English

• 1. INTRODUCTION

  Recently, the presence of antibiotic residues in water and their harmful effects on ecological environment and human health have aroused worldwide attention. TC (tetracycline hydrochloride) as a kind of antibiotic is usually used in the pharmaceutical industry for humans, animal husbandry and aquaculture to fight infections, which has been considered as one of the most significant pollutants in water environments^{[1, 2]}. Thus, removing TC from water environments by exploiting new control and treatment technologies becomes more and more urgent. Among various elimination methods for TC, semiconductor-based photocatalysis has attracted great attention due to its low energy consumption, high efficiency, nanotoxicity and green advantages^{[3, 4]}. According to previous reports, many semiconductor photocatalysts such as TiO₂^[5], Ga₂O₃^[6], ZnSb₂O₄^[7], ZnO^[8], C₃N₄^[9] and ZnIn₂S₄^[10] have been used in the removal of TC from wastewater.

  It is well-known that p-block metal (Sb, In, Ge, Sn, Ga) composite oxides have brought about widespread attention as a class of potential high-efficiency photocatalysts because of its unique crystal and electronic structures^[11]. The internal fields as a result of the dipole moment inside the distorted GeO₄ tetrahedron or MO₆ (M = Sb, In, Sn, or Ga) octahedra and the highly dispersive conduction band due to the hybridization of s and p orbitals of metal elements may be beneficial to the separation of photogenerated electron-hole pairs^{[12, 13]}. Especially, the antimony composite oxides, like M₂Sb₂O₇ (M = Ca or Sr), MSb₂O₆ (M = Ca or Zn), MSbO₃ (M = Na or Ag), BiSbO₄, Bi₃SbO₇ and ZnSb₂O₄, have been reported to be photocatalysts for water splitting and organic pollutants degradation^[14-22]. The valence of Sb ions in most of these compounds is +5. In our previous research, we found that the areal photoactivity of ZnSb₂O₄ was remarkably higher than that of ZnSb₂O₆^[7]. Strontium antimonate composite oxides have many forms including SrSb₂O₆, Sr_1.36Sb₂O₆, Sr₂Sb₂O₇, etc. Both Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ have been reported to be photocatalysts for water splitting^{[23, 24]}. Sr₂Sb₂O₇ also exhibits excellent photocatalytic activity in the decomposition of organic pollutants in gas and liquid phase^[25]. However, their application for TC degradation has never been explored to the best of our knowledge.

  In this study, the performances of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ synthesized by a hydrothermal method in the photocatalytic degradation of TC under UV light irradiation were investigated. The influences of synthesis conditions, electronic and crystal structures, specific surface area, and separation of charge carriers on the photocatytic activities of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ were systemically studied. The purpose of this study aims to draw a direct correlation between different strontium antimonate composite oxides and their photocatalytic activities, which is of great significance for the screening and design of p-block metal composite oxide photocatalyst with improved performance.

  2. EXPERIMENTAL

  2.1 Synthesis

  Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ were synthesized by a hydrothermal method^{[23, 26]}. In a typical procedure, Sr(CH₃COO)₂·0.5H₂O (1.07 g, 5.00 mmol), Sb₂O₅ (0.81 g, 2.50 mmol) and 70 mL deionized water were added to a 100 mL Teflon-lined stainless-steel autoclave reactor, mechanically stirred for 20 min. The pH of the resulting mixture was adjusted by 4 mol/L nitric acid solution or sodium hydrate solution under constant stirring. The precursor solution was heated at different temperature in an oven for different reaction time. The produced precipitate was washed with distilled water and absolute ethanol for several times and dried in air at 70 ℃. Finally, samples Sr_1.36Sb₂O₆ or Sr₂Sb₂O₇-T-t-n were obtained, where T is the reaction temperature, t the reaction time, and n the pH value or the OH^- concentration of precursor solution (n mol/L).

  2.2 Characterization

  Phase identifications of Sr₂Sb₂O₇ and Sr₂Sb_1.36O₆ were taken by a Bruker D8 Advance X-ray diffractometer using CuKα radiation operated at the accelerating voltage of 40 KV and the applied current of 40 mA. Surface morphology of all samples was obtained by scanning electron microscopy (SEM) recorded on a Regulus 8100 field emission (Japan). The transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were conducted with a JEOL model JEM 2010 EX instrument operated at the accelerating voltage of 200 kV. The light absorption of samples was measured with a Varian Cary 500 UV-vis spectrophotometer (UV-vis DRS) using BaSO₄ as the reference. The Brunauer-Emmett-Teller (BET) specific surface area was measured by BELSORP-mini II instrument. Samples were pretreated at 120 ^oC for 2 h in vacuum to remove moisture and other gases before the test. Photocurrent measurements were recorded in a threeelectrode cell using an electrochemical workstation (Versa STAT3, Princeton Instruments, America). A Pt plate and Ag/AgCl electrode were employed as the counter and reference electrodes, respectively. The working electrode was prepared on fluorine-doped tin oxide (FTO) glass that was cleaned by ultrasonication in ethanol for 30 min. Typically, 5 mg of the photocatalyst was ultrasonicated in 0.5 mL of N, N-dimethylformamide (DMF) to disperse it evenly to get a slurry. After that, the homogeneous paste was spread on the conductive surface of the FTO glass and then dried in air. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the three-electrode cell in the presence of 0.1 M KCl solution containing 10 mM K₃[Fe(CN)₆] under open-circuit potential conditions. ESR signals of the radical spin-trapped by DMPO were examined with a Bruker ESP 300E spectrometer. The settings for the ESR spectrometer were: center field, 3510.00 G; microwave frequency, 9.79 GHz; power, 5.05 mW.

  2.3 Photocatalytic activity measurements

  The photocatalytic reactions of the samples were performed in a quartz tube with 4.7 cm inner diameter and 16.5 cm length. Four 4W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5) were equipped as illuminating source. The photocatalyst (150 mg) was added to the vessel containing 150 mL 50 mg/L TC aqueous solution. The mixture was stirred for 1 h in the dark to ensure the establishment of adsorption-desorption equilibrium before irradiation. During irradiation, a 4 mL suspension was collected from the mixture at certain time intervals and centrifuged. The residual contaminant concentration was analyzed by a Shimadzu UV-1750 UV-Vis-NIR spectrophotometer at the detection wavelength of 357 nm.

  3. RESULTS AND DISCUSSION

  In order to obtain the optimal conditions for the hydrothermal synthesis of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇, the influences of the pH value or the OH^- concentration of precursor solution and the reaction temperature and time on their photocatytic activities were investigated. The XRD patterns of Sr_1.36Sb₂O₆-180 ℃-24 h-n (n = 1 and 5) synthesized at different pH (Fig. 1a) can be indexed to Sr_1.36Sb₂O₆ (JCPDS card 81-0735). The UV light-driven photocatalytic activities of Sr_1.36Sb₂O₆-180 ℃-24 h-n (n = 1 and 5) were explored using the degradation of TC as a model. The temporal concentration changes of TC were monitored by measuring the UV-vis absorption of TC solution at 357 nm to determine the catalytic activities. As shown in Fig. 1b, the photocatalytic activity of Sr_1.36Sb₂O₆-180 ℃-24 h-5 is higher than that of Sr_1.36Sb₂O₆-180-24-1. The XRD patterns of Sr_1.36Sb₂O₆-T ℃-24 h-5 (T = 60, 80, 100, 120, 150) reveals In this survey, we found that a mixture of Sr_1.36Sb₂O₆ and Sb₂O₅ can be obtained at low reaction temperature (60 and 80 ℃) and pure Sr_1.36Sb₂O₆ is achieved between 100 and 150 ℃ (Fig. 1c). Fig. 1d shows the conversion of TC over Sr_1.36Sb₂O₆-T ℃-24 h-5 (T = 100, 120 and 150) and Sr_1.36Sb₂O₆-100 ℃-24 h-5 exhibits the highest photocatalytic activity for the degradation of TC. The XRD spectrum of Sr_1.36Sb₂O₆-100 ℃-6 h-5 shows that in addition to the diffraction peaks of Sr_1.36Sb₂O₆, a diffraction peak attributed to Sb₂O₅ appears, indicating that pure Sr_1.36Sb₂O₆ can not be produced with short reaction time. Pure Sr_1.36Sb₂O₆ can be formed as the reaction time is more than or equal to 12 h. (Fig. 1e) Sr_1.36Sb₂O₆-100 ℃-24 h-5 has the best photocatalytic performance and the degradation efficiency of TC over it is found to be 80.7% under UV light irradiation in 180 min (Fig. 1f).

  Figure 1

  

  Figure 1.  XRD patterns of samples: (a) Sr_1.36Sb₂O₆-180 ℃-24 h-n (n = 1 and 5), (c) Sr_1.36Sb₂O₆-T ℃-24 h-5 (T = 60, 80, 100, 120, 150), (e) Sr_1.36Sb₂O₆-100 ℃-t h-5 (t = 2, 12, 24 and 48); Temporal changes of concentration of TC monitored by the UV-vis absorption spectra at 357 nm on samples: (b) Sr_1.36Sb₂O₆-180 ℃-24 h-n (n = 1 and 5), (d) Sr_1.36Sb₂O₆-T ℃-24 h-5 (T = 60, 80, 100, 120, 150), (f) Sr_1.36Sb₂O₆-100 ℃-t h-5 (t = 2, 12, 24 and 48); (_▼) Sb₂O₅

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  Both of the XRD spectra of Sr₂Sb₂O₇-180 ℃-24 h-n (n = 1 and 2) obtained with the different OH^- concentration of precursor solution are shown in Fig. 2a. It can be observed that both samples are in good agreement with the phase of Sr₂Sb₂O₇ (JCPDS card 78-1774). Compared with Sr₂Sb₂O₇-180 ℃-24 h-1, Sr₂Sb₂O₇-180 ℃-24 h-2 exhibits enhanced degradation efficiency (Fig. 2b). Fig. 2c shows the XRD patterns of the Sr₂Sb₂O₇-T ℃-24 h-2 (T = 100, 120, 150, 180 and 200). All the diffraction peaks can be indexed to pure Sr₂Sb₂O₇. The photocatalytic degradation efficiency of the samples obtained at different reaction temperature follows the sequence: 150℃ > 180℃ > 200℃ > 120℃ > 100℃ (Fig. 2d). Fig. 2e shows the XRD patterns of the as-prepared Sr₂Sb₂O₇-150℃-t h-2 (t = 6, 12, 24 and 36) with different reaction time. When the reaction time is 6 h, a mixture of Sr₂Sb₂O₇ and Sb₂O₅ can be obtained. With the reaction time over or equal to 12 h, no obvious diffraction peaks of impure phases are found, indicating the high purity of as-prepared Sr₂Sb₂O₇ samples. The results of TC degradation over Sr₂Sb₂O₇-150℃-t h-2 (t = 12, 24 and 36) are given in Fig. 2f. Sr₂Sb₂O₇-150℃-24 h-2 displays the highest photocatalytic activity and 99.7% of TC is removed after 30 min UV light irradiation.

  Figure 2

  

  Figure 2.  XRD patterns of samples: (a) Sr₂Sb₂O₇-180 ℃-24 h-n (n = 1 and 2), (c) Sr₂Sb₂O₇-T ℃-24 h-2 (T = 100, 120, 150, 180 and 200), (e) Sr₂Sb₂O₇-150 ℃-t h-2 (t = 6, 12, 24 and 36); Temporal changes of concentration of TC monitored by the UV-vis absorption spectra at 357 nm on samples: (b) Sr₂Sb₂O₇-180 ℃-24 h-n (n = 1 and 2), (d) Sr₂Sb₂O₇-T ℃-24 h-2 (T = 100, 120, 150, 180 and 200), (f) Sr₂Sb₂O₇-150 ℃-t h-2 (t = 6, 12, 24 and 36); (_●) Sb₂O₅, (*) Sr(OH)₂

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  Based on the above results, the optimal conditions for the hydrothermal synthesis of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ are determined to be: the pH value 5 and the OH^- concentration of precursor solution 2 mol/L, hydrothermal temperature 100 and 150 ℃, reaction time 5 and 24 h, respectively. The Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150℃-24 h-2 samples prepared under optimal conditions exhibit a remarkably different photocatalytic activity for the degradation of TC. In order to reveal the relationship between composition, structure and photocatalytic performance, the morphology, surface areas, optical absorption property, the main active species in photocatalytic reaction, the charge carriers' transfer and separation behaviors, electronic and crystal structures of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ prepared under optimal conditions are characterized systematically.

  The morphology and microstructures of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ were investigated by SEM, TEM and HRTEM, respectively. In the SEM image in Fig. 3a, the as-prepared Sr_1.36Sb₂O₆ consists entirely of octahedra with diameters of 30~80 nm. This indicates that the octahedra can be successfully synthesized on a large scale. The average diameter of the octahedra, measured in a typical TEM image of Sr_1.36Sb₂O₆ (Fig. 3b), is approximately 50 nm. In the HRTEM image (Fig. 3c), the clear lattice fringes indicate the good crystallization of Sr_1.36Sb₂O₆. The spacing of the lattice fringes is 0.30 nm, which means that they belong to the (222) plane of Sr_1.36Sb₂O₆. The typical SEM image of Sr₂Sb₂O₇ (Fig. 3d) shows that the sample consists of small particles. The TEM image of Sr₂Sb₂O₇ is shown in Fig. 3e, which indicates that the average size of nanoparticles is around 9 nm. Clear diffraction patterns with interdistance d = 0.30 nm can be seen in the HRTEM image of the sample (Fig. 3f), which corresponds to the (220) plane of Sr₂Sb₂O₇. Additionally, the crystallite size of Sr_1.36Sb₂O₆ is much larger than that of Sr₂Sb₂O₇, which may lead to the significantly different BET surface areas. The specific surface areas of the samples are determined with the N₂ absorption-desorption isotherm curves (Fig. 4). The BET specific surface area of Sr₂Sb₂O₇ is 29.6 m²·g^-1, which is 1.6 times larger than that of Sr_1.36Sb₂O₆ (18.9 m²·g^-1). However, the photocatalytic performance of Sr₂Sb₂O₇ is much better than that of Sr_1.36Sb₂O₆, indicating the specific surface area is not the main factor affecting their photocatalytic activity.

  Figure 3

  

  Figure 3.  Images of the Sr_1.36Sb₂O₆-100 ℃-24 h-5 sample: (a) SEM image; (b) TEM image; (c) HRTEM image; Images of the Sr₂Sb₂O₇-150℃-24 h-2 sample: (d) SEM image; (e) TEM image; (f) HRTEM image

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  Figure 4

  

  Figure 4.  Nitrogen adsorption-desorption isotherm for Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  The UV-vis diffuse reflectance spectroscopy of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ is shown in Fig. 5. The absorptions for Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ locate at ca. 292 and 323 nm, corresponding to a band gap of about 4.2 and 3.8 eV, respectively. It is well known that the wide band gap endows the photogenerated holes and electrons with strong redox ability^{[27, 28]}. Sr_1.36Sb₂O₆ has a small band gap energy compared with Sr₂Sb₂O₇, and thus it possesses lower photocatalytic activity.

  Figure 5

  

  Figure 5.  Diffuse reflectance absorption spectra of Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  To testify the active species involved in different photocatalytic systems, EPR spin trap technique was carried out. As shown in Fig. 6, the typical peaks of DMPO-OH• and DMPO-O₂^-• under the UV light irradiation are observed. The generation amount of OH• and O₂^-• in Sr₂Sb₂O₇ system is more than that in Sr_1.36Sb₂O₆, which may be one of the reasons for the higher photocatalytic degradation efficiency^[29-31].

  Figure 6

  

  Figure 6.  ESR signals of DMPO-trapped •OH for Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  The charge carriers' transfer and separation behaviors were investigated by the photoelectrochemical properties. As the photocurrent response curves and EIS Nyquist plots of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ shown in Fig. 7, Sr₂Sb₂O₇ shows stronger photocurrent density and smaller EIS arc radius than Sr_1.36Sb₂O₆, indicating higher charge transfer and separation efficiency on the surface of Sr₂Sb₂O₇, which leads to the higher photocatalytic performance^{[32, 33]}.

  Figure 7

  

  Figure 7.  (a) Photocurrents, (b) Electrochemical impedance spectroscopy (EIS) Nynquist plots of Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  The activity of a photocatalyst is closely related to the distorted tetrahedral or octahedral units in its crystal structure^{[34, 35]}. In our previous study we found that the different photocatalytic activities of ZnSb₂O₄ and ZnSb₂O₆ were strongly associated with their crystal structures, and the distortion of SbO₃ tetrahedra in ZnSb₂O₄ contributed to its high photocatalytic activity^[7]. Fig. 8 shows the schematic representation of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ structures. The SbO₆ tetrahedra in Sr_1.36Sb₂O₆ crystal are composed of six Sb–O bonds with the same bond length (Sb–O, 1.974 Å×6). However, there are two kinds of distorted Sb–O octahedra in Sr₂Sb₂O₇ crystal. One is the stretched SbO₆ octahedra which are composed of two long and four short Sb–O bonds (Sb–O: 2.005 Å×2, Sb–O: 1.971 Å×4), and the other is the flattened SbO₆ octahedra made up of two short and four long Sb–O bonds (Sb–O: 1.922 Å×2, Sb–O: 2.021 Å×4). We think that the distortion of SbO₆ octahedra is the main reason for the remarkable difference of photocatalytic activity between Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇. The internal electric field formed by dipole moment of distorted SbO₆ octahedra can be helpful to the separation of electron and hole, thus leading to the improvement of photocatalytic activity.

  Figure 8

  

  Figure 8.  Schematic representation of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ crystal structures

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  4. CONCLUSION

  In the present work, p-block metal composite oxides Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ synthesized by a hydrothermal method were applied to TC photocatalytic degradation for the first time. The Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2 samples prepared under optimal conditions exhibited remarkably different photocatalytic activities. The degradation efficiency of TC over Sr_1.36Sb₂O₆-100 ℃-24 h-5 was found to be 80.7% in 180 min and 99.7% of TC was removed in 30 min using Sr₂Sb₂O₇-150 ℃-24 h-2 as a photocatalyst under UV light irradiation. The difference of photocatalytic performance between Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ was mainly attributed to their different electronic and crystal structures that led to the differences on the redox ability of photogenerated holes and electrons, charges transfer and separation efficiency, and generation amount of active species. Our work will facilitate the development of novel semiconductor photocatalytic materials using p-block metal composite oxides for organic pollutants degradation.

  
  
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  Figure 1  XRD patterns of samples: (a) Sr_1.36Sb₂O₆-180 ℃-24 h-n (n = 1 and 5), (c) Sr_1.36Sb₂O₆-T ℃-24 h-5 (T = 60, 80, 100, 120, 150), (e) Sr_1.36Sb₂O₆-100 ℃-t h-5 (t = 2, 12, 24 and 48); Temporal changes of concentration of TC monitored by the UV-vis absorption spectra at 357 nm on samples: (b) Sr_1.36Sb₂O₆-180 ℃-24 h-n (n = 1 and 5), (d) Sr_1.36Sb₂O₆-T ℃-24 h-5 (T = 60, 80, 100, 120, 150), (f) Sr_1.36Sb₂O₆-100 ℃-t h-5 (t = 2, 12, 24 and 48); (_▼) Sb₂O₅

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  Figure 2  XRD patterns of samples: (a) Sr₂Sb₂O₇-180 ℃-24 h-n (n = 1 and 2), (c) Sr₂Sb₂O₇-T ℃-24 h-2 (T = 100, 120, 150, 180 and 200), (e) Sr₂Sb₂O₇-150 ℃-t h-2 (t = 6, 12, 24 and 36); Temporal changes of concentration of TC monitored by the UV-vis absorption spectra at 357 nm on samples: (b) Sr₂Sb₂O₇-180 ℃-24 h-n (n = 1 and 2), (d) Sr₂Sb₂O₇-T ℃-24 h-2 (T = 100, 120, 150, 180 and 200), (f) Sr₂Sb₂O₇-150 ℃-t h-2 (t = 6, 12, 24 and 36); (_●) Sb₂O₅, (*) Sr(OH)₂

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  Figure 3  Images of the Sr_1.36Sb₂O₆-100 ℃-24 h-5 sample: (a) SEM image; (b) TEM image; (c) HRTEM image; Images of the Sr₂Sb₂O₇-150℃-24 h-2 sample: (d) SEM image; (e) TEM image; (f) HRTEM image

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  Figure 4  Nitrogen adsorption-desorption isotherm for Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  Figure 5  Diffuse reflectance absorption spectra of Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  Figure 6  ESR signals of DMPO-trapped •OH for Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  Figure 7  (a) Photocurrents, (b) Electrochemical impedance spectroscopy (EIS) Nynquist plots of Sr_1.36Sb₂O₆-100 ℃-24 h-5 and Sr₂Sb₂O₇-150 ℃-24 h-2

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  Figure 8  Schematic representation of Sr_1.36Sb₂O₆ and Sr₂Sb₂O₇ crystal structures

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