Interconversion of One-Dimensional Thiogallates Cs2[Ga2(S2)2−xS2+x] (x = 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 x in the Cs2Sx melts has an influence on the crystalline reaction product. From sulfur-rich melts (x > 7), CsGaS3 is formed, whereas sulfur-poor melts (x < 7) lead to the formation of Cs2Ga2S5. In situ investigations using hightemperature 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-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 S2 2− content. ■ INTRODUCTION Group 13 chalcogenometallates containing alkali metal cations MxTyQz (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. 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 TQ4 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 CsGaQ3, 3,4 Cs2Ga2Q5, 5,6 CsAlTe3 7 and several chalcogenoborates show 1D anionic chains containing Q2 2− or Q3 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. 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 adjusting the sulfur content, as shown by Dürichen and Bensch as well as by Palchik et al. In this paper, we report on the facilitated synthesis of the thiogallates Cs2Ga2S5 5 and CsGaS3, 3 as well as the reactivity of thiogallates in molten polysulfide fluxes and at elevated temperatures. In order to elucidate the product formation in the polysulfide melts, the reactions were investigated in situ by high-temperature Raman spectroscopy. The high-temperature degradation processes of CsGaS3 and Cs2Ga2S5 were further studied in situ by high-temperature X-ray diffraction techniques. Furthermore, the optical properties and the influence of disulfide units on the band gaps of these semiconductors were investigated by UV/vis-diffuse reflectance spectroscopy and DFT calculations. ■ RESULTS AND DISCUSSION As the following investigations all involve the three thiogallates CsGaS3, 3 Cs2Ga2S5, 5 and CsGaS2-mC16, 17 the crystal structures of these solids will be briefly discussed beforehand. Crystallographic data of these substances are listed in Table 1. All three compounds feature polymeric anionic chains as displayed in Figure 1. These chains can be converted into each other by substitution of chalcogenide ions by dichalcogenide ions Received: June 16, 2017 Revised: July 19, 2017 Published: July 31, 2017 Article pubs.acs.org/crystal © 2017 American Chemical Society 4887 DOI: 10.1021/acs.cgd.7b00840 Cryst. Growth Des. 2017, 17, 4887−4892 according to the formula ∞ 1 [Ga2(S2)2−xS2+x 2−] (x = 0, 1, 2). The cesium cations in all three compounds form a cubic diamond analogous topology. During our investigations of the formation of Cs2Ga2S5, we noticed that the crystallization of the product seemed to be favored by the in situ formation of cesium polysulfides when using the alkali metal azide route. This polysulfide was identified as Cs2S6 by Raman spectroscopy (see the Supporting Information, Figure S1). After removal of the polysulfide with N,N′-dimethylformamide (DMF), the powdered samples always contained traces of gallium sulfide as impurities. Furthermore, we never managed to prepare a sample of CsGaS3 by using the alkali metal azide route 5,6,17 or an exact reproduction of the polysulfide-flux synthesis described by Suseela Devi et al. Usage of the alkali azide route always resulted in the formation of CsGaS2-mC16. Therefore, we systematically investigated the synthesis of thiogallates using cesium polysulfide melts. Polysulfide-Flux Reactions. On the basis of the observation that Cs2S6 is involved in the formation of Cs2Ga2S5, we decided to investigate mixtures of Cs2S6 and different gallium sources (Ga, GaS, Ga2S3) with different stoichiometric ratios. These samples were annealed in evacuated quartz glass ampules at 530 °C for 7 days analogous to our synthesis of Cs2Ga2S5. 5 The excess polysulfide was then removed by using DMF or acetone and water. Other batches of the same reaction mixtures were annealed at different temperatures between 300 and 500 °C in order to investigate the influence of the temperature. Ex Situ Investigations. After removal of the excess polysulfide, the dried samples were characterized by using powder X-ray diffraction. Interestingly, the crystalline reaction product in all samples was either Cs2Ga2S5 or CsGaS3. As the same results were obtained regardless of the annealing temperature, the decisive factor for the product formation had to be the stoichiometric composition of the melt. It should be noted, however, that higher temperatures lead to a faster quantitative conversion of the gallium source. Similar investigations of niobates and germanates revealed that the length of the polysulfide chain x in the Cs2Sx melt, specifically the ratio Cs2S/S, is important for the product formation. This fact is also true for the synthesis of the thiogallates under discussion; see Table S1 of the Supporting Information. The ratio Cs2Sx/Ga has no influence on the crystalline reaction product, since gallium is consumed as long as it is available. Therefore, any gallium source can be chosen as long as the Cs2S/S ratio of the melt remains correct. To further test this assumption, additional sulfur was added to the “sulfurpoor” mixtures originally leading to Cs2Ga2S5. For the mixtures with a Cs2S/S ratio < 0.143(2), CsGaS3 was formed instead of Cs2Ga2S5, further confirming the importance of the polysulfide content (more specifically, the basicity and oxidation power) of the melts. On the basis of this knowledge, the possibility of a direct conversion of CsGaS2 or Cs2Ga2S5 to CsGaS3 was tested. For that purpose, new mixtures of Cs2S6 and the ternary compounds were prepared (see Table S1). Additional sulfur had to be added in order to bring the ratio Cs2S/S below 0.14. The samples were annealed at 500 °C for 7 days. Once again, the formation of CsGaS3 was observed in the sulfur-rich melts (Cs2S/S < 0.143), whereas Cs2Ga2S5 was obtained from the sulfur-poor mixtures (Cs2S/S > 0.143). The same observation was made using either polymorph of CsGaS2 as gallium source for the polysulfide fluxes. In Situ X-ray Investigations. We employed diffraction techniques in order to investigate the processes in the molten polysulfides, especially the formation of the crystalline products, in situ. A study of the reaction using high-temperature X-ray powder diffraction, however, failed due to technical difficulties. These measurements were performed on a STOE Stadi P diffractometer equipped with a capillary furnace, and a capillary in vertical orientation. Upon melting, the samples moved out of the incident X-ray beam and were thus no longer accessible for the in situ experiments. We therefore decided to use a different approach after encountering these difficulties. In Situ Raman Spectroscopy. Raman spectroscopy turned out to be a suitable technique to study the processes in cesium polysulfide melts. The unambiguous assignment of the observed vibrational frequencies was possible since all crystalline compounds (Cs2S6, Ga2S3, CsGaS2-mC16, Cs2Ga2S5, and CsGaS3) could be characterized beforehand (see the Supporting Information, Figures S2−S5). The spectra for Cs2Ga2S5 (Figure S2) and CsGaS2-mC16 (Figure S3) are identical to our previously reported spectra. No Raman spectrum of CsGaS3 (Figure S4) has been published prior to this work. The vibration spectra of Cs2S6 (Figure S5) are in good agreement with the values reported by Ziemann et al. and the calculations performed by Steudel et al. Furthermore, we were able to overcome the difficulties observed in the X-ray diffraction experiments by constructing a special heatable sample holder. This holder allowed the usage of the same quartz glass ampules, which were used for the ex situ investigations (diameter 5 mm). A construction scheme of the sample holder and the spectrometer are shown in the Supporting Information in Figures S6 and S7. Details on the components in the spectrometer are given in the Experimental Section. Table 1. Crystallographic Data for CsGaS3, 3 Cs2Ga2S5, 5 and CsGaS2-mC16 17 CsGaS3 Cs2Ga2S5 CsGaS2-mC16 crystal system monoclinic monoclinic monoclinic space group P21/c C2/c C2/c a/Å 7.558(3) 12.586(1) 7.388(1) b/Å 12.502(7) 7.143(1) 12.128(1) c/Å 6.411(5) 12.344(1) 5.899(1) β/deg 107.75(4) 108.34(1) 113.21(1) V/Å 576.9(6) 1053.6(1) 458.8(3) Z 4 4 4 ρ/g·cm−3 3.440 3.564 3.647 Figure 1. Polymeric thiogallate chains in (a) CsGaS2-mC16, (b) Cs2Ga2S5, and (c) CsGaS3. Crystal Growth & Design Article DOI: 10.1021/acs.cgd.7b00840 Cryst. Growth Des. 2017, 17, 4887−4892 4888 The main goal of these in situ experiments was to determine the moment the crystalline reaction product is formed (upon heating, during the annealing, upon cooling, etc.). Furthermore, we wanted to study the formation of small building blocks, i.e., GaS4 5− tetrahedra, in the melt prior to crystallization. The investigated samples were binary mixtures of Cs2S6 and Ga2S3 with a stoichiometric ratio of 2:1 and 5:1. These mixtures reproducibly yielded CsGaS3 and Cs2Ga2S5, respectively. The starting materials were thoroughly ground to ensure optimum intermixture. Furthermore, the experiments were performed in the temperature region from 20 to 400 °C. This procedure should not affect the final product as the ex situ experiments showed. During all three stages of the polysulfide reaction (heating, annealing, and cooling), several Raman spectra were measured. No change of the vibrational bands was observed upon heating of the powdered samples below the melting point of Cs2S6. Above the melting point of Cs2S6, only weak bands of vibrations assigned to Ga2S3 were detected. The general shape of the spectrum did not change during annealing of the samples at 400 °C. After 24 h, the heater was turned off and spectra were measured in intervals of 1 min (temperature was manually registered). The excessive molten polysulfide started to crystallize, and the corresponding vibrational bands of Cs2S6 were observed during this cooling period. The spectra, however, contained additional vibrational bands not originating from either Cs2S6 or Ga2S3 (Figure 2). A comparison of the Raman spectra of the reaction mixture (Cs2S6 + Ga2S3) with the spectra of the pure product phases revealed that crystalline CsGaS3 and Cs2Ga2S5 were formed in the respective sample (Figure 2). X-ray diffraction patterns of the samples showed an almost complete conversion of the gallium source, as only weak reflections of residual Ga2S3 were observed. This is likely due to the relatively short reaction times during the experiments, as pure samples could be obtained after longer annealing. The Raman spectroscopic investigations revealed no additional details on the processes taking place in the polysulfide melts. However, we managed to identify one important fact of these polysulfide-flux reactions. The crystalline product Cs2Ga2S5 or CsGaS3 does not form during annealing of the polysulfide melt. The product rather crystallizes upon cooling below the melting point of the flux. Due to the increasing intensities of the product bands in the course of cooling, one can assume that the amount of the product phase increases. Summary. The directed synthesis of the thiogallates Cs2Ga2S5 and CsGaS3 is possible in cesium polysulfide fluxes. The crucial parameter determining which of these two products is formed is the length of the polysulfide chains x in the Cs2Sx melts. This is represented by the ratio Cs2S/S (Table S1). The chosen gallium source has no influence on the crystalline reaction product. Therefore, even the conversion of ternary compounds like CsGaS2 is possible. Furthermore, the yield is only limited by the amount of the gallium source, as long as enough cesium polysulfide is provided. High-Temperature Degradation. After the successful preparation of pure samples of Cs2Ga2S5 and CsGaS3, we decided to study the thermal behavior of these compounds. A differential thermal analysis (DTA) was performed in order to determine the melting points of these compounds. However, no thermal effect was observed in the temperature region from 20 to 1000 °C in both cases. However, it is quite unlikely that polymeric thiogallates with disulfide units are stable to such high temperature. Therefore, we investigated the substances using high-temperature X-ray powder diffraction techniques. After the experiments, small amounts of elemental sulfur could be observed in all capillaries. Upon heating, a change in the diffraction pattern of both compounds can be observed. The full decomposition pathway can be observed for the compound CsGaS3 (Figure 3). Figure 2. (a) Comparison of the Raman spectra of the reaction mixture (Cs2S6 + Ga2S3) with the spectra of pure CsGaS3 and Cs2Ga2S5, respectively. (b) Series of measurements during cooling of a mixture of Cs2S6:Ga2S3 = 2:1. (c) Series of measurements during cooling of a mixture of Cs2S6:Ga2S3 = 5:1. The weak peak at ∼540 cm−1 results from the sample holder or the spectrometer and is present in all spectra. Crystal Growth & Design Article DOI: 10.1021/acs.cgd.7b00840 Cryst. Growth Des. 2017, 17, 4887−4892 4889 The compound starts to decompose to Cs2Ga2S5 at temperatures above ca. 400 °C. Cs2Ga2S5 is the sole detectable crystalline phase in the temperature region from 440 to 540 °C. If the temperature is raised above 540 °C, the compound decomposes to CsGaS2-mC16. This phase is stable up to the melting point of about 1130 °C. One sulfur atom of the disulfide units S2 2− is removed from the anionic chains during each degradation step. Even though the crystal structures of CsGaS3, Cs2Ga2S5, and CsGaS2-mC16 seem to be related (Table 1), it is not possible to transform the atomic sites of the three structures using allowed crystallographic transformations. Furthermore, no intermediate crystalline or amorphous phase was detected. The high-temperature decomposition of CsGaS3 was also investigated in the temperature region from 25 to 800 °C by using thermogravimetric analysis (TGA). This analysis also revealed a two-step degradation of the compound (Supporting Information, Figure S8) with weight losses of 4.6% and 7.1% (calculated loss of 1 equiv of S = 5.3% based on the mass of CsGaS3). The temperatures for the thermal degradation of CsGaS3 determined by different methods slightly differ, which is attributed to different heating rates and experimental environments. Overview. When the results of the polysulfide-flux syntheses and the high-temperature decomposition are combined, a “cycle” for the interconversion of the compounds CsGaS2-mC16, Cs2Ga2S5, and CsGaS3 can be drawn (Figure 4). Optical Properties. The optical band gaps of the thiogallates CsGaS3, Cs2Ga2S5, and CsGaS2-mC16 were reported as 3.00, 3.26, and 3.27 eV, respectively. During our investigations, however, we used our newly attained knowledge on the synthesis of these thiogallates, and we prepared pure samples of these compounds. Optical band gaps were thus redetermined by diffuse reflectance spectroscopy. The absorption data were calculated using a modified Kubelka−Munk function (Figure S9). Table 2 lists the band gaps of all three compounds. We previously reported a value of 3.26 eV for the band gap of Cs2Ga2S5, which most likely resulted from impurities of CsGaS2-mC16. This can be explained with a partial decomposition of the sample upon annealing at 530 °C. The reported optical band gap of 3.00 eV for CsGaS3 by Suseela Devi et al. possibly originates from α-Ga2S3 (see Figure S10), which was used as a starting material for the synthesis of CsGaS3. 3 The electronic structures of these thiogallates were analyzed by relativistic DFT calculations using the generalized gradient approximation (GGA) according to Perdew−Burke−Ernzerhof (PBE). For the total energy and band structure calculations based on the experimentally determined structures, the fullpotential local orbital code FPLO was applied. The calculated band gaps are listed in Table 2. The orbital projected density of states (PDOS) for CsGaS3, Cs2Ga2S5, and CsGaS2-mC16 is shown in Figure 5. As the bonding behavior in Cs2Ga2S5 and CsGaS2-mC16 was already discussed beforehand, 5,17 only a brief comparison shall be presented. The bonding character of Cs in these compounds is mainly ionic, which can be concluded from the unoccupied Cs-6s states. Therefore, the alkali metal does not influence the band gap in these structures. Ga−S interactions within the GaS4 tetrahedra of all three thiogallates cause a splitting into valence and conduction band. The band gaps are mostly determined by the energies of the S-3p states. The influence of the sulfur ions Figure 3. Evolution of the X-ray powder pattern during thermal decomposition of CsGaS3 in the temperature region from 20 to 800 °C (Mo-Kα1 radiation). Figure 4. Schematic representation of the different possibilities of interconversion of the thiogallates CsGaS3, Cs2Ga2S5, and CsGaS2mC16 using the high-temperature decomposition route (red arrows) and the polysulfide route (orange arrows). Table 2. Experimental Optical Band Gaps of CsGaS3, Cs2Ga2S5, and CsGaS2-mC16. 17 The Calculated Gaps (DFT, GGA) Are Also Listed compound experimental value (eV) calculated value (eV)