Thermoelectric Properties of Undoped High Purity Higher Manganese Silicides Grown by Chemical Vapor Transport

Semiconducting higher manganese silicides (HMS), with a nominal composition of MnSi 1.73, are particularly promising thermoelectric materials because of their elemental abundance, nontoxicity, and reported ZT of around 0.4 at 800 K for undoped samples. However, embedded MnSi impurities naturally form during the melt growth of HMS materials. The influences of such naturally occurring MnSi impurities within bulk HMS have yet to be carefully studied. Herein, we report the synthesis of high-purity MnSi-free single crystals of HMS by chemical vapor transport and the thermoelectric properties of consolidated HMS samples prepared by spark plasma sintering (SPS). The high purity of the HMS crystals is verified by scanning and transmission electron microscopy, electron diffraction, and synchrotron high-resolution X-ray diffraction. Despite successfully growing high purity HMS single crystals, we find that MnSi will nevertheless precipitate from HMS after SPS processing. In-situ sychrotron high-resolution X-ray diffraction experiments show that HMS are unstable at high temperatures. Despite the precipitation of MnSi inclusions within the HMS materials, we show that samples prepared from undoped single crystals of HMS exhibit higher hole mobilities owing to their higher purity, resulting in an improved maximum ZT of 0.52 ± 0.08 at 750 K.