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

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 Sm₂B₂O₇ (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 Sm₂B₂O₇ oxides can be engineered from pyrochlore to fluorite structure, accompanied with an order-disorder transition. The pyrochlore Sm₂(Nb_0.2Sn_0.2Ti_0.2Y_0.2Zr_0.2)₂O₇ and fluorite Sm₂(Nb_0.2Ta_0.2Y_0.2Yb_0.2Zr_0.2)₂O₇ 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 Sm₂(Nb_0.2Ta_0.2Y_0.2Yb_0.2Zr_0.2)₂O₇ 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 Sm₂B₂O₇ (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) can be promising candidates for thermal barrier coatings and thermally insulators.