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 (La3–xTe4), 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 (Nd3–xTe4), another rare-earth telluride with a structure similar to La3–xTe4. Density functional theory (DFT) calculations predicted a significant increase in the Seebeck coefficient over La3–xTe4 at equivalent vacancy concentrations because of 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 Nd3–xTe4 at various carrier concentrations. These measurements were compared to La3–xTe4 in order to elucidate the impact of the four 4f electrons of Nd on the transport properties of Nd3–xTe4. A zT of 1.2 was achieved at 1273 K for Nd2.78Te4, which is a 10% improvement over that of La2.74Te4.