Interplay of Metal-Atom Ordering, Fermi Level Tuning, and Thermoelectric Properties in Cobalt Shandites Co3M2S2 (M = Sn, In)

A combination of structural, physical, and computational techniques including powder X-ray and neutron diffraction, SQUID magnetometry, electrical and thermal transport measurements, DFT calculations, and Sn-119 Mossbauer and X-ray photoelectron spectroscopies has been applied to Co3Sn2-xInxS2 (0 <= x <= 2) in an effort to understand the relationship between metal-atom ordering and physical properties as the Fermi level is systematically varied. While solid solution behavior is found throughout the composition region, powder neutron diffraction reveals that indium preferentially occupies an interlayer site over an alternative kagome-like intralayer site. DFT calculations indicate that this ordering, which leads to a lowering of energy, is related to the differing bonding properties of tin and indium. Spectroscopic data suggest that throughout the composition range 0 <= x <= 2, all elements adopt oxidation states that are significantly reduced from expectations based on formal charges. Chemical substitution enables the electrical transport properties to be controlled, through tuning of the Fermi level within a region of the density of states which comprises narrow bands of predominantly Co d-character. This leads to a compositionally induced double metal-to-semiconductor-to-metal transition. The marked increase in the Seebeck coefficient as the semiconducting region is approached leads to a substantial improvement in the thermoelectric figure of merit, ZT, which exhibits a maximum of ZT = 0.32 at 673 K. At 425 K, the figure of merit for phases in the region 0.8 <= x <= 0.85 is among the highest reported for sulfide phases, suggesting these materials may have applications in low-grade waste heat recovery.