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

Synthesis, Crystal Structure, and Physical Properties of Two Polymorphs of CsGaSe₂, and High-Temperature X‑ray Diffraction Study of the Phase Transition Kinetics

The light gray selenogallate CsGaSe₂-<i>mC</i>64 was obtained by reaction of stoichiometric amounts of CsN₃, GaSe, and Se at elevated temperatures. Its crystal structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic space group <i>C</i>2/<i>c</i> (No. 15) with <i>a</i> = 11.043(2) Å, <i>b</i> = 11.015(4) Å, <i>c</i> = 16.810(2) Å, β = 99.49(1) °, <i>V</i> = 2016.7(8) ų, and <i>Z</i> = 16 (powder data, ambient temperature). Its crystal structure features anionic layers _∞²[Ga₄Se₈^4–] consisting of corner-sharing Ga₄Se₁₀ supertetrahedra. The compound undergoes a first-order phase transition at temperatures of 610 ± 10 °C. The high-temperature phase CsGaSe₂-<i>mC</i>16 also crystallizes in the monoclinic space group <i>C</i>2/<i>c</i> (No. 15) with <i>a</i> = 7.651(3) Å, <i>b</i> = 12.552(4) Å, <i>c</i> = 6.170(3) Å, β = 113.62(4)°, <i>V</i> = 542.9(5) ų, and <i>Z</i> = 4 (powder data, ambient temperature). The crystal structure of the high-temperature phase consists of SiS₂ analogous chains _∞¹[GaSe₂^–]. <i>In situ</i> high-temperature X-ray diffraction experiments were performed to study this phase transition. The crystallization kinetics of the phase transitions were studied using Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory for isothermal crystallization processes. The activation energy of the phase transition was determined using the Arrhenius equation. Furthermore, the compound was studied by vibrational and diffuse reflectance spectroscopy

In Situ X‑ray Diffraction Study of the Thermal Decomposition of Selenogallates Cs₂[Ga₂(Se₂)_2–<i>x</i>Se_2+<i>x</i>] (<i>x</i> = 0, 1, 2)

The selenogallates CsGaSe₃ and Cs₂Ga₂Se₅ release gaseous selenium upon heating. An in situ high-temperature X-ray powder diffraction analysis revealed a two-step degradation process from CsGaSe₃ to Cs₂Ga₂Se₅ and finally to CsGaSe₂. During each step, one Se₂^2– unit of the anionic chains in Cs₂[Ga₂(Se₂)_2–<i>x</i>Se_2+<i>x</i>] (<i>x</i> = 0, 1, 2) decomposes, and one equivalent of selenium is released. This thermal decomposition can be reverted by simple addition of elemental selenium and subsequent annealing of the samples below the decomposition temperature. The influence of the diselenide units in the anionic selenogallate chains on the optical properties and electronic structures was further studied by UV/vis diffuse reflectance spectroscopy and relativistic density functional theory calculations, revealing increasing optical band gaps with decreasing Se₂^2– content