Copper Hyper-Stoichiometry: The Key for the Optimization of Thermoelectric Properties in Stannoidite Cu8+xFe3-xSn2S12

A univalent copper hyper-stoichiometric stannoidite Cu8+xFe3-xSn2S12 with 0 <= x <= 0.5 has been synthesized using mechanical alloying followed by spark plasma sintering. The X-ray diffraction analysis combined with Fe-57 and Sn-119 Mossbauer investigations has allowed the charge distribution of the cationic species on the various sites to be established and suggests the possibility of a small tin deficiency. The transport properties show a remarkable crossover from a semiconducting to a metal-like behavior as the copper content increases from x = 0 to x = 0.5, whereas correlatively the Seebeck coefficient decreases moderately, with S values ranging from 310 to 100 mu V/K. The thermal conductivity decreases as the temperature increases showing low values at high temperature, far below those reported in related stannite materials. The investigation of the thermoelectric properties shows that the ZT figure of merit is dramatically enhanced by the copper hyper-stoichiometry by a factor of 5 going from 0.07 for x = 0 to 0.35 for x = 0.5 at 630 K. This thermoelectric behavior is interpreted on the basis of a model involving the Cu-S framework as the conducting electronic network where the Fe2+/Fe3+ species play the role of hole reservoir.