High-pressure modulation of band gap and microstructure in N-type high-entropy strontium titanate for enhanced thermoelectric performance

In thermoelectrics, optimizing both carrier and phonon transport is crucial for enhancing thermoelectric performance. Strontium titanate, a representative N-type oxide thermoelectric material, often exhibits inferior figure of merit (zT) due to its large band gap that limits carrier concentration, and high lattice thermal conductivity, attributed to strong Ti-O covalent bonds. Conventional approaches, such as aliovalent doping to increase carrier concentration or introducing structural defects to reduce lattice thermal conductivity, are insufficient as they fail to decouple the interdependent electrical and thermal properties. Herein, we introduce a high-pressure synthesis technique that concurrently modulates both the band gap and microstructure. This approach effectively enhances the carrier concentration by narrowing the band gap and increases the effective mass of the density of states through enhanced solubility limit of rare earth elements, significantly improving the power factor. Additionally, high-pressure condition induces microstructural defects, including point defects, dislocations, lattice distortions, and nanoscale grains, which promote broad-wavelength phonon scattering and minimize lattice thermal conductivity. Consequently, a peak zT value of 0.25 at 973 K is attained in high-entropy (Sr0.2La0.2Nd0.2Sm0.2Eu0.2)TiO3 synthesized at 5 GPa, representing a 5.3-fold improvement over undoped strontium titanate. This work highlights the pivotal role of high-pressure synthesis in decoupling the carrier and phonon transport in thermoelectrics.