Crystal structure, phase transition and properties of indium(III) sulfide
Poly- and single-crystalline samples of In0.67□0.33In2S4 thiospinel were obtained by various powder metallurgical and chemical vapor transport methods, respectively. All synthesized samples contained β-In0.67□0.33In2S4 modification only, independent from synthesis procedure. High-resolution powder X-ray diffraction (PXRD) experiments at 80 K enabled observation of split tetragonal reflections (completely overlapped at room temperature), which proves the correctness of the crystal structure model accepted for the β-polymorph. Combining single-crystal XRD, transmission electron microscopy and selected-area electron diffraction, the presence of three twin domains in as-grown crystals is confirmed. A high temperature PXRD study revealed both abrupt (in full width at half maximum) and gradual (in intensity of satellites, c/a ratio and unit-cell volume) changes in the vicinity of the α-β phase transition. On the other hand, a clear thermal effect in heat capacity, the magnitude of enthalpy/entropy change and the temperature dependence of electrical resistivity, associated with hysteresis, hints towards the 1st order type of the transition. Three scenarios, based on Rietveld refinement analysis, are considered for the description of the crystal structure evolution from β- to α-modification, including the (3+3)D-modulated cubic structure at 693 K as an intermediate state during the β-α transformation. Seebeck coefficient, electrical resistivity and thermal conductivity are not only influenced by phase transition, but also by annealing conditions (S-poor or S-rich atmosphere). Density functional calculations predicts n-type semiconducting behavior of In0.67□0.33In2S4, as well as instability of a fictitious InIn2S4 thiospinel.