Structural stability and thermoelectric performance of high quality synthetic and natural pyrites (FeS2)

Synthetic bulk and natural pyrite from the hydrothermal mine in Schönbrunn (Saxony, Germany) are confirmed to be stoichiometric compounds with the composition FeS2 and to be stable up to ~600 K by combined chemical, spectroscopic and X-ray diffraction analyses. Natural pyrite with negligibly small amount (< 0.6 wt.%) of well-defined transition metal carbonates impurities revealed characteristics of a nondegenerate semiconductor and is considered as a model system for investigation of thermoelectric performance. In the temperature range 50-600 K both natural and synthetic high quality bulk FeS2 samples show electrical resistivity varying within (220)-(5×10-3) Ω m and Seebeck coefficients of (4)-(-450) μV K-1. The thermal conductivity is large (~40 W m-1 K-1 at 300 K) and exclusively due to phononic contribution. It reveals a well pronounced maximum centered at ~75 K for natural pyrite (grain size ≤ 5 mm), which becomes almost completely suppressed in the sintered bulk samples due to increase of the point defects concentration and additional scattering on the grain boundaries (grain size ≤ 100 μm). The thermoelectric efficiency of a pure pyrite with ZT ~ 10-6 at 600 K is indeed by a factor of ~1000 worse than those reported earlier for some minerals and synthetic samples.