Interconversion of One-Dimensional Thiogallates Cs₂[Ga₂(S₂)_2–xS_2+x] (x = 0, 1, 2) by Using High-Temperature Decomposition and Polysulfide-Flux Reactions

Abstract

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 x in the Cs 2 S x melts has an influence on the crystalline reaction product. From sulfur-rich melts (x > 7), CsGaS 3 is formed, whereas sulfur-poor melts (x < 7) lead to the formation of Cs 2 Ga 2 S 5 . In situ investigations using hightemperature Raman spectroscopy revealed that the crystallization of these solids takes place upon cooling of the melts. Upon heating, CsGaS 3 and Cs 2 Ga 2 S 5 release gaseous sulfur due to the degradation of S 2 2− units. This decomposition of CsGaS 3 to Cs 2 Ga 2 S 5 and finally to CsGaS 2 -mC16 was further studied in situ 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 S 2 2− content. ■ INTRODUCTIONGroup 13 chalcogenometallates containing alkali metal cations M x T y Q z (M = alkali metal, T = triel, Q = chalcogen) are interesting materials due to their semiconducting properties. These materials are used, e.g., in gas sensors or detectors for high-energy radiation. 1 The crystal structures of most compounds in the ternary systems M−T−Q consist of oligomeric or polymeric one-, two-, or three-dimensional (1D, 2D, 3D) anions formed by condensed TQ 4 tetrahedra. 2 Among the large number of chalcogenotrielates, however, only a few compounds featuring polychalcogenide units are known. Incorporation of polychalcogenide units into the anionic structures leads to more exotic structural motives. The crystal structures of mixed-valent chalcogenometallates like CsGaQ 3 , 3,4 Cs 2 Ga 2 Q 5 , 5,6 CsAlTe 3 7 and several chalcogenoborates 8−11 show 1D anionic chains containing Q 2 2− or Q 3 2− units (Q = S, Se). The mixed-valent nature of the chalcogen in these compounds should significantly influence the properties of these semiconductors. Therefore, it is necessary to understand the reactive behavior of these compounds and the conditions necessary for their formation in order to design compounds with desired properties. Molten polysulfide fluxes are an established method for the synthesis of substances featuring polysulfide anions. 12−14 Due to an enhanced diffusion rate at comparably low temperatures in these fluxes, the formation of thermodynamically metastable compounds is favored as compared to high-temperature reactions. Furthermore, the properties of these fluxes (basicity, viscosity, etc.) can be fine-tuned by adjusti...

Influence of Alkali Metal Substitution on the Phase Transition Behavior of CsGaQ2 (Q = S, Se)

“…This observation can easily be interpreted by DFT simulations performed by Feng et al for KGaSe 2 [8] and by us for the cesium compounds [6,7,13] and RbGaSe 2 [11]. While these calculated values are in good agreement with experimental data for the sulfides, significant deviations (>1 eV) between the experimental and calculated band gaps were obtained for all selenides [7][8][9][10][11].…”

Layered and Cubic Semiconductors AGaM′Q4 (A+ = K+, Rb+, Cs+, Tl+; M′4+ = Ge4+, Sn4+; Q2– = S2–, Se2–) and High Third-Harmonic Generation

“…In the monoclinic polymorphs A Ga M ′ Q 4 - mP 56, this 9-fold coordination is more akin to a 7 + 2 fold coordination with seven shorter distances d ( A - Q ) < 4 Å and two slightly longer distances 4 Å < d ( A - Q ) < 4.2 Å. All the observed distances in these compounds are in good agreement with already known AMM ′ Q 4 compounds ,, and binary and ternary gallium, − germanium, ,− and tin − compounds, respectively.…”

In Situ X-ray Diffraction Study of the Thermal Decomposition of Selenogallates Cs2[Ga2(Se2)2–xSe2+x] (x = 0, 1, 2)

“…Similar to other chalcogenotrielates investigated by us ,,,− or other groups, , the alkali metal does not significantly influence the optical band gaps of these compounds, as the unoccupied Cs-6 s states indicate a mainly ionic bonding between Cs + and the chalcogenometalate units. Interactions of gallium and selenium within the GaSe 4 tetrahedra lead to a splitting into valence and conduction band.…”

“…Even though most crystalline phases in these systems contain the elements in their most stable oxidation states, a small number of mixed-valent indates and compounds containing polychalcogenide units are known. − Incorporation of such polychalcogenide units into the anionic structures leads to the formation of more complex, uncommon one-dimensional structures like CsGa Q 3 , , Cs 2 Ga 2 Q 5 , , CsAlTe 3 , several per chalcogenoborates − and -antimonates, − or the neutral chains in P 2 S 7 . The polychalcogenide units Q 2 2– or Q 3 2– in such compounds have a significant influence on the semiconducting properties of these substances …”

“…We recently reported on our investigations of ternary thiogallates CsGaS 3 , Cs 2 Ga 2 S 5 , and CsGaS 2 . , The crystal structures of these compounds contain anionic chains ∞ 1 [Ga 2 (S 2 ) 2– x S 2+ x 2– ] ( x = 0, 1, 2), which release gaseous sulfur upon heating due to decomposition of the disulfide units . An in situ high-temperature X-ray diffraction analysis of this process revealed a stepwise degradation from CsGaS 3 to Cs 2 Ga 2 S 5 to CsGaS 2 - mC 16.…”