Mechanochemical synthesis and high temperature thermoelectric properties of calcium-doped lanthanum telluride La_3-xCa_xTe₄

The thermoelectric properties from 300-1275 K of calcium-doped La_3-xTe₄ are reported. La_3-xTe₄ is a high temperature n-type thermoelectric material with a previously reported zT_max ∼ 1.1 at 1273 K and x = 0.23. Computational modeling suggests the La atoms define the density of states of the conduction band for La_3-xTe₄. Doping with Ca²⁺ on the La³⁺ site is explored as a means of modifying the density of states to improve the power factor and to achieve a finer control over the carrier concentration. High purity, oxide-free samples are produced by ball milling of the elements and consolidation by spark plasma sintering. Calcium substitution upon the lanthanum site was confirmed by a combination of Rietveld refinements of powder X-ray diffraction data and wave dispersive spectroscopy. A zT_max ∼ 1.2 is reached at 1273 K for the composition La_2.2Ca_0.78Te₄ and the relative increase compared to La_3-xTe₄ is attributed to the finer carrier concentration.