Interstitial Copper Doping and Thermally Activated Rattling in La_{3- x}Te₄ for Enhanced Thermoelectric Performance

La_{3- x}Te₄ materials are ideal candidates for next-generation radioisotope thermoelectric generators due to their excellent thermoelectric performance and high-temperature stability. For decades, researchers have used substitutional doping or vacancy modulation to tune carrier concentration, but these methods can only tune it without optimizing the conduction band structure or suppressing lattice thermal conductivity. Interstitial doping strategy breaks this deadlock by enabling simultaneous electronic and thermal regulation without mutual interference. Herein, a series of Cu-doped La_2.74Cu_xTe₄ (x = 0, 0.01, 0.05, 0.1, 0.15) samples were synthesized. Cu incorporation elevates the Seebeck coefficient without degrading significantly the power factor, while simultaneously suppressing lattice thermal conductivity via anharmonic vibrational behavior that strengthens low-frequency acoustic phonon modes and intensifies phonon-phonon scattering. Among the synthesized compositions, La_2.74Cu_0.05Te₄ achieves a peak thermoelectric figure of merit of 1.58 at 1073 K, representing a 34% improvement over the undoped La_2.74Te₄. Furthermore, the material exhibits a notable average zT value of 1.5 within the operational temperature range of 873-1073 K. When compared to the previously reported state-of-the-art lanthanum telluride-based thermoelectrics, this represents a significant improvement of 71.3% in the average zT value.