Evaluating the Energy Conversion Performance of Rare-Earth-Based Cu₃<i>RE</i>Te₃ Thermoelectric Materials with Strong Electronic Correlations

A key challenge in thermoelectric materials development is achieving low thermal conductivity without compromising the electrical performance. Rare-earth tellurides, due to their complex crystal chemistry and intrinsic defects, offer a pathway to optimize these conflicting parameters. In this work, we investigated the structural and thermoelectric properties of a series of ternary rare-earth copper tellurides, Cu3RETe3 (RE = Er, Ho, Tb). Our findings reveal that the specific rare-earth element plays a critical role in determining the crystal structure of these compounds. Notably, the Er- and Ho-containing phases predominantly crystallize in the orthorhombic Pmn21 structure, whereas the Tb analogue adopts a trigonal R-3 structure. Owing to this structural difference, the Tb-based compound exhibits approximately twice the effective mass─and, correspondingly, a 2-fold increase in the Seebeck coefficient─attributed to band convergence, as confirmed by theoretical calculations. All materials exhibit intrinsically low lattice thermal conductivity, attributed to strong lattice anharmonicity and point defect scattering, particularly pronounced in Cu3TbTe3. Hall effect and Seebeck measurements indicate p-type semiconducting behavior with carrier concentrations on the order of 1020 cm–3. First-principles calculations show the presence of strong electronic correlations, and GGA+U method is necessary to confirm semiconducting electronic structures and support experimental trends in carrier mobility and Seebeck coefficient. Among the compounds, Cu3HoTe3 achieves the highest peak thermoelectric figure of merit (ZT ≈ 0.9 at 873 K), while Cu3TbTe3 delivers the highest average performance (ZTave = 0.4 over the temperature range of 298–873 K). These findings highlight the potential of Cu3RETe3 compounds as efficient rare-earth-based thermoelectric materials for energy conversion applications.