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[Journal of Crystal Growth 647 (2024) 127878_R.pdf](https://mdr.nims.go.jp/filesets/e50c1a31-b2cc-4ffd-81dc-9a6f4d6a4a5c/download)

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[Masaru Nakamura](https://orcid.org/0000-0001-9729-847X), [Yoshitaka Matsushita](https://orcid.org/0000-0002-4968-8905)

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[Optical transmittance range of High-Temperature stable phase germanium dichalcogenide (GeS2) single crystal grown by the Bridgman method](https://mdr.nims.go.jp/datasets/14a807dd-5d5e-4310-9240-8f787be6c261)

## Fulltext

1  Optical Transmittance Range of High-Temperature Stable Phase Germanium Dichalcogenide (GeS2) Single Crystal Grown by the Bridgman Method  Masaru Nakamura1*, Yoshitaka Matsushita1 National Institute of Materials Science (NIMS), 1-1 Namiki, Tukuba, Inaraki 305044, Japan  Corresponding Author *Masaru Nakamura, E-mail address: nakamura.masaru@nims.go.jp.      mailto:nakamura.masaru@nims.go.jp2  ABSTRACT A high-temperature (HT) stable phase germanium dichalcogenide (GeS2) was grown in an evacuated quartz ampoule to investigate its transparency range. First, as a starting material for the crystal growth of HT-GeS2, GeS2 glass containing HT-GeS2 crystals was synthesized using our previously improved process for synthesizing polycrystalline sulfide compounds in a conventional horizontal furnace. The HT-GeS2 crystal was grown using the Bridgman method under a temperature gradient of 20–30 ℃/cm. The grown HT-GeS2 single crystal was easily cleaved along the growth direction, and this cleaved surface was identified as the (001) face using X-ray diffraction (XRD). Single-crystal XRD analysis confirmed the growth of an HT-GeS2 single crystal with a monoclinic structure (space group P21/c) and lattice parameters of a = 6.7061(4) Å, b = 16.0877(13) Å, and c = 11.4242(11) Å, and β = 91.069 (8)°. The optical transmittance spectra of the HT-GeS2 single crystal were measured in the visible and infrared (IR) regions, revealing a transparency range of 0.36–22.5 μm. Differential thermal analysis revealed the melting point of HT-GeS2 as 841 ± 1 ℃. The HT-GeS2 single crystal is expected to be a new candidate for IR optical crystal. Keywords: A2. Bridgman technique, Growth from melt, Single crystal growth, B1. Sulfides, B3. Infrared devices 3  1. Introduction The demand for infrared (IR) light sources (wavelength: 2.5 to 25 µm) is surging in various fields such as medicine, environmental measurement, materials processing, and optical communications. Materials capable of transmitting in the far-IR region above 10 µm are mainly limited to halogenated (nonfluoride) and chalcogenated materials. A zinc telluride (ZnTe), zinc selenide (ZnSe), and zinc sulfide (ZnS) crystals are promising candidate crystals in this region (transparency region: 0.59–25 µm (ZnTe) [1], 0.47–19 µm (ZnSe) [1], and 0.35–23 µm (ZnS) [1]). However, these materials do not melt at ambient pressure, making the growth of crystals from the melt difficult. High-pressure and high-temperature (HT) growth conditions are required to grow these crystals from melts [2]. Se and Te are present in small amounts in the environment, whereas S is abundant. Therefore, we focused on the sulfide crystals, particularly GeS2 crystals. GeS2 has been studied for many years. In 1886, Winkler successfully synthesized GeS2 for the first time [3]. Phase diagrams of the Ge–S system have been published [4–7]. Based on these phase diagrams, GeS2 is found to melt congruently at 821 ℃ [6] and 840 ℃ [4] at ambient pressure. In addition, GeS2 has been found to occur in the phase transition at 600 ℃ [4,6] from the HT to the low-temperature (LT) stable phase. However, there are 4  few studies on the physical properties of GeS2 crystals, although there are many studies on glasses in the Ge–S system. In addition, fewer studies have been conducted on HT-GeS2 crystals were fewer compared to LT-GeS2 crystals. Transparency range is one of the most important parameters in IR applications. However, data on the transparency range of HT-GeS2 crystal have not been published. Although these two papers described the crystal growth of the HT-GeS2 single crystal using the Bridgman method, transmittance data near the IR region of the HT-GeS2 crystal have not been reported. In these two studies, the optical transmittance of the HT-GeS2 single crystals was measured in the range near the absorption edge, owing to the bandgap [8,9].  Therefore, in this study, we investigated the transmittance range of an HT-GeS2 single crystal. First, as a starting material for the crystal growth of HT-GeS2, GeS2 glass containing HT-GeS2 crystals was synthesized using our previously improved process for the synthesis of polycrystalline sulfide compounds in a conventional horizontal furnace. A single crystal of HT-GeS2 was grown in an evacuated quartz ampoule using the Bridgman method. The optical transmittance spectra of the HT-GeS2 single crystal were measured in the visible and IR regions.  5  2. Experimental Procedure 2.1. Preparation of Starting Material for Crystal Growth of HT-GeS2  First, we synthesized a GeS2 glass, containing GeS2 crystals, as the starting material for the crystal growth of HT-GeS2. This process is similar to our previously improved method for synthesizing sulfide polycrystals [10,11]. The raw materials used for this process were commercially available Ge metal of (99.999%) 5N equivalent purity (resistivity > 50 Ω·cm, Mitsubishi Materials Electronic Chemicals Co. Ltd. Akita, Japan) and purified S powder. A commercially available S powder (reagent-grade S powder; Kanto Chemical Co. Inc. Tokyo, Japan) was purified by a three-cycle distillation process developed in-house [12]. The compositional ratio of the preparation was Ge:S = 33.3 mol% : 66.7 mol% (stoichiometric composition), for a total of 5.0 g. The raw materials were placed in a quartz ampoule with an outer diameter of 13 mm, evacuated to approximately 10-3 Pa using a diffusion pump, and vacuum-sealed. The vacuum-sealed quartz ampoule was approximately 170 mm long and more than half the length of the furnace. Figure 1 shows the schematic of the equipment used to prepare the starting material for the crystal growth of HT-GeS2. These materials in the ampoule were pulled to one side of the ampoule and this part was placed in a single-zone horizontal furnace at 6  approximately 1080 ℃ in the center. Immediately after insertion, the sulfur evaporated, and its vapor moved in the opposite direction of the ampoule, which was outside the furnace; thus, the section was water-cooled to reduce its vapor pressure. The reaction between Ge and S started after approximately 1 h. The content of sulfur vapor decreased as the reaction between Ge and S progressed; thus, the ampoule was gradually inserted deeper into the furnace. When the reaction was complete, the entire ampoule was placed in a furnace, and water cooling was completed. The reaction between Ge and S completed after approximately 1 h from its initiation. Notably, the quartz ampoule did not crack during this water-cooling process. The temperature was then lowered to about 1000 °C and held for about 12 h, after which the ampoule was removed from the furnace and quenched to room temperature. The sample synthesized by this process was characterized using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM).  2.2. Growth of HT-GeS2 Single Crystal  Once the identification of the samples synthesized by the above method was complete, the quartz ampoule with the pointed tip for crystal growth (outer diameter 13 mmφ) was used to synthesize the above samples. Then, the ampoule was shortened as 7  much as possible and used for crystal growth after the sample synthesis to eliminate the need to transfer the synthesized sample from the synthesis ampoule to the growth ampoule. The HT-GeS2 single crystals were grown using the vertical Bridgeman method. The growth temperature was based on the reported melting point of the phase diagrams (821℃ [6] and 840 ℃ [4]). The ampoule was set at the center of the furnace at room temperature. The furnace temperature was elevated to approximately 935 ℃ and held for approximately 4 h to achieve a homogeneous melt. The temperature was lowered to 900 ℃ before crystal growth began. Based on these reported melting points, the temperature gradient for the growth furnace was determined to be 20–30 °C/cm. The growth conditions were a pull-down speed of 0.5–1.0 mm/h, no rotation, and no seed crystal. At the end of the crystal-growth process, the furnace temperature was maintained, and the ampoule was removed from the furnace to room temperature to suppress the phase transition from an HT stable phase to an LT stable phase.  2.3. Characterization Powder XRD was used to identify the synthesized sample as the starting material for crystal growth. The diffractogram was recorded on a MiniFlex (Rigaku, Tokyo, Japan) X-ray diffractometer, using Cu Kα radiation at 40 kV and 15 mA, in the 2θ scan range of 8  5°–90° at a scan rate of 2°/min and scan step of 0.02°. This diffractometer was also used to perform an XRD analysis of a platelet crystal with a cleaved surface to identify the face of the cleaved surface. SEM (JSM-6500F, JEOL Ltd., Tokyo, Japan) and SEM-energy-dispersive X-ray spectrometry (SEM-EDX; JED-2300, JEOL) analyses were performed to identify the starting material for crystal growth. Single-crystal XRD was performed to precisely analyze the crystal structures of the grown crystals. Data collection for crystal structure analysis was performed at 296 K using a RIGAKU Synergy Custom DW diffractometer with Mo K radiation ( = 0.71073 Å). Cell refinement and data reduction were performed using the d*trek package of the CrystalClear software suite [13]. Preliminary structures were solved using SHELXT [14] and refined by full-matrix least-squares on F2 using SHELXL-2016 [15] in the WinGX program package [16]. The optical transmission spectra of the grown single crystal were measured at room temperature using an ultraviolet (UV)–visible–near-IR spectrophotometer (Lambda 900, PerkinElmer, USA) and a Fourier transform IR spectrometer (FT/IR-6800, JASCO, Tokyo, Japan). A 0.1-mm-thick sample with cleaved surfaces on both sides was used for these measurements. 9  Finally, the melting point of HT-GeS2 was confirmed using a grown HT single crystal by differential thermal analysis (DTA). The melting point is the most important information regarding crystal growth. Next, 0.3405 g of the grown HT-GeS2 single crystal was vacuum-sealed in a quartz ampoule with an outer diameter of 8.0 mm in the same manner as described for the synthesis process. The heating rate for the DTA measurement was 10 °C/min up to 1000 °C, and an empty quartz ampoule was used as a reference. The details of the in-house apparatus used for DTA measurements are described in our previous paper [11].  3. Results and Discussion The synthesized sample used as the starting material for the crystal growth of HT-GeS2 is shown in Figure 2. The overall color was transparent yellow, with brown dots scattered throughout. The powder XRD patterns of the sample are shown in Figure 3. The XRD pattern without the brown dots (red line) shows no diffraction peaks, indicating an amorphous (glass) phase. In contrast, the XRD pattern with brown dots shows a diffraction peak (black line), indicating the crystalline phase and the peak position is consistent with the 002 reflection from the HT-GeS2 phase. In Figure 3, the observed powder XRD pattern 10  differs from the ideal powder XRD pattern. This phenomenon is probably due to the strong preferential orientation during powdering. Because the GeS2 crystal has a layered structure, this phenomenon seems plausible. Therefore, GeS2 has attracted considerable attention as a two-dimensional (2D) material [17–19]. In addition, SEM observations were conducted to investigate this sample; an SEM image of the sample is shown in Figure 4, in which the dendritic shape is considered as a brown dot. To determine the compositional difference between the dendritic shape and the locations not included in the dendritic shape, SEM-EDX analysis was performed. The composition of the dendritic shape was found to be the same as that of the locations not included in the dendritic shape, indicating that the dendritic shape represented an HT-GeS2 dendritic crystal. These results indicate that the GeS2 glass containing the HT-GeS2 crystals was the synthesized sample used as the starting material for the crystal growth of HT-GeS2. No oxide phase was detected within the accuracy of the powder XRD measurements. By contrast, the presence of oxygen in the sample not be precisely estimated using SEM-EDX because the sample could not be placed in the SEM-EDX apparatus without exposing it to the atmosphere. The grown HT-GeS2 crystal cleaved along the growth direction is shown in Figure 5. The grown crystals were brown and transparent. Because a photograph of HT-11  GeS2 crystals has not yet been reported, we cannot compare the feathers of the HT-GeS2 crystal. No cracks were observed in the crystal despite quenching from a temperature close to the growth temperature to room temperature to suppress the transition from the HT phase to the LT phase. The grown crystals were easily cleaved. This phenomenon may be natural because GeS2 has attracted considerable interest as a 2D material [17–19]. In addition, the grown crystals could be easily removed from the quartz ampoule, even though no carbon-coated quartz ampoule was used. The cleavage plane extends from the early stage of crystal growth to the end of crystal growth, suggesting that the crystal is a single crystal. The XRD pattern of the cleaved surface is shown in Figure 6. Since only 00l peaks were observed, the cleavage plane was determined to be the (001) plane. This result shows that HT-GeS2 single crystals can be grown without using seed crystals and that the growth direction is parallel to the (001) plane. Although the temperature gradient of 20–30 °C/cm in this growth condition was gentler than those reported in previous studies (180 [8] and 45 °C/cm [9]), the HT-GeS2 single crystal could be grown without a seed crystal. This feature is advantageous for growing large crystals. The details of the crystal data, data collection, and structure refinement from the single-crystal XRD analysis are summarized in Table 1. The fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) and geometric 12  parameters (Å, °) are given in Tables 2 and 3, respectively. These results indicate that the grown crystal is the HT-GeS2 single crystal with the monoclinic structure (space group P21/c) and lattice parameters of a = 6.7061(4) Å, b = 16.0877(13) Å, c = 11.4242(11) Å and β = 91.069 (8)°, which are almost identical to those of a previously reported structure [20,21]. The optical transmittance spectra of the HT-GeS2 single crystal in the visible and IR regions are shown in Figure 7(a) and (b), respectively. From these data, the transmittance range of the HT-GeS2 single crystal is found to be 0.358–22.5 μm, except for the absorption near 12.5 μm. This transparency range of the HT-GeS2 is found to be comparable to those of ZnTe (0.59–25 µm), ZnSe (0.47–19 µm), and ZnS (0.35–23 µm) [1]. In addition, the results of the crystal structure analysis show that the space group of this crystal is not centrosymmetric; therefore, it is expected to have nonlinear optical effects in the IR region. From Figure 7(a), the band gap of GeS2 was estimated to be Eg = 3.46 eV as determined by UV absorption (358 nm) (thickness: 100 μm). This value of the band gap is slightly larger than those previously reported (Eg = 3.2 eV (thickness: 14 μm) [8] and Eg = 3.092, 3.245 eV (thickness: 50 μm) [9]); this difference may be attributed to the purity of the samples. Golubkov et al. [8] described a moderate degree of amorphization of the sample, whereas Wu et al. [9] demonstrated the growth of HT-13  GeS2 and LT-GeS2 crystals. These data therefore suggest that the HT-GeS2 single crystal is a promising new optical single-crystal candidate for use in the IR region. The cause of this absorption is yet to be elucidated. Finally, the DTA thermogram obtained using the HT-GeS2 single crystal is shown in Figure 8. In this figure, only one endothermic peak is observed. This peak is thought to be associated with the melting of the HT-GeS2 single crystal. The melting point of HT-GeS2 is determined to be 841 ± 1 °C; Ge–S diagrams of two melting point datasets were published in 1990 (840 °C) [4] and 1963 (821 °C) [6]. This value is in good agreement with the Ge–S diagram data reported by Okamoto [4]. The precise melting point data of HT-GeS2 will be useful for the crystal growth of HT-GeS2.  4. Conclusion To investigate the transparency range of HT-GeS2, an HT-GeS2 single crystal was grown in an evacuated quartz ampoule. First, the GeS2 glass contained in the HT-GeS2 crystals was synthesized as the starting material for crystal growth using our previously improved process for synthesizing sulfide polycrystals. The single crystal of HT-GeS2 was grown by the Bridgman method under a temperature gradient of 20–30 ℃/cm. The grown HT-GeS2 single crystal was easily cleaved along the growth 14  direction, and this cleaved surface was identified as the (001) face using XRD. Single-crystal XRD was carried out to precisely determine the crystal structure, revealing that the grown crystal was the HT-GeS2 single crystal with the monoclinic structure (space group P21/c) and lattice parameters of a = 6.7061(4) Å, b = 16.0877(13) Å, and c = 11.4242(11) Å, and β = 91.069 (8)°. The optical transmittance spectra of the HT-GeS2 single crystal were measured in the visible and IR regions, revealing a transparency range of 0.36–22.5 μm. The HT-GeS2 single crystal is expected to be a new candidate for optical crystals in the IR region. DTA confirmed that the melting point of the as-grown HT-GeS2 single crystal was determined to be 841 ± 1 °C. In the near future, we would like to clarify the cause of the absorption near the 12.5 μm in the optical spectra of the HT-GeS2 single crystal, eliminate this absorption, and investigate nonlinear optical effects.  Acknowledgment This work was supported by JSPS KAKENHI (grant number 24K08274). We would like to thank Ms. Miwako Takano of NIMS for the SEM-EDX analysis.   References 15  [1] F. Charra, G. Gurzadyan, In Nonlinear Dielectric Susceptibilities; Landolt-Börnstein, Group III Condensed Matter, Vol. 30B; Nelson, D. F., Eds.; SpringerMaterials, Springer-Verlag, Berlin, Heidelberg, 2000. DOI: https://doi.org/10.1007/10134958_42 [2] T. Hattori, Y. Homma, A. Mituishi, M. Tacke, Indices of Refraction of ZnS, ZnSe, ZnTe, CdS, and CdTe in the Far Infrared. Opt. Comm. 1973, 7, 229 – 232. DOI: https://doi.org/10.1016/0030-4018(73)90015-1 [3] C. Winkler, Mittheilungen über das Germanium, J. für Praktische Chem. 1886, 34, 177–229. DOI: https://doi.org/10.1002/prac.18860340122 [4] H. Okamoto, Ge–S (Germanium–Sulfur) Binary Alloy Phase Diagrams. In ASM Handbook Alloy Phase Diagrams, Vol. 12; Materials Park, OH, USA, 1990; pp. 3528–3531, ASM International, ASM Alloy Phase Diagrams Database. https://matdata.asminternational.org/apd/, 2023 [5] W. Viaene, G. H. Moh, Das Binäre System Germanium-Schwefel und p-T-Relationen im Druckbereich bis 5 kba r. Neues Jahrb. Mineral. Abh. 1973, 119, 113–144, ASM International, ASM Alloy Phase Diagrams Database. https://matdata.asminternational.org/apd/, 2023 https://doi.org/10.1007/10134958_42https://ui.adsabs.harvard.edu/link_gateway/1973OptCo...7..229H/doi:10.1016/0030-4018(73)90015-1https://onlinelibrary.wiley.com/authored-by/Winkler/Clemenshttps://onlinelibrary.wiley.com/journal/15213897bhttps://doi.org/10.1002/prac.18860340122https://matdata.asminternational.org/apd/https://matdata.asminternational.org/apd/16  [6] C. H. Liu, A. S. Pashinkin, Novoselova, A. V. An Investigation of the Germanium-Sulphur System, Dokl. Akad. Nauk SSSR 1963, 151, 1335–1338, ASM International, ASM Alloy Phase Diagrams Database. https://matdata.asminternational.org/apd/, 2023 [7] H. Okamoto, Ge–S (Germanium–Sulfur) J. Phase Equilib. 2002, 23, 194, ASM International, ASM Alloy Phase Diagrams Database. https://matdata.asminternational.org/apd/, 2023 [8] A. V. Golubkov, G. B. Dubrovski, A. I. Shelykh, Preparation and Properties of GeS2 Single Crystals. Semiconductors 1998, 32, 734–735. [9] C. C. Wu, C. H. Ho, J. Y. Wu, S. L. Lin, Y. S. Huang, Characterization of Ge(Se1–xSx)2 Series Layered Crystals Grown by Vertical Bridgman Method. J. Cryst. Growth 2005, 281, 377–383. DOI: 10.1016/j.jcrysgro.2005.04.001 [10] M. Nakamura, H. Nakamura, Y. Matsushita, K. Shimamura, N. Ohashi, Growth of Germanium Monosulfide (GeS) Single Crystal by Vapor Transport From Molten GeS Source Using a Two-Zone Horizontal Furnace. J. Cryst. Growth 2020, 547, 125813. DOI: https://doi.org/10.1016/j.jcrysgro.2020.125813 [11] M. Nakamura, H. Nakamura, K. Shimamura, N. Ohashi, Growth and Characterization of a Gallium Monosulfide (GaS) Single Crystal Using the Bridgman https://matdata.asminternational.org/apd/https://matdata.asminternational.org/apd/https://doi.org/10.1016/j.jcrysgro.2020.12581317  Method. J. Cryst. Growth 2021, 573, 126303. DOI: https://doi.org/10.1016/j.jcrysgro.2021.126303 [12] M. Nakamura, H. Nakamura, M. Imura, S. Otani, K. Shimamura, N. Ohashi, SnS Crystal Grown Using Horizontal Gradient Freeze Method and its Electrical Properties. J. Alloy Compd. 2014, 591, 326–328. DOI: https://doi.org/10.1016/j. jallcom.2013.12.230 [13] CrystalClear, Rigaku Corporation, Tokyo, Japan (2005). [14] G. M. Sheldrick, SHELXT–Integrated Space-Group and Crystal-Structure Determination. Acta Crystallogr. A: Found. Adv. 2015, 71 (1), 3–8. DOI: https://doi.org/10.1107/S2053273314026370 [15] G. M. Sheldrick, Crystal Structure Refinement With SHELXL. Acta Crystallogr. C: Struct. Chem. 2015, 71 (1), 3–8. 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DOI: http://dx.doi.org/10.1016/j.jallcom.2014.12.259 [19] W. Jiangjing, R. Ider, Z. Ling, L.Lu, L. D. Volker, Z. Baiyu, T. Lin, D. Hongchu, J. Chunlin, Q. Xiaofeng, W. Matthias, M. Riccardo, Z. Wei, Unconventional Two-Dimensional Germanium Dichalcogenides. Nanoscale 2018, 10, 7363. DOI: 10.1039/c8nr01747f [20] M. Rubenstein, G. Roland, A Monoclinic Modification of Germanium Disulfide, GeS2. Acta Crystallogr. B: Struct. Crystallogr. Cryst. Chem. 1971, 27 (2), 505–506. DOI: https://doi.org/10.1107/S0567740871002450 [21] Von G. Dittmar, H. Schäfer, Die Kristallstruktur von HT-GeS2. Acta Crystallogr. B: Struct. Crystallogr. Cryst. Chem. 1975, 31 (8), 2060–2064. DOI: https://doi.org/10.1107/S0567740875006851   https://doi.org/10.1107/S0567740871002450https://doi.org/10.1107/S056774087500685119  Figure Captions Figure 1. Schematic of the equipment used to prepare the starting material for the HT-GeS2 crystal growth. Figure 2. Synthesized sample used as the starting material for the crystal growth of HT-GeS2. Figure 3. Powder XRD patterns of the sample used as the starting material for the crystal growth of HT-GeS2; XRD pattern without brown dots and that with brown dots (black line). Figure 4. SEM image of the sample used as the starting material for the crystal growth of HT-GeS2. Figure 5. Grown HT-GeS2 crystal cleaved along the growth direction. Figure 6. XRD pattern measured from the cleaved surface of the grown HT-GeS2 crystal. Figure 7. Optical transmittance spectra of the HT-GeS2 single crystal in the visible (a) and IR regions (b). Figure 8. DTA thermogram of the HT-GeS2 single crystal.    20  Table 1. Crystal data, collected data, and structure refinement details. Crystal data Chemical formula GeS2 Mr 136.71 Crystal system, space group Monoclinic, P21/c Temperature (K) 296 a, b, c (Å) 6.7061 (4), 16.0877 (13), 11.4242 (11)  (°) 91.069 (8) V (Å3) 1232.29 (17) Z 16 Radiation type Mo K  (mm-1) 10.96 Crystal size (mm) 0.49 × 0.17 × 0.07 Data collection No. of reflections 2510 No. of parameters 109 Refinement R[F2 > 2(F2)], wR(F2), S 0.090,  0.263,  1.04 max, min (e Å-3) 2.59, -1.50           21  Table 2. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2).  x y z Uiso*/Ueq Ge3 0.67153 (18) 0.34855 (9) 0.28012 (14) 0.0356 (5) Ge2 0.17398 (19) 0.30746 (8) 0.27810 (15) 0.0350 (5) Ge4 0.3400 (2) 0.49735 (9) 0.20585 (15) 0.0377 (5) Ge1 -0.1568 (2) 0.15315 (8) 0.22174 (15) 0.0373 (5) S4 0.4212 (5) 0.3314 (2) 0.4016 (4) 0.0408 (9) S6 0.9280 (5) 0.3320 (2) 0.3999 (4) 0.0411 (9) S5 0.6728 (5) 0.2567 (2) 0.1377 (4) 0.0430 (9) S3 0.1773 (5) 0.3906 (2) 0.1239 (4) 0.0445 (9) S1 -0.2218 (6) 0.0372 (2) 0.1228 (4) 0.0439 (10) S7 0.6651 (5) 0.4743 (2) 0.2013 (4) 0.0475 (11) S2 0.1677 (5) 0.1772 (2) 0.2151 (5) 0.0471 (11) S8 0.2711 (6) 0.6127 (2) 0.1060 (4) 0.0462 (10)                22  Table 3. Geometric parameters (Å, º). Ge3—S6 2.194 (4) Ge4—S1ii 2.220 (4) Ge3—S5 2.198 (4) Ge4—S8 2.222 (4) Ge3—S7 2.215 (4) Ge4—S3 2.231 (4) Ge3—S4 2.215 (4) Ge4—Ge1ii 2.918 (2) Ge2—S4 2.191 (4) Ge1—S2 2.213 (4) Ge2—S3 2.212 (4) Ge1—S1 2.220 (4) Ge2—S6i 2.213 (4) Ge1—S8iii 2.222 (4) Ge2—S2 2.216 (4) Ge1—S5i 2.228 (4) Ge4—S7 2.213 (4)       S6—Ge3—S5 111.47 (15) S8—Ge4—Ge1ii 48.98 (11) S6—Ge3—S7 111.95 (15) S3—Ge4—Ge1ii 125.11 (12) S5—Ge3—S7 108.27 (18) S2—Ge1—S1 108.28 (15) S6—Ge3—S4 100.86 (16) S2—Ge1—S8iii 115.93 (18) S5—Ge3—S4 113.17 (15) S1—Ge1—S8iii 97.86 (15) S7—Ge3—S4 111.05 (15) S2—Ge1—S5i 110.57 (15) S4—Ge2—S3 112.81 (15) S1—Ge1—S5i 108.34 (16) S4—Ge2—S6i 97.35 (16) S8iii—Ge1—S5i 114.71 (16) S3—Ge2—S6i 114.31 (15) S2—Ge1—Ge4iii 125.41 (12) S4—Ge2—S2 112.65 (15) S1—Ge1—Ge4iii 48.92 (11) S3—Ge2—S2 108.27 (18) S8iii—Ge1—Ge4iii 48.95 (10) S6i—Ge2—S2 111.26 (15) S5i—Ge1—Ge4iii 123.32 (11) S7—Ge4—S1ii 115.91 (18) Ge2—S4—Ge3 101.05 (18) S7—Ge4—S8 108.86 (16) Ge3—S6—Ge2iv 102.39 (18) S1ii—Ge4—S8 97.89 (15) Ge3—S5—Ge1iv 101.11 (17) S7—Ge4—S3 109.59 (15) Ge2—S3—Ge4 98.24 (17) 23  S1ii—Ge4—S3 114.38 (16) Ge4iii—S1—Ge1 82.17 (15) S8—Ge4—S3 109.40 (16) Ge4—S7—Ge3 98.93 (15) S7—Ge4—Ge1ii 124.80 (12) Ge1—S2—Ge2 99.63 (15) S1ii—Ge4—Ge1ii 48.92 (10) Ge4—S8—Ge1ii 82.07 (15)                 24    Figure 1. Schematics of the equipment used to prepare the starting material for the crystal growth of HT-GeS2.       25                          Figure 2. Synthesized sample used as the starting material for the crystal growth of HT-GeS2.           10 mm 26    Figure 3. Powder XRD patterns of the sample used as the starting material for the crystal growth of HT-GeS2; XRD pattern without brown dots and that with brown dots (black line).              27   Figure 4. SEM image of the sample used as the starting material for the crystal growth of HT-GeS2.              28              Figure 5. Grown HT-GeS2 crystal cleaved along the growth direction.            .          10 mm29    Figure 6. XRD pattern measured from the cleaved surface of the grown HT-GeS2 crystal.               30    Fig. 7(a)  Figure 7. Optical transmittance spectra of the HT-GeS2 single crystal in the visible (a) and IR regions (b).            31      Fig. 7(b)  Figure 7. Optical transmittance spectra of the HT-GeS2 single crystal in the visible (a) and IR regions (b).        32    Figure 8. DTA thermogram of the HT-GeS2 single crystal.