First-principles investigations on the thermoelectric properties of high-entropy Ca_(1-x)/3Sr_(1-x)/3Ba_(1-x)/3La_xTiO₃ (x = 0.1, 0.25, 0.4, 0.55, 0.7) ceramics

A high thermoelectric figure of merit (ZT) is desirable for oxide thermoelectric materials. High entropy is an effective approach for enhancing the electrical conductivity and reducing thermal conductivity of materials used in thermoelectric devices. First-principles calculations are used to investigate the thermoelectric properties of high-entropy Ca_(1-x)/3Sr_(1-x)/3Ba_(1-x)/3La_xTiO₃ (x = 0.1, 0.25, 0.4, 0.55, 0.7) ceramics. As La^{3 +} concentrations rises, the lattice distortion leads to an initial decrease followed by an increase in the bandgap and effective mass, which in turn causes the Seebeck coefficient of the high-entropy system to exhibit a similar trend of first decreasing and then increasing. The increase in La³⁺ content causes changes in the carrier concentration and oxygen vacancy concentration within the high-entropy system, resulting in an initial decline followed by a subsequent rise in electrical conductivity. The competition between the Seebeck coefficient and electrical conductivity shows that the power factor is highest of 0.496 mW/(mK²) (1300 K) at x = 0.1. As the La³⁺ content increases, the distortion of the lattice structure and the nonharmonic vibration of phonons initially decrease and then slightly increase thermal conductivity. Ca_0.15Sr_0.15Ba_0.15La_0.55TiO₃ exhibits the lowest thermal conductivity, which can be as low as 1.312 W/(m·K) at 1300 K, along with 1.611 W/(m·K) according to Clark model and 1.641 W/(m·K) according to Cahill model. With elevated power factor and reduced thermal conductivity, the ZT of Ca_0.1Sr_0.1Ba_0.1La_0.7TiO₃ is the highest of 0.449 (1300 K), and the average ZT can also reach 0.165. As the configuration entropy increases, the ZT gradually decreases. It is demonstrated that high entropy can effectively improve the ZT of the material. However, this does not imply that indiscriminately seeking high configuration entropy has a significant impact on the ZT of the material. This work lays the foundation for reducing the thermal conductivity of SrTiO₃-based high-entropy ceramics and improving their thermoelectric properties.