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

Abstract

• 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. Materials with ultralow thermal conductivity and good thermal stability are of great interest in numerous applications such as energy storage and conversion devices, and thermal insulation components. In this work, a family of high-entropy Sm 2 B 2 O 7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides with highly disordered cations on the B-site has been synthesized by introducing large atomic-size mismatch, mass and charge disorder. Through tuning the composition, the high-entropy Sm 2 B 2 O 7 oxides can be engineered from pyrochlore to fluorite structure, accompanied with an order-disorder transition. The pyrochlore Sm 2 (Nb 0.2 Sn 0.2 Ti 0.2 Y 0.2 Zr 0.2 ) 2 O 7 and fluorite Sm 2 (Nb 0.2 Ta 0.2 Y 0.2 Yb 0.2 Zr 0.2 ) 2 O 7 exhibit low thermal conductivities of 1.35 W·m −1 ·K −1 and 1.23 W·m −1 ·K −1 , respectively, indicating their good thermal insulation. In addition, the high-entropy fluorite Sm 2 (Nb 0.2 Ta 0.2 Y 0.2 Yb 0.2 Zr 0.2 ) 2 O 7 also shows average thermal expansion coefficient of 10.2 × 10 −6 °C −1 and high-temperature stability even after thermal exposure at 1600 °C in air for 30 h. These results indicate that the high-entropy Sm 2 B 2 O 7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) can be promising candidates for thermal barrier coatings and thermally insulators.