Indium thiospinel In1-xxIn2S4 - structural characterization and thermoelectric properties

A detailed study of polycrystalline indium-based In1-xxIn2S4 (x = 0.16, 0.22, 0.28, and 0.33) thiospinel is presented ( - vacancy). Comprehensive investigation of synthesis conditions, phase composition and thermoelectric properties was performed by means of various diffraction, microscopic and spectroscopic methods. Single-phase alpha- and beta-In1-xxIn2S4 were found in samples with 0.16 <= x <= 0.22 and x = 0.33 (In2S3), respectively. In contrast, it is shown that In0.720.28In2S4 contains both alpha- and beta-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined alpha- and beta-In1-xxIn2S4 is conducted for the first time. alpha-In1-xxIn2S4 (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200| mu V K-1) and moderate charge carrier mobility on the level of similar to 20 cm(2) V-1 s(-1) at room temperature (RT). Decreases in charge carrier concentration (increase of electrical resistivity) and thermal conductivity (even below 0.6 W m(-1) K-1 at 760 K) for larger In-content are observed. Although beta-In0.670.33In2S4 (beta-In2S3) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (similar to 10(4) cm(2) V-1 s(-1) at RT). These findings indicate that structural disorder in the alpha-modification affects both electronic and thermal properties in this thiospinel. The reduction of thermal conductivity counterbalances a lowered power factor and, thus, the thermoelectric figure of merit ZT(max) = 0.2 at 760 K is nearly the same for both alpha- and beta-In1-xxIn2S4.