Microstructure and thermoelectric properties of the medium-entropy BiSbTe$_{1.5}$Se$_{1.5}$ and PbSbTeSe compounds prepared by reactive spark plasma sintering

Single-phased samples of the medium-entropy BiSbTe$_{1.5}$Se$_{1.5}$ (low-temperature thermoelectric with electron con- ductivity) and PbSnTeSe (mid-temperature thermoelectric with hole conductivity) compounds have been prepared by reactive spark plasma sintering of powder mixtures consisting of relevant elemental metals. The samples prepared are grained. Plate-like grains with average grain size of $\sim$3.3 $\mu$m are observed in BiSbTe$_{1.5}$Se$_{1.5}$, whereas irregularly-shaped grains with average grain size of $\sim$18.9 $\mu$m are typical for PbSnTeSe. Highest value of the thermoelectric figure-of-merit of the samples is equal to $\sim$0.43 (at 500 K for BiSbTe$_{1.5}$Se$_{1.5}$) and $\sim$0.35 (at 725 K for PbSnTeSe).