Synthesis and characterization of vacancy-doped neodymium telluride for thermoelectric applications

Thermoelectric materials exhibit a voltage under an applied thermal gradient and are the heart of radioisotope thermoelectric generators (RTGs), which are the main power system for space missions such as Voyager I, Voyager II, and the Mars Curiosity rover. However, materials currently in use enable only modest thermal-to-electrical conversion efficiencies near 6.5% at the system level, warranting the development of material systems with improved thermoelectric performance. Previous work has demonstrated large thermoelectric figures of merit for lanthanum telluride (La_3- Te₄), a high-temperature n-type material, achieving a peak zT value of 1.1 at 1275 K at an optimum cation vacancy concentration. Here we present an investigation of the thermoelectric properties of neodymium telluride (Nd_3- Te₄), another rare-earth telluride with a similar structure to La_3- Te₄. Density functional theory (DFT) calculations predicted a significant increase in the Seebeck coefficient over La_3- Te₄ at equivalent vacancy concentrations due to an increased density of states (DOS) near the Fermi level from the 4f electrons of Nd. The high temperature electrical resistivity, Seebeck coefficient, and thermal conductivity were measured for Nd_3- Te₄ at various carrier concentrations. These measurements were compared to La_3- Te₄ in order to elucidate the impact of the four 4f electrons of Nd on the transport properties of Nd_3- Te₄. A zT of 1.2 was achieved at 1273 K for Nd₂.₇₈Te₄, which is a 10% improvement over that of La₂.₇₄Te₄.