Interconversion of One-Dimensional Thiogallates Cs₂[Ga₂(S₂)_2–<i>x</i>S_2+<i>x</i>] (<i>x</i> = 0, 1, 2) by Using High-Temperature Decomposition and Polysulfide-Flux Reactions

The potential of cesium polysulfide-flux reactions for the synthesis of chalcogenogallates was investigated by using X-ray diffraction and Raman spectroscopy. An investigation of possible factors influencing the product formation revealed that only the polysulfide content <i>x</i> in the Cs₂S_<i>x</i> melts has an influence on the crystalline reaction product. From sulfur-rich melts (<i>x</i> > 7), CsGaS₃ is formed, whereas sulfur-poor melts (<i>x</i> < 7) lead to the formation of Cs₂Ga₂S₅. <i>In situ</i> investigations using high-temperature Raman spectroscopy revealed that the crystallization of these solids takes place upon cooling of the melts. Upon heating, CsGaS₃ and Cs₂Ga₂S₅ release gaseous sulfur due to the degradation of S₂^2– units. This decomposition of CsGaS₃ to Cs₂Ga₂S₅ and finally to CsGaS₂-<i>mC</i>16 was further studied <i>in situ</i> by using high-temperature X-ray powder diffraction. A combination of the polysulfide reaction route and the high-temperature decomposition leads to the possibility of the directed interconversion of these thiogallates. The presence of disulfide units in the anionic substructures of these thiogallates has a significant influence on the electronic band structures and their optical properties. This influence was studied by using UV/vis-diffuse reflectance spectroscopy and DFT simulations, revealing a trend of smaller band gaps with increasing S₂^2– content