Synthesis and Study of the Effect of Ba2+ Cations Substitution with Sr2+ Cations on Structural and Optical Properties of Ba2−xSrxZnWO6 Double Perovskite Oxides (x = 0.00, 0.25, 0.50, 0.75, 1.00)

The effect of Sr2+ substitution on the morphology, crystal structure, and optical properties of double perovskite oxide Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) were investigated. Scanning electronic microscopy demonstrated that all samples have similar microstructure morphology but differ in the range of grain sizes. X-ray diffraction measurements indicated that these materials crystallize in a (Fm-3m) cubic crystal structure, and also confirmed the tolerance factor. Rietveld analysis revealed that the lattice parameter decreased from 8.11834 to 8.039361 Å when the substitution of Ba2+ with Sr2+ cations increased from zero to 100%. Fourier transform infrared (FTIR) and Raman spectroscopies displayed a symmetric stretching vibration of WO6 octahedra at 825 cm−1, and an anti-symmetric stretching mode of WO6 was observed by FTIR at 620 cm−1. A strong peak at 420 cm−1 was also observed in the Raman spectra and is due to the W–O–W bending vibration modes. UV-Vis diffuse reflectance spectroscopy was carried out for the series, and the band gap energy decreased from 3.27 eV for Ba2ZnWO6 to 3.02 and 3.06 eV for Ba1.75Sr0.25ZnWO6 and Ba1.5Sr0.5ZnWO6, respectively. The excitation and emission photoluminescence properties were investigated at room temperature.

A variety of devices and methods have previously been used to find and characterize new double perovskite compounds at high temperatures. Zaraq et al. [1] synthesized SrCaCoTeO 6 and SrCaNiTeO 6 compounds and used X-ray diffraction (XRD) and scanning electron microscopy (SEM) to describe their crystal structure and phase transition. Chufeng Lau et al. [2] used XRD, photoelectron spectroscopy,

Scanning Electron Microscopy
The ESM images of the Ba 2−x Sr x ZnWO 6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) series are displayed in Figure 1a-e. The morphologies of all samples are identical and they appear to be highly homogeneous with no impurities. It is observed that, in all samples, the size of the particles is large and they are aggregated in groups, which is due to the higher preparation temperature. Chunfeng Lan et al. [2] observed an equivalent effect of temperature in the morphology of La 2 NiMnO 6 double perovskite oxide. Furthermore, the images reveal the presence of fine fragments that are produced during the preparation grinding. Each of the samples has grains of various sizes, i.e., Ba 2 ZnWO 6 has 1-3 µm grains, Ba 1.75 Sr 0.25 ZnWO 6 has 1-5 µm grains, Ba 1.5 Sr 0.5 ZnWO 6 has 1.5-4 µm grains, Ba 1.25 Sr 0.75 ZnWO 6 has 2-8 µm grains, and BaSrZnWO 6 has 2-7 µm grains. An Energy-dispersive X-ray spectroscopy (EDX) analysis is conducted with each sample using the SEM images. Figure 1e presents the energy dispersive X-ray spectrum from the element that formed the BaSrZnWO 6 sample. All EDX graphs confirm that all samples contain elements of the raw material preparation composition and a proportion approximating the input quantities to configure each sample with a small error, which refers to the homogeneity and crystal purity.

X-ray Powder Diffraction
The X-ray diffraction data of the perovskite oxide compounds are essential in determining the crystalline structure of the samples in terms of the Bravais lattice, atomic position, lattice parameter, and space group. Many studies refer to the importance of the study of material structure since they govern the other properties of the materials [17]. The XRD of the Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) double perovskite oxide series prepared by the solid-state reaction is shown in Figure 2. The BaWO4 and Ba2WO5 phases displayed as minor peaks at low intensity in the XRD pattern shown in Figure 1 are attributed to the impurities in the Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) structure around 26° and 28° [18]. A plus sign and a star are used to depict them in Figure 1 when 2 is around 26° and 28° for BaWO4 and Ba2WO5, respectively. The XRD data of each sample in the series are refined by the Rietveld method using the FullProf program. Table 1 displays the atom

X-ray Powder Diffraction
The X-ray diffraction data of the perovskite oxide compounds are essential in determining the crystalline structure of the samples in terms of the Bravais lattice, atomic position, lattice parameter, and space group. Many studies refer to the importance of the study of material structure since they govern the other properties of the materials [17]. The XRD of the Ba 2−x Sr x ZnWO 6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) double perovskite oxide series prepared by the solid-state reaction is shown in Figure 2. The BaWO 4 and Ba 2 WO 5 phases displayed as minor peaks at low intensity in the XRD pattern shown in Figure 1 are attributed to the impurities in the Ba 2−x Sr x ZnWO 6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) structure around 26 • and 28 • [18]. A plus sign and a star are used to depict them in Figure 1 when 2θ is around 26 • and 28 • for BaWO 4 and Ba 2 WO 5 , respectively. The XRD data of each sample in the series are refined by the Rietveld method using the FullProf program. Table 1 Figure 3 shows the XRD refinement of BaSrZnWO 6 , which is represented by a (Fm-3m) cubic structure with lattice parameters a = b = c = 8.039361 Å and α = β = γ = 90 • . Identical results are obtained for Ba 2 ZnWO 6 using single-crystal X-ray and neutron diffraction [19]. Furthermore, the Ba 2−x Sr x MgTeO 6 series was found in a (Fm-3m) cubic crystal structure [20]. The crystallite size was calculated from the Full width at half maximum (FWHMs) at the major peaks at (220) for the Ba 2−x Sr x ZnWO 6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) double perovskite series using the Scherer equation [21], which was observed to vary between 47.41 and 105.9 nm for the samples.
where D is the crystalline size, λ is the wavelength (1.5405 Å), β is the full width at half maximum, and θ is the diffraction angle. The tolerance factor was found to be between 0.998 and 1.007, which can be calculated by coordinates of all the samples obtained in the (Fm-3m) cubic crystal structure. Figure 3 shows the XRD refinement of BaSrZnWO6, which is represented by a (Fm-3m) cubic structure with lattice parameters a = b = c = 8.039361 Å and α = β = γ = 90°. Identical results are obtained for Ba2ZnWO6 using single-crystal X-ray and neutron diffraction [19]. Furthermore, the Ba2−xSrxMgTeO6 series was found in a (Fm-3m) cubic crystal structure [20]. The crystallite size was calculated from the Full width at half maximum (FWHMs) at the major peaks at (220) for the Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) double perovskite series using the Scherer equation [21], which was observed to vary between 47.41 and 105.9 nm for the samples.
where D is the crystalline size, λ is the wavelength (1.5405 Å), β is the full width at half maximum, and θ is the diffraction angle. The tolerance factor was found to be between 0.998 and 1.007, which can be calculated by         Table 2 displays the tolerance factor and parameter of the crystal structure of Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) using the Rietveld method of refinement. The unit cell volume decreases with an increasing substitution as a result of the larger ionic radius of the Ba 2+ cation than that of Sr 2+ .   Table 2 displays the tolerance factor and parameter of the crystal structure of Ba 2−x Sr x ZnWO 6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) using the Rietveld method of refinement. The unit cell volume decreases with an increasing substitution as a result of the larger ionic radius of the Ba 2+ cation than that of Sr 2+ . The FTIR spectra identify the crystal structure of the perovskite structure materials that have characteristic absorption bands in the 850-400 cm −1 region [22]. The strong high-energy anti-symmetric stretching mode of the WO 6 octahedral displayed at 620 cm −1 is due to the higher charge of the tungsten cations. The symmetric stretching vibration of the WO 6 octahedral appears as a high-intensity band at about 825 cm −1 . Figure 4 shows the transmittance of the Ba 2−x Sr x ZnWO 6 double perovskite series versus wave number, and all the samples confirm the molecular bands on the perovskite oxide structure [17]. characteristic absorption bands in the 850-400 cm −1 region [22]. The strong high-energy anti-symmetric stretching mode of the WO6 octahedral displayed at 620 cm −1 is due to the higher charge of the tungsten cations. The symmetric stretching vibration of the WO6 octahedral appears as a high-intensity band at about 825 cm −1 . Figure 4 shows the transmittance of the Ba2−xSrxZnWO6 double perovskite series versus wave number, and all the samples confirm the molecular bands on the perovskite oxide structure [17].

Raman Spectroscopy
The Raman spectra of the samples are shown in Figure 5 for Ba2−xSrxZnWO2 (x = 0.00, 0.25, 0.50, 0.75, 1.00). For all samples, the Raman modes are classified into two types of lattice vibration-the W-O-W bending vibration in the 200-500 cm −1 region and the W-O stretching mode between 700 and 950 cm −1 . This result was observed in many studies on double perovskite oxides [22,23]. Ba has a larger ionic radius (149 Å) than that of Sr (132 Å). When the Ba substitution increases, the effect of the Ba radius is reflected by a decrease in both the bending and stretching modes of the W-O bonds. A redshift in the Raman energy is also observed.

Raman Spectroscopy
The Raman spectra of the samples are shown in Figure 5 for Ba 2−x Sr x ZnWO 2 (x = 0.00, 0.25, 0.50, 0.75, 1.00). For all samples, the Raman modes are classified into two types of lattice vibration-the W-O-W bending vibration in the 200-500 cm −1 region and the W-O stretching mode between 700 and 950 cm −1 . This result was observed in many studies on double perovskite oxides [22,23]. Ba has a larger ionic radius (149 Å) than that of Sr (132 Å). When the Ba substitution increases, the effect of the Ba radius is reflected by a decrease in both the bending and stretching modes of the W-O bonds. A redshift in the Raman energy is also observed.   Figure 6 presents the diffuse reflectance spectrum of the Ba 2−x Sr x ZnWO 6 series at room temperature in the 200-800 nm range. The strong absorption band observed at 300-450 nm refers to the absorption edge in tungsten due to the charge transfer transition of W 6+ − O 2− in the lattice from the highest filled molecular orbital 2p of oxygen to the lowest empty molecular orbital 5d of tungsten [14,19,24]. The absorption band has a blue shift with an increasing substitution ratio of Ba 2+ with Sr 2+ cations.  Figure 6 presents the diffuse reflectance spectrum of the Ba2−xSrxZnWO6 series at room temperature in the 200-800 nm range. The strong absorption band observed at 300-450 nm refers to the absorption edge in tungsten due to the charge transfer transition of − in the lattice from the highest filled molecular orbital 2p of oxygen to the lowest empty molecular orbital 5d of tungsten [14,19,24]. The absorption band has a blue shift with an increasing substitution ratio of Ba 2+ with Sr 2+ cations.  The absorption coefficient can be calculated for the Ba 2−x Sr x ZnWO 6 series from the diffuse reflectance data using the Kubelka-Munk function [25]:

UV-Visible Diffuse Reflectance Spectroscopy
where F(R ∞ ) is the KM function, α is the absorption coefficient, s is the scattering coefficient, and R is the reflection coefficient. The absorbance (f (R)hυ) in relation to wavelength is shown in Figure 7.  [19] according to the relationship E g = 1240/λ (λ is the absorption edge wavelength and E g is the band gap [26]). In addition, the band gap energy was calculated for the samples using the Tauc plot [18], as shown in Figure 8, according to Equation (4).
where hν is the incident photon energy, A is a proportional constant, E g is the band gap energy, and n takes values of 2 or 0.5 for indirect and direct transitions, respectively. Table 3 illustrates the bang gap energy according to the absorption edge and Tauc plot. The results of UV-vis diffuse reflectance and the optical energy gap of the sample series indicate that they can be classified as semiconductor materials [19,27].
where ℎ is the incident photon energy, A is a proportional constant, is the band gap energy, and n takes values of 2 or 0.5 for indirect and direct transitions, respectively. Table 3 illustrates the bang gap energy according to the absorption edge and Tauc plot. The results of UV-vis diffuse reflectance and the optical energy gap of the sample series indicate that they can be classified as semiconductor materials [19,27].
Initially, the materials were used as purchased from Alfa Acer with a purity of 99.99%. Several different treatments of the samples were conducted to achieve the crystal structure. The raw materials were mixed and ground in an agate mortar with acetone, kept in crucibles, and subsequently heated in air at 800 • C for 12 h twice. The sample pellets were prepared in a round shape and heated at 1000 • C twice, following which the same procedure was repeated at 1200 • C. Between the steps for the heating treatment, the sample was ground for 2 h with acetone to increase the homogeneity of the sample at a rate of 10 • C per minute during the heating and cooling process.

Sample Characterization
A Jeol JSM-6360 (JEOL Inc. Peabody, MA, USA) and high-resolution Stereo Scan LEO 440 SEM (LEO, Austin, TX, USA) were used to investigate the morphology and determine the homogeneity of the samples, as well as to obtain crystal-scale crystallization. At room temperature, the XRD data were recorded with a Bruker: D8 Advance (Bruker-Axs, Madison, WI, USA) using CuKα radiation (λ = 1.5406 Å) with a nickel filter. At 40 kV and 40 mA, data were collected for 2θ at 0.02-step sizes and 5-s count times in the 20 • -80 • range. The XRD data were analyzed using the Rietveld refinement method with the FullProf Suite program [31]. The crystalline size (D) [32] was calculated using the Debi-Scherer equation for all samples. At room temperature, the transmittance mode was investigated for all samples using the Satellite FTIR 5000 (ARCoptix S.A, Neuchatel, Switzerland) (with a wavelength range of 400-4000 cm −1 ) [33], where the important bands and peaks of the perovskite structure can be assigned. Using FTIR spectroscopy collected using the KBr pellet method, the material was mixed with KBr at 1:100 ratios for the FTIR measurement in the range of 400-2000 cm −1 . The Raman spectra were collected in Raman HR (Stellarnet Inc., Tampa, FL, USA), using the high resolution Raman spectrometer with a range of 200-2200 cm −1 at 785 nm with a resolution of 4 cm −1 . The Raman probe attaches to the laser via FC/APC and the spectrometer via SMA 905, and has integrated Raman filters and optics with a working distance to the sample of 4.5 mm, configured for the 785 nm laser. A UV-Vis spectrophotometer (UV-2550, Shimadzu, Chiyoda-ku, Tokyo) using BaSO 4 as a reference is used to calculate the UV-Vis diffuse reflectance spectrum at room temperature. In addition, the UV-Vis reflectance spectrum is converted to absorbance using the Kubella-Munk method to estimate the edge of absorption and band gap of the Ba 2−x Sr x ZnWO 6 double perovskite powder series. A Perkin Elmer LS55 fluorescence spectrometer (Perkin-Elmer, Wokingham, UK) was used to investigate the emission and excitation of the Ba 2−x Sr x ZnWO 6 double perovskite series at room temperature.

Conclusions
The Ba 2−x Sr x ZnWO 6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) double perovskite series was prepared by the solid-state reaction technique. In addition, techniques such as X-ray diffraction, scanning electronic microscopy, Fourier transform infrared spectra, Raman spectra, UV-Vis diffuse reflectance, photoluminescence excitation, and emission spectra were investigated. SEM revealed that the prepared Ba 2−x Sr x ZnWO 6 samples crystallized at micrometer scales, where the crystal structure of the samples was determined by XRD as a cubic Fm-3m space group. The lattice parameter decreased with an increase in the proportion of substitution from x = 0 to 1, where the result of FTIR also confirmed the double perovskite structure. The Raman spectra of W-O bonds indicated a systematic decrease and re-shifting with an increasing Ba substitution. Strong UV-Vis absorption was found between 350 and 410 nm and smaller optical bandgap energy, 3.02 eV, was found for Ba 1.75 Sr 0.25 ZnWO 6 compared to the Sr-free sample. The excitation photoluminescence spectra displayed broad bands between 260 and 320 nm, which were assigned to the charge transfer band of Ba 2−x Sr x ZnWO 6 . Ba 2−x Sr x ZnWO 6 displays photoluminescence in the near-UV and visible region. This feature makes Ba 2−x Sr x ZnWO 6 a potential semiconductor in optoelectronics applications.