High-entropy Sm2B2O7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides with highly disordered B-site cations for ultralow thermal conductivity

•A family of high-entropy Sm2B2O7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides with highly disordered cations on the B-sites has been synthesized by introducing large atomic-size mismatch, and mass and charge disorder at B-sites.•The high-entropy Sm2(Nb0.2Sn0.2Ti0.2Y0.2Zr0.2)2O7 and Sm2(Nb0.2Ta0.2Y0.2Yb0.2Zr0.2)2O7 exhibit ultralow thermal conductivity of 1.35 W•m−1•K−1 and 1.23 W•m−1•K−1, respectively.•The ultralow thermal conductivity of Sm2B2O7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides can be attributed to the highly disorder on the B-sites including atomic-size mismatch, mass fluctuations and strain field fluctuations together with oxygen vacancies.•The high-entropy fluorite Sm2(Nb0.2Ta0.2Y0.2Yb0.2Zr0.2)2O7 also shows average thermal expansion coefficient of 10.2 °C−1 and high-temperature stability up to 1600 °C in air.