Strategic Integration of Band Structure and Entropy Engineering Achieves Unprecedented Thermoelectric Performance in <i>n</i> ‐Type CaTiO ₃ ‐Based Compounds

ABSTRACT Enhancing the thermoelectric performance of n ‐type CaTiO 3 ‐based compounds necessitates simultaneously reducing thermal conductivity while maintaining excellent electrical transport properties. To address this challenge, we synergistically integrate band structure engineering with multiscale hierarchical structural design in this work. Defect engineering simultaneously sharpens the conduction band and weakens bond polarity, significantly enhancing electrical transport properties. By rational electronic and structural designs, the delicate trade‐off between carrier mobility and lattice thermal conductivity is achieved. This strategy leads to a drastic suppression of κ tot with only a moderate decrease in carrier mobility, collectively resulting in a substantially enhanced ZT . Consequently, a record‐high ZT of 0.43 at 1073 K and a record‐high ZT ave of 0.25 over the temperature range of 323–1073 K are achieved in the medium‐entropy Ca 0.70 La 0.20 Ba 0.10 Ti 0.95 Nb 0.05 O 3 ceramic. These results demonstrate how defect engineering synergistically optimizes electrical and phonon transport properties, offering broad application across thermoelectric systems.