Evaluating the Energy Conversion Performance of Rare-Earth-Based Cu3RETe3 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 (ZT_ave = 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.