Mechanochemical synthesis and high temperature thermoelectric properties of calcium-doped lanthanum telluride La3−xCaxTe4

The thermoelectric properties from 300-1275 K of calcium-doped La3-xTe4 are reported. La3-xTe4 is a high temperature n-type thermoelectric material with a previously reported zT(max) similar to 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 La3-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) similar to 1.2 is reached at 1273 K for the composition La2.2Ca0.78Te4 and the relative increase compared to La3-xTe4 is attributed to the finer carrier concentration.