¹·K⁻¹. Building upon these insights and employing a valence combination strategy, three new single-phase HEFOs with low thermal conductivity were successfully designed and synthesized, namely, (La_0.28Y_0.28Ce_0.18Zr_0.18W_0.08)O_2−δ, (La_0.3Y_0.3Ce_0.2Nb_0.1Ta_0.1)O_2−δ, and (Yb_0.52Ce_0.12Zr_0.12Sn_0.12Nb_0.12)O_2−δ. This design approach will provide a valuable reference for the design of other high-entropy oxides.