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 (Sr0.25Ca0.25Ba0.25RE0.25)TiO3 (RE=Nd, Sm, Eu, Gd, Dy, Ho) ceramics were designed to systematically investigate the effects of critical average ionic radius (rA̅), atomic size disorder factor (δAr), and the mass difference factor (δAm) 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 (Sr0.25Ca0.25Ba0.25Nd0.25)TiO3, (Sr0.25Ca0.25Ba0.25Sm0.25)TiO3, and (Sr0.25 Ca0.25Ba0.25Eu0.25)TiO3 by a solid-state reaction method were reported. When rA̅ ＜1.377 Å, indicator of δAr≥ 12.86, and δAm≥ 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.