Lanthanum telluride: Mechanochemical synthesis of a refractory thermoelectric material

Recent experimental work on lanthanum telluride has confirmed its significant potential as an n-type material for high temperature thermoelectric (TE) power generation application. The phase of interest, La _3-xTe₄, has a Th₃P₄ defect structure where x is the lanthanum vacancy with values ranging between 0 and 1/3. Thermoelectric properties change rapidly with x since the carrier concentration, n, is proportional to the (1-3x) parameter. Controlling the Te to La stoichiometry in lanthanum telluride is thus vital to achieving the optimum self-doping level for the highest dimensionless figure of merit ZT value. We report on a significant improvement in reproducibly preparing this refractory compound over prior lengthy and unwieldy high temperature experimental techniques developed in the 1980's. Mechanochemical processes are utilized to synthesize precise stoichiometries of lanthanum telluride at room temperature, enabling improved characterization, analysis and modeling of its transport properties as a function of the number of La vacancies. We report TE properties for a large range of the allowed compositions, with ZT values greater than 1.0 obtained at 1275 K for several compositions. In addition to stoichiometric optimization within the pure compound, chemical substitutions can enhance performance by decreasing the lattice thermal conductivity and tuning the electrical properties for maximum ZT values at lower temperatures; preliminary studies indicate that the addition of ytterbium increases ZT. Some properties pertaining to device development are discussed. Specifically, lanthanum telluride has a low sublimation rate, and a coefficient of thermal expansion that closely matches a p-type rare earth compound analog (the Yb ₁₄MnSb₁₁ Zintl compound).