Rational Design Vacancies and Crystallographic Distortion via Stepwise Optimization Strategy Enables High...

CuGaTe₂ possesses a large Seebeck coefficient but suffers from low carrier concentration and high lattice thermal conductivity, resulting in mediocre thermoelectric performance. Here, density-functional-theory calculations are integrated with experiments to devise a stepwise optimization route that couples vacancy engineering with crystallographic distortion. First, controlled Ga-vacancy doping drives the Fermi level deep into the valence band, increasing the hole concentration without seriously degrading carrier mobility. Next, simultaneous Ag substitution on the Cu site and trace In substitution on the Ga site introduce pronounced mass/size disorder and local off-centered Ag atoms that soften chemical bonds, lower the Debye frequency, and intensify acoustic-optical phonon coupling. These combined effects depress the room-temperature lattice thermal conductivity from ≈7.96 to ≈2.35 W m^-1 K^-1 while maintaining an enhanced power factor of ≈13.0 µW cm^-1 K^-2 at 823 K. Consequently, Cu_0.85Ag_0.15Ga_0.965In_0.025Te₂ attains a peak figure of merit of ≈1.11 at 823 K and an average figure of merit of ≈0.51 between 300 and 823 K-247% higher than pristine CuGaTe₂ (≈0.32, 823 K). This work demonstrates that the deliberate coupling of vacancy creation with lattice-distortion alloying provides a powerful guideline for designing high-performance thermoelectric materials.