Size disorder as a descriptor for predicting reduced thermal conductivity in medium- and high-entropy pyrochlore oxides

•18 compositionally-complex (medium- and high-entropy) pyrochlore oxides fabricated.•Mixing of cations at A- and/or B-sites leads to reduced thermal conductivity.•Thermal conductivity correlates well with a modified size disorder parameter.•A new descriptor proposed for designing low-k medium- and high-entropy ceramics.•Medium-entropy ceramics can outperform their high-entropy counterparts. High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 22 single-phase pyrochlores were examined. Up to 35% reductions in thermal conductivity were achieved in medium- and high-entropy compositions with retained moduli, thereby attaining insulative yet stiff properties for thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter δSize*, thereby suggesting the importance of severe lattice distortion. Thus, this δSize* is suggested as a useful descriptor for designing thermally-insulative compositionally-complex ceramics, where medium-entropy ceramics can outperform their high-entropy counterparts. [Display omitted]