Indium thiospinel In_1-x□_xIn₂S₄- structural characterization and thermoelectric properties

A detailed study of polycrystalline indium-based In_1-x□_xIn₂S₄ (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 α- and β-In_1-x□_xIn₂S₄ were found in samples with 0.16 ≤x≤ 0.22 and x = 0.33 (In₂S₃), respectively. In contrast, it is shown that In_0.72□_0.28In₂S₄ contains both α- and β-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined α- and β-In_1-x□_xIn₂S₄ is conducted for the first time. α-In_1-x□_xIn₂S₄ (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200|μV K^-1) and moderate charge carrier mobility on the level of ∼20 cm² 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 β-In_0.67□_0.33In₂S₄ (β-In₂S₃) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (∼10⁴ cm² V^-1 s^-1 at RT). These findings indicate that structural disorder in the α-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 α- and β-In_1-x□_xIn₂S₄.