Structural peculiarities and thermoelectric study of iron indium thiospinel

The homogeneity range of ternary iron indium thiospinel at 873 K is investigated. A detailed study was focused on two distinct series (y = z): (i) a previously reported charge-balanced (In0.67+0.33y square0.33-0.33y)tetr[In2-zFez]octS4 (A1-series; square stands for vacancy) and (ii) a new charge-unbalanced (In0.67+y square0.33-y)tetr[In2-zFez]octS4 (A2-series). Fe atoms are confirmed to exclusively occupy an octahedral position in both series. An unusual reduction of the unit-cell parameter with increasing Fe-content is explained by differences in ionic radii between Fe and In as well as by an additional electrostatic attraction originating from charge imbalance (latter only in A2-series). The studied compound is n-type semiconductor and its charge carrier concentration increases or decreases for larger Fe-content within the A1- and A2-series, respectively. The thermal conductivity kappatot is significantly reduced upon increasing vacancy-concentration, whereas the change of power factor is insufficient to drastically improve the thermoelectric figure of merit.