High-entropy MTiO₃ perovskite oxides with glass-like thermal conductivity for thermoelectric applications

High-entropy oxides with complex compositions can be designed to lower the thermal conductivity of SrTiO₃-based thermoelectric materials to optimize their properties. High-entropy (Sr_0.25Ca_0.25Ba_0.25RE_0.25)TiO₃ (RE=Nd, Sm, Eu, Gd, Dy, Ho) ceramics were designed to systematically investigate the effects of critical average ionic radius (r_A̅), atomic size disorder factor (δ_A^r), and the mass difference factor (δ_A^m) of A-site elements on the single-phase formation, disorder microstructure, and the thermoelectric properties. The successful synthesis of single-phase perovskite-type ceramics of (Sr_0.25Ca_0.25Ba_0.25Nd_0.25)TiO₃, (Sr_0.25Ca_0.25Ba_0.25Sm_0.25)TiO₃, and (Sr_0.25 Ca_0.25Ba_0.25Eu_0.25)TiO₃ by a solid-state reaction method were reported. When r_A̅ ＜1.377 Å, indicator of δ_A^r≥ 12.86, and δ_A^m≥ 18.6, the pyrochlore phase of Gd₂Ti₂O₇, Dy₂Ti₂O₇, and Ho₂Ti₂O₇ precipitated. Importantly, the obtained temperature- independent low glass-like lattice thermal conductivity of 2.27 W/(m∙K) was significantly reduced by 68.2–21.4% compared with the majority values ranging from 7.16 W/(m∙K) at 300 K to 2.89 W/(m∙K) at 1073 K of SrTiO₃-based polycrystal system. The temperature independence of thermal conductivity over the whole range of 323–1073 K realizes the concept of ‘phonon-glass electron crystal’. Evidence strongly confirmed that large lattice distortion, TiO₆ octahedral twist, dislocations, and complex strain field coexisted by Raman, HRTEM combined with GPA analysis in the high-entropy perovskite structures.