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Structural and optical properties of A2YVO6 (A = Mg, Sr) double perovskite oxides
Results in Physics 15 (2019) 102589
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Structural and optical properties of A2YVO6 (A = Mg, Sr) double perovskite oxides
T
⁎
Yousef A. Alsabaha,b,e, Ali. Taj Eldenb, Mohamad S. AlSalhia,c, , Abdelrahman A. Elbadawib,f, Mohamed A. Siddigb,d a
Research Chair in Laser Diagnosis of Cancers, College of Science, King Saud University, Riyadh, Saudi Arabia Department of Physics, Faculty of Science and Technology, Al Neelain University, Khartoum, Sudan c Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi Arabia d Department of Physics, Faculty of Science, Albaha University, Albaha, Saudi Arabia e Department of Physics, Faculty of Education and Applied Science, Hajjah University, Hajjah, Yemen f Faculty of Basic Studies, Future University, Khartoum, Sudan b
A B S T R A C T
New double perovskite oxides A2YVO6 (A = Mg, Sr) were prepared and synthesized using conventional methods via a solid-state reaction route. The samples were studied by X-ray powder diffraction (XRD) and the prepared Mg2YVO6 and Sr2YVO6 samples were shown to have monoclinic crystal structures belonging to the (P2/ m) space group. The crystallite size in the samples was found to decrease from 56.320 to 40.480 nm upon increasing the A-site ionic radius. Scanning electron microscopy (SEM) results demonstrated that the sample microstructure consisted of homogenous crystallites of approximately 6.5-µm in size, and energy-dispersive X-ray (EDX) results revealed elements ratios in samples that were identical to the expected values. FTIR results allowed the identification of particular perovskite structural features from vibrational peaks in the range of 400–4000 cm−1, and optical structure in the range of 190–600 nm was obtained from ultraviolet–visible (UV–vis) spectroscopy. The identity of the element at the A site was found to affect the band-gap energy, which decreased from 2.9 to 2.4 eV with the change from Mg2YVO6 to Sr2YVO6, indicating that the A2YVO6 (A = Mg, Sr) samples may possess insulating characteristics.
Introduction Double perovskite materials have long been attractive to materials science, physics, chemistry, and materials engineering researchers because of their interesting physical and chemical properties [1–3]. The properties of double perovskite oxide materials may be ordered depending on the degree of A-site cation doping, for example in Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) [4], Sr2MgMoO6−δ [5], LaCu3Fe4O12 [6], and (La0.8Sr0.2)2FeMnO6−δ [7], or B-site cation doping, such as in La2CoMnO6 (A = Co, Ni) [8], Ba2Zn1−-xNixWO6 (0 ≤ x ≤ 1) [9], and A2BWO6 (A = Sr, Ba; B = Co, Ni, Zn) [10]. Furthermore, the manufactures of perovskites are required to be environmentally friendly due to their ability to dealing with periodic table for materials preparation, like Ag3–2xCuxPO4 powders [11], αAg2–2xZnxWO4 (0 ≤ x ≤ 0.25) [12]. Maughan et al. [13] used X-ray powder diffraction (GSAS/EXPGUI) to study crystal structure and investigate electrical resistivity, and used ultraviolet–visible (UV–vis) diffuse reflectance and photoluminescence (PL) spectroscopy to study optical properties and band gaps as well as magnetoresonance in Sr2FeMoO6. Sarma et al. [14] analyzed the X-ray diffraction data for Cs2InAgCl6 by the Rietveld method, and then verified these results via
⁎
Mössbauer studies. Volonakis et al. [15] studied Cs2InAgX6 (X = Cl, Br, I), halide double perovskites based on bismuth and silver, and they showed that the band gaps of the materials corresponded to wavelengths in the visible region. In addition, they used first principles calculations to confirm the optical properties, which included small variations in the absorption region for the Cl sample. Organic perovskite materials are often used to produce perovskite solar cells such as the (HC(NH2)2)0.83Cs0.17Pb(I0.6Br0.4)3 perovskite [16] and mixed halide MAPbI3–xClx [17] solar cells. All types of perovskite compounds have played an important role in recent applies research and their demand in future looks likely to continue increasing because of their transport characters [4]. This property, in particular, is important, and the perovskite oxide compounds have uses in a wide variety of different industrial and scientific applications, such as magnetic resonance, microwaves, photocatalysis, sunglasses, and solar cells, motivating investigations in the fields of fundamental and applied perovskite research [9]. Accordingly, the aim of the present work was to synthesis and study the structural and optical properties of a new double perovskite, A2YVO6 in which A = Mg, Sr. In particular, the effect of substitutions at the A site was investigated and analyzed. In this article, we report the synthesis by solid-state
Corresponding author at: Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi Arabia. E-mail address: [email protected] (M.S. AlSalhi).
https://doi.org/10.1016/j.rinp.2019.102589 Received 24 May 2019; Received in revised form 11 August 2019; Accepted 12 August 2019 Available online 16 August 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 15 (2019) 102589
Y.A. Alsabah, et al.
Fig. 1. Shows the Rietveld pattern of Mg2YVO6 double perovskite.
Fig. 2. Shows the Rietveld pattern of Sr2YVO6 double perovskite.
and mixed in order to prepare A2YVO6 (A = Mg, Sr). The raw materials of the A2YVO6 (A = Mg, Sr) double perovskites samples were weighted by a sensitive balance (KERN electronic balance, capacity: 120 g, Readability: 0.1 mg). Then, an agate mortar and pestle was used to grind the mixture for each sample for a period of 2 h, during which time acetone was added every 30 min. The mixtures were placed in alumina crucibles. The crucibles containing the samples were placed in an oven at a temperature of 800 °C (CARBOLITE –CWF 1200 °C “Serial No. 20–302426”, manufactured in England) for 24hrs and subsequently, the samples were cooled and ground for 3hrs. The samples were then heated for 24hrs, once more, this time at a temperature of 1000 °C, and then they subjected to a second 3hrs period of cooling and grinding. A final cycle of cooling and grinding was then carried out, this time during the 24hrs heating period the temperature
reaction method of A2YVO6, where A = Mg, Sr, a new double perovskite material. Different treatments were used in order to prepare this material as a single phase. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and UV–vis spectroscopy were used as analytical tools to study the structural and optical properties of A2YVO6 (A = Mg, Sr) double perovskite samples.
Materials and methods New samples of double perovskite oxides were synthesized by solidstate reaction methods using different treatments in order to obtain a single phase. Stoichiometric amounts of Mg2Co3, Sr2Co3, Y2O3, and V2O5 were all purchased from Alfa Aesar (purity: 99.9% in each case) 2
Results in Physics 15 (2019) 102589
Y.A. Alsabah, et al.
Table 1 Rietveld refined parameters from powder X-ray diffraction data of A2YVO6 where (A = Mg,Sr) double perovskite. A2YVO6 (A = Mg, Sr) space group (P2/m) Element
Coordinates
Mg2YVO6
Sr2YVO6
Mg2+/Sr2+
X Y Z X Y Z X Y Z X Y X
0.42035 4.58642 0.46741 0.00001 0.00001 0.00001 1.31270 2.23775 0.01877 0.52938 1.43251 0.09381
0.41166 1.20755 0.32208 0.00001 0.00001 0.00001 1.28315 2.21890 0.00266 0.52664 1.25433 0.18234
Y3+
V5+
O2–
was set to 800 °C. Finally, when the mixtures reached room temperature, the samples of A2YVO6 (A = Mg, Sr) were obtained. The following chemical equation was used to determine the weights of the raw materials.
⎧ 2MgCo3 ⎫ + 1 Y2O3 + 1 V2O5 → ⎧ M g 2YV O6 ⎫ + 2Co2 ↑ ⎨ ⎨ 2 2 ⎩ Sr2YV O6 ⎬ ⎭ ⎩ 2SrCo3 ⎬ ⎭
(1)
The crystal structure of calcined samples were Examined by X-ray diffraction (Shimadzu, MAX_X, XRD-7000) [18], using Cu Kα radiation with scanning speed of 1000°/min, and the crystallite size was calculated from the main peak for sample using the Debye–Scherer equation [19] as follows:
D=
0.94λ β cosθ
(2)
where D is the crystallite size, λ is the wavelength (1.5405 Å), β is the full width at half maximum, and ?? is the diffraction angle. The crystalline structure of the samples was also predicted by the tolerance factor (t) was also investigated; in general, for double perovskites A2B′B′′O6, this can be written as [20]:
t=
(rA + rO) r r 2 ( 2B‵ + B2‵‵ + rO )
Fig. 3. SEM images of (a) Mg2YVO6 and (b) Sr2YVO6 double perovskites.
where F(R∞) , h, ν, Εg, and A are the Kubelka–Munk (K–M) function, Plank’s constant, the light frequency, band gap, and proportionality constant, respectively.
(3) Results and discussion
where rO, rA, rB′, and rB′′ are the ionic radii of the oxygen anion and of the A, B′, and B′′ cations, respectively. The morphology and elemental weight content of the samples were investigated by SEM with EDX spectroscopy using a TESCAN VEGA3 instrument (manufactured by Shimadzu company, Japan) [21]. The chemical groups present in the samples were identified by FTIR spectroscopy in the wavelength range of 400–4000 cm−1 using a Satellite FTIR spectrometer (“serial No. 20010102”; voltage: +5/+15/-15 VDC, manufactured in the U.S.A) [4]. The UV–vis absorption spectrum was obtained using a UV mini1240 CE instrument (“serial No. A10934081718” Sm 220-240 V-50/ 60HZ −160VA, manufactured in Germany). Hydrochloric acid HCl was used as a reference for 100% absorbance. Then, the energy gap was measured for all samples using the Tauc plot method, according to the following expression [22]:
[F (R∞) hν )]n = A (hν − Eg )
X-ray diffraction analysis XRD analysis is a very important tool for the study of the structural and optical properties of alloys and compounds. Figs. 1 and 2 show the X-ray powder diffraction analysis results for A2YVO6 (A = Mg, Sr), which were carried out using a Rietveld method using the FullProf Suite [23]. The atomic positions of the elements are listed in Table 1. The crystal phases, lattice parameters, space groups, tolerance factors, R factors, and crystallite sizes of the samples are detailed in Table 2. A monoclinic lattice belonging to the P2/m space group was obtained for the all samples, and the lattice parameters were found to be a = 10.738954 Å, b = 4.719255 Å, c = 6.786163 Å, α = 90°, β = 118.445°, and γ = 90° for Mg2YVO6 and a = 21.171469 Å, b = 4.657803 Å, c = 20.203579 Å, α = 90°, β = 91.629°, and γ = 90° for Sr2YVO6. This result differs from that reported by Chia-Hui et al.
(4)
Table 2 Lattice parameters, unit cell volume, and tolerance factor for A2YVO6 where (A = Mg, Sr) double perovskite. Empirical formula
Space group
a(Å)
b(Å)
c(Å)
α
Β
γ
V (Å3)
D (nm)
T
RWP
RP
χ2
Mg2YVO6 Sr2YVO6
P 2/m P 2/m
10.738954 21.171469
4.719255 4.657803
6.786163 20.203579
90 90
118.445 91.629
90 90
343.921 1992.326
56.320 40.480
0.707 0.807
4.43 3.10
6.72 4.15
12.1 3.82
3
Results in Physics 15 (2019) 102589
Y.A. Alsabah, et al.
Fig. 4. EDX results for (a) Mg2YVO6 and (b) Sr2YVO6 double perovskites.
Fig. 5. The FTIR combined for the samples of (Mg2YVO6, Sr2YVO6) double perovskite.
Fig. 6. UV–visible combined for the samples of Mg2YVO6 and Sr2YVO6 absorption.
[24]. They obtained tetragonal structures, in the I41/amd space group for the Mg2YVO6. This difference can be attributed to the different conditions of synthesis and sintering temperatures. Similarly, Supelano et al. [25] attributed the difference in space group for their La1xMgxMnO3 ceramic system with previous reported work to different conditions of synthesis. The crystallite size in the samples was found to decrease, from 56.320 to 40.480 nm, as the ionic radius of the A site increased from 86 to 118 Å for Mg2+ and Sr2+ cations, respectively [26]. In addition, the tolerance factor calculations for the samples were confirmed, using the
X-ray diffraction results, and were found to be 0.707 and 0.807 for Mg2YVO6 and Sr2YVO6, respectively. These values verify that the samples crystallized into monoclinic structures [27], and the difference in the tolerance factor was attributed to the difference between the Asite ionic radii of the samples, specifically, 86 Å for Mg2+ and 118 Å for the Sr2+ cations. Scanning electron microscopy and energy-dispersive X-ray analysis SEM images of Mg2YVO6 and Sr2YVO6 samples are exhibited in 4
Results in Physics 15 (2019) 102589
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appearing at around 1115 cm−1 indicates the presence of the C–O group [32,33], while at 1380 cm−1, the peak suggests that CH2 groups are present in the samples [34]. In the case of Mg2YVO6, the existence of methylene (CH2) groups is also suggested by the symmetric stretching peak that appears near 2920 cm−1 [35] which are confirmed by the XRD and EDX results. A peak seen at 1630 cm−1 was assigned to a strong vibrational band of water (H–O–H) [36] and results from the presence of absorbed moisture when the samples prepared for the FTIR spectra studied. Ultraviolet–visible spectroscopic analysis The UV–Vis spectra of the samples were investigated with the aim of probing their optical properties. Fig. 4 shows a plot of the UV–vis absorption intensity for the samples versus wavelength (λ). That observed for both Mg2YVO6 and Sr2YVO6 samples, the Coefficient of absorption value α ≥ 106 cm−1 at approximately 379 nm. This may correspond to a direct electronic transition, the properties of which are important since they are responsible for electrical conduction. The absorption intensity is considered to be generally large, and this is due to the high concentration of the solution. As shown in Fig. 6, commercial TiO2 (P25) absorbed only UV light (λ less than 420 nm). The Mg2YVO6 and Sr2YVO6 samples showed absorption edges at about 365 and 370 nm, respectively, indicating that the Mg2YVO6 and Sr2YVO6 samples absorb a band of visible light with edges in the UV wavelength regions [37,38], which is possibly due to levels that appeared between the valence and conducting bands as result of defects in the samples. Mg2YVO6 was found to have stronger absorbance in UV region, while in case of Sr2YVO6, absorption was nearly exclusively in the visible region [39,40]. The optical absorption performance of an insulator was evaluated based on the band-gap energy. The band-gap energies of the samples were calculated using the Tauc equation [41]. The band-gap energy of Mg2YVO6 was found to be 2.9 eV, while the value in the case of Sr2YVO6 was found to be 2.48 eV (see Fig. 7). The energy gap, therefore, decreases as the size of the ionic radius of the A-site (A = Mg, Sr) atoms increases, perhaps because the increase in size reduces the transition distance for the valence electrons. Saad et al. [42] showed that the insulating direct band gap of 2.901 eV in the spin-down channel for Ba2CrTaO6 directly below the valence band is dominated by Cr(3d) orbitals. Xiao et al. [43] found that the band gap energies in Sr2CaWxMo1–xO6 materials increased from 2.948 to 3.908 eV upon increasing the value of x from zero to 1, since the ionic radius of tungsten is less than that of molybdenum. This is a similar case to the band gab of A2YVO6, since the ionic radii A-site increase from 86 to 118 Å for Mg2+ and Sr2+ cations, respectively.
Fig. 7. Bandgap Tauc plot for the samples of Mg2YVO6, Sr2YVO6 double perovskite.
Fig. 3. The morphology of the Mg2YVO6 and Sr2YVO6 samples appears to be highly homogenous, with large particles visible throughout the images of both samples. Alsabah et al. [9] studied the morphology of the Ba2Zn1–xNixWO6 (x = 0.00–1.00) series and observed a similar degree of homogeneity. Some aggregation of particles into clusters can be attributed to the higher-temperature calcination procedure used for our perovskite samples [28]. Furthermore, the SEM images of the Mg2YVO6 and Sr2YVO6 samples revealed grain sizes of approximately 6.5 µm in both cases. Chia-Hui et al. [24] obtained a dense crystalline Mg2YVO6 sample with an average grain size of less than 2 µm when the specimen was sintered at 1230 °C. The EDX results for the Mg2YVO6 and Sr2YVO6 samples accompanying the SEM data are shown in Fig. 4. These verify that the samples contain the same elements as the raw materials. Furthermore, the EDX data also shows that there is a correlation between the element proportions in the final products with that of the starting (raw) materials. The difference between these two sets of elemental ratios is very small, and may be a result of residual carbon in the samples after the reactions, especially in the Mg2YVO6 specimen, as defects; indeed, a few defects in the XRD patterns and the raised fragments in the SEM images support this hypothesis. Fourier-transform infrared spectroscopic analysis Fourier-transform infrared spectroscopy was used in order to verify the synthesis of the prepared Mg2YVO6 and Sr2YVO6 samples. The FTIR spectra for the samples are shown in Fig. 5. The two samples, Mg2YVO6 and Sr2YVO6, have simple, very similar spectra, following the typical pattern of perovskite material FTIR spectra. However, a slight broadening of all bands without shift was observed in case of Mg2YVO6. The FTIR spectra of the samples shows two strong and well-defined absorption bands typical of perovskite materials. The O–V–O stretching vibration mode appears at 448 cm−1, and the mode near 470 cm−1 corresponds to the B2g vibrations of the VO4−3 ion. In addition, the (Eg) mode is broadened, having a doublet structure of which the second component is around 560 cm−1 [24]. Two main absorptions peaks were clearly split and that probably as a consequence of the (V– O6) octahedral series of double perovskite series. The strong high-energy band centered around 640 cm−1 may be assigned to the anti-symmetric stretching vibration of VO6 [29,30]. The MgO stretching vibrations in the range from 600 to 800 cm−1 partially overlap with V–O stretching vibrations. Another interesting point raised by these spectra is the presence of a high-intensity band near the peaks at 820 and 880 cm−1 that assigned to the symmetric stretching vibrations of the VO6 octahedron [30,31]. Some of the weak peaks found in the FTIR spectra may perhaps be attributed to impurities of the raw materials. The band
Conclusion A2YVO6 (A = Mg, Sr) compounds were successfully prepared using a solid-state route. Characterization and optical property investigation were undertaken using X-ray diffraction analysis as well as FTIR and UV–Vis spectroscopy. Mg2YVO6 and Sr2YVO6 were demonstrated to have monoclinic crystal structures, belonging to the (P2/m) space group, and the crystallite sizes in the samples was found to decrease from 56.320 to 40.480 nm upon increasing the ionic radius at the A site. SEM data revealed that the samples had a microstructure consisting of homogenous crystallites and the EDX data showed that was obvious a correlation between the proportions of the elements in the final product and that of the raw material elements. FTIR absorption spectroscopy was used as a method for monitoring the formation of the perovskite phase. The spectra of the two samples were found to be similar and the formation of the A2YVO6 (A = Mg, Sr) double perovskite oxide was confirmed. The UV spectra results indicated that the samples have intense absorbance peaks at 365 and 370 nm, with associated band-gap energies of 2.9 and 2.48 eV for Mg and Sr, respectively. The samples 5
Results in Physics 15 (2019) 102589
Y.A. Alsabah, et al.
were demonstrated to possess insulating characteristics. The spectra of the Mg2YVO6 and Sr2YVO6 samples revealed that the different element affects the size of the optical band gap.
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Structural and optical properties of A2YVO6 (A = Mg, Sr) double perovskite oxides
T
⁎
Yousef A. Alsabaha,b,e, Ali. Taj Eldenb, Mohamad S. AlSalhia,c, , Abdelrahman A. Elbadawib,f, Mohamed A. Siddigb,d a
Research Chair in Laser Diagnosis of Cancers, College of Science, King Saud University, Riyadh, Saudi Arabia Department of Physics, Faculty of Science and Technology, Al Neelain University, Khartoum, Sudan c Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi Arabia d Department of Physics, Faculty of Science, Albaha University, Albaha, Saudi Arabia e Department of Physics, Faculty of Education and Applied Science, Hajjah University, Hajjah, Yemen f Faculty of Basic Studies, Future University, Khartoum, Sudan b
A B S T R A C T
New double perovskite oxides A2YVO6 (A = Mg, Sr) were prepared and synthesized using conventional methods via a solid-state reaction route. The samples were studied by X-ray powder diffraction (XRD) and the prepared Mg2YVO6 and Sr2YVO6 samples were shown to have monoclinic crystal structures belonging to the (P2/ m) space group. The crystallite size in the samples was found to decrease from 56.320 to 40.480 nm upon increasing the A-site ionic radius. Scanning electron microscopy (SEM) results demonstrated that the sample microstructure consisted of homogenous crystallites of approximately 6.5-µm in size, and energy-dispersive X-ray (EDX) results revealed elements ratios in samples that were identical to the expected values. FTIR results allowed the identification of particular perovskite structural features from vibrational peaks in the range of 400–4000 cm−1, and optical structure in the range of 190–600 nm was obtained from ultraviolet–visible (UV–vis) spectroscopy. The identity of the element at the A site was found to affect the band-gap energy, which decreased from 2.9 to 2.4 eV with the change from Mg2YVO6 to Sr2YVO6, indicating that the A2YVO6 (A = Mg, Sr) samples may possess insulating characteristics.
Introduction Double perovskite materials have long been attractive to materials science, physics, chemistry, and materials engineering researchers because of their interesting physical and chemical properties [1–3]. The properties of double perovskite oxide materials may be ordered depending on the degree of A-site cation doping, for example in Ba2−xSrxZnWO6 (x = 0.00, 0.25, 0.50, 0.75, 1.00) [4], Sr2MgMoO6−δ [5], LaCu3Fe4O12 [6], and (La0.8Sr0.2)2FeMnO6−δ [7], or B-site cation doping, such as in La2CoMnO6 (A = Co, Ni) [8], Ba2Zn1−-xNixWO6 (0 ≤ x ≤ 1) [9], and A2BWO6 (A = Sr, Ba; B = Co, Ni, Zn) [10]. Furthermore, the manufactures of perovskites are required to be environmentally friendly due to their ability to dealing with periodic table for materials preparation, like Ag3–2xCuxPO4 powders [11], αAg2–2xZnxWO4 (0 ≤ x ≤ 0.25) [12]. Maughan et al. [13] used X-ray powder diffraction (GSAS/EXPGUI) to study crystal structure and investigate electrical resistivity, and used ultraviolet–visible (UV–vis) diffuse reflectance and photoluminescence (PL) spectroscopy to study optical properties and band gaps as well as magnetoresonance in Sr2FeMoO6. Sarma et al. [14] analyzed the X-ray diffraction data for Cs2InAgCl6 by the Rietveld method, and then verified these results via
⁎
Mössbauer studies. Volonakis et al. [15] studied Cs2InAgX6 (X = Cl, Br, I), halide double perovskites based on bismuth and silver, and they showed that the band gaps of the materials corresponded to wavelengths in the visible region. In addition, they used first principles calculations to confirm the optical properties, which included small variations in the absorption region for the Cl sample. Organic perovskite materials are often used to produce perovskite solar cells such as the (HC(NH2)2)0.83Cs0.17Pb(I0.6Br0.4)3 perovskite [16] and mixed halide MAPbI3–xClx [17] solar cells. All types of perovskite compounds have played an important role in recent applies research and their demand in future looks likely to continue increasing because of their transport characters [4]. This property, in particular, is important, and the perovskite oxide compounds have uses in a wide variety of different industrial and scientific applications, such as magnetic resonance, microwaves, photocatalysis, sunglasses, and solar cells, motivating investigations in the fields of fundamental and applied perovskite research [9]. Accordingly, the aim of the present work was to synthesis and study the structural and optical properties of a new double perovskite, A2YVO6 in which A = Mg, Sr. In particular, the effect of substitutions at the A site was investigated and analyzed. In this article, we report the synthesis by solid-state
Corresponding author at: Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, Saudi Arabia. E-mail address: [email protected] (M.S. AlSalhi).
https://doi.org/10.1016/j.rinp.2019.102589 Received 24 May 2019; Received in revised form 11 August 2019; Accepted 12 August 2019 Available online 16 August 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 15 (2019) 102589
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Fig. 1. Shows the Rietveld pattern of Mg2YVO6 double perovskite.
Fig. 2. Shows the Rietveld pattern of Sr2YVO6 double perovskite.
and mixed in order to prepare A2YVO6 (A = Mg, Sr). The raw materials of the A2YVO6 (A = Mg, Sr) double perovskites samples were weighted by a sensitive balance (KERN electronic balance, capacity: 120 g, Readability: 0.1 mg). Then, an agate mortar and pestle was used to grind the mixture for each sample for a period of 2 h, during which time acetone was added every 30 min. The mixtures were placed in alumina crucibles. The crucibles containing the samples were placed in an oven at a temperature of 800 °C (CARBOLITE –CWF 1200 °C “Serial No. 20–302426”, manufactured in England) for 24hrs and subsequently, the samples were cooled and ground for 3hrs. The samples were then heated for 24hrs, once more, this time at a temperature of 1000 °C, and then they subjected to a second 3hrs period of cooling and grinding. A final cycle of cooling and grinding was then carried out, this time during the 24hrs heating period the temperature
reaction method of A2YVO6, where A = Mg, Sr, a new double perovskite material. Different treatments were used in order to prepare this material as a single phase. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and UV–vis spectroscopy were used as analytical tools to study the structural and optical properties of A2YVO6 (A = Mg, Sr) double perovskite samples.
Materials and methods New samples of double perovskite oxides were synthesized by solidstate reaction methods using different treatments in order to obtain a single phase. Stoichiometric amounts of Mg2Co3, Sr2Co3, Y2O3, and V2O5 were all purchased from Alfa Aesar (purity: 99.9% in each case) 2
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Table 1 Rietveld refined parameters from powder X-ray diffraction data of A2YVO6 where (A = Mg,Sr) double perovskite. A2YVO6 (A = Mg, Sr) space group (P2/m) Element
Coordinates
Mg2YVO6
Sr2YVO6
Mg2+/Sr2+
X Y Z X Y Z X Y Z X Y X
0.42035 4.58642 0.46741 0.00001 0.00001 0.00001 1.31270 2.23775 0.01877 0.52938 1.43251 0.09381
0.41166 1.20755 0.32208 0.00001 0.00001 0.00001 1.28315 2.21890 0.00266 0.52664 1.25433 0.18234
Y3+
V5+
O2–
was set to 800 °C. Finally, when the mixtures reached room temperature, the samples of A2YVO6 (A = Mg, Sr) were obtained. The following chemical equation was used to determine the weights of the raw materials.
⎧ 2MgCo3 ⎫ + 1 Y2O3 + 1 V2O5 → ⎧ M g 2YV O6 ⎫ + 2Co2 ↑ ⎨ ⎨ 2 2 ⎩ Sr2YV O6 ⎬ ⎭ ⎩ 2SrCo3 ⎬ ⎭
(1)
The crystal structure of calcined samples were Examined by X-ray diffraction (Shimadzu, MAX_X, XRD-7000) [18], using Cu Kα radiation with scanning speed of 1000°/min, and the crystallite size was calculated from the main peak for sample using the Debye–Scherer equation [19] as follows:
D=
0.94λ β cosθ
(2)
where D is the crystallite size, λ is the wavelength (1.5405 Å), β is the full width at half maximum, and ?? is the diffraction angle. The crystalline structure of the samples was also predicted by the tolerance factor (t) was also investigated; in general, for double perovskites A2B′B′′O6, this can be written as [20]:
t=
(rA + rO) r r 2 ( 2B‵ + B2‵‵ + rO )
Fig. 3. SEM images of (a) Mg2YVO6 and (b) Sr2YVO6 double perovskites.
where F(R∞) , h, ν, Εg, and A are the Kubelka–Munk (K–M) function, Plank’s constant, the light frequency, band gap, and proportionality constant, respectively.
(3) Results and discussion
where rO, rA, rB′, and rB′′ are the ionic radii of the oxygen anion and of the A, B′, and B′′ cations, respectively. The morphology and elemental weight content of the samples were investigated by SEM with EDX spectroscopy using a TESCAN VEGA3 instrument (manufactured by Shimadzu company, Japan) [21]. The chemical groups present in the samples were identified by FTIR spectroscopy in the wavelength range of 400–4000 cm−1 using a Satellite FTIR spectrometer (“serial No. 20010102”; voltage: +5/+15/-15 VDC, manufactured in the U.S.A) [4]. The UV–vis absorption spectrum was obtained using a UV mini1240 CE instrument (“serial No. A10934081718” Sm 220-240 V-50/ 60HZ −160VA, manufactured in Germany). Hydrochloric acid HCl was used as a reference for 100% absorbance. Then, the energy gap was measured for all samples using the Tauc plot method, according to the following expression [22]:
[F (R∞) hν )]n = A (hν − Eg )
X-ray diffraction analysis XRD analysis is a very important tool for the study of the structural and optical properties of alloys and compounds. Figs. 1 and 2 show the X-ray powder diffraction analysis results for A2YVO6 (A = Mg, Sr), which were carried out using a Rietveld method using the FullProf Suite [23]. The atomic positions of the elements are listed in Table 1. The crystal phases, lattice parameters, space groups, tolerance factors, R factors, and crystallite sizes of the samples are detailed in Table 2. A monoclinic lattice belonging to the P2/m space group was obtained for the all samples, and the lattice parameters were found to be a = 10.738954 Å, b = 4.719255 Å, c = 6.786163 Å, α = 90°, β = 118.445°, and γ = 90° for Mg2YVO6 and a = 21.171469 Å, b = 4.657803 Å, c = 20.203579 Å, α = 90°, β = 91.629°, and γ = 90° for Sr2YVO6. This result differs from that reported by Chia-Hui et al.
(4)
Table 2 Lattice parameters, unit cell volume, and tolerance factor for A2YVO6 where (A = Mg, Sr) double perovskite. Empirical formula
Space group
a(Å)
b(Å)
c(Å)
α
Β
γ
V (Å3)
D (nm)
T
RWP
RP
χ2
Mg2YVO6 Sr2YVO6
P 2/m P 2/m
10.738954 21.171469
4.719255 4.657803
6.786163 20.203579
90 90
118.445 91.629
90 90
343.921 1992.326
56.320 40.480
0.707 0.807
4.43 3.10
6.72 4.15
12.1 3.82
3
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Fig. 4. EDX results for (a) Mg2YVO6 and (b) Sr2YVO6 double perovskites.
Fig. 5. The FTIR combined for the samples of (Mg2YVO6, Sr2YVO6) double perovskite.
Fig. 6. UV–visible combined for the samples of Mg2YVO6 and Sr2YVO6 absorption.
[24]. They obtained tetragonal structures, in the I41/amd space group for the Mg2YVO6. This difference can be attributed to the different conditions of synthesis and sintering temperatures. Similarly, Supelano et al. [25] attributed the difference in space group for their La1xMgxMnO3 ceramic system with previous reported work to different conditions of synthesis. The crystallite size in the samples was found to decrease, from 56.320 to 40.480 nm, as the ionic radius of the A site increased from 86 to 118 Å for Mg2+ and Sr2+ cations, respectively [26]. In addition, the tolerance factor calculations for the samples were confirmed, using the
X-ray diffraction results, and were found to be 0.707 and 0.807 for Mg2YVO6 and Sr2YVO6, respectively. These values verify that the samples crystallized into monoclinic structures [27], and the difference in the tolerance factor was attributed to the difference between the Asite ionic radii of the samples, specifically, 86 Å for Mg2+ and 118 Å for the Sr2+ cations. Scanning electron microscopy and energy-dispersive X-ray analysis SEM images of Mg2YVO6 and Sr2YVO6 samples are exhibited in 4
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appearing at around 1115 cm−1 indicates the presence of the C–O group [32,33], while at 1380 cm−1, the peak suggests that CH2 groups are present in the samples [34]. In the case of Mg2YVO6, the existence of methylene (CH2) groups is also suggested by the symmetric stretching peak that appears near 2920 cm−1 [35] which are confirmed by the XRD and EDX results. A peak seen at 1630 cm−1 was assigned to a strong vibrational band of water (H–O–H) [36] and results from the presence of absorbed moisture when the samples prepared for the FTIR spectra studied. Ultraviolet–visible spectroscopic analysis The UV–Vis spectra of the samples were investigated with the aim of probing their optical properties. Fig. 4 shows a plot of the UV–vis absorption intensity for the samples versus wavelength (λ). That observed for both Mg2YVO6 and Sr2YVO6 samples, the Coefficient of absorption value α ≥ 106 cm−1 at approximately 379 nm. This may correspond to a direct electronic transition, the properties of which are important since they are responsible for electrical conduction. The absorption intensity is considered to be generally large, and this is due to the high concentration of the solution. As shown in Fig. 6, commercial TiO2 (P25) absorbed only UV light (λ less than 420 nm). The Mg2YVO6 and Sr2YVO6 samples showed absorption edges at about 365 and 370 nm, respectively, indicating that the Mg2YVO6 and Sr2YVO6 samples absorb a band of visible light with edges in the UV wavelength regions [37,38], which is possibly due to levels that appeared between the valence and conducting bands as result of defects in the samples. Mg2YVO6 was found to have stronger absorbance in UV region, while in case of Sr2YVO6, absorption was nearly exclusively in the visible region [39,40]. The optical absorption performance of an insulator was evaluated based on the band-gap energy. The band-gap energies of the samples were calculated using the Tauc equation [41]. The band-gap energy of Mg2YVO6 was found to be 2.9 eV, while the value in the case of Sr2YVO6 was found to be 2.48 eV (see Fig. 7). The energy gap, therefore, decreases as the size of the ionic radius of the A-site (A = Mg, Sr) atoms increases, perhaps because the increase in size reduces the transition distance for the valence electrons. Saad et al. [42] showed that the insulating direct band gap of 2.901 eV in the spin-down channel for Ba2CrTaO6 directly below the valence band is dominated by Cr(3d) orbitals. Xiao et al. [43] found that the band gap energies in Sr2CaWxMo1–xO6 materials increased from 2.948 to 3.908 eV upon increasing the value of x from zero to 1, since the ionic radius of tungsten is less than that of molybdenum. This is a similar case to the band gab of A2YVO6, since the ionic radii A-site increase from 86 to 118 Å for Mg2+ and Sr2+ cations, respectively.
Fig. 7. Bandgap Tauc plot for the samples of Mg2YVO6, Sr2YVO6 double perovskite.
Fig. 3. The morphology of the Mg2YVO6 and Sr2YVO6 samples appears to be highly homogenous, with large particles visible throughout the images of both samples. Alsabah et al. [9] studied the morphology of the Ba2Zn1–xNixWO6 (x = 0.00–1.00) series and observed a similar degree of homogeneity. Some aggregation of particles into clusters can be attributed to the higher-temperature calcination procedure used for our perovskite samples [28]. Furthermore, the SEM images of the Mg2YVO6 and Sr2YVO6 samples revealed grain sizes of approximately 6.5 µm in both cases. Chia-Hui et al. [24] obtained a dense crystalline Mg2YVO6 sample with an average grain size of less than 2 µm when the specimen was sintered at 1230 °C. The EDX results for the Mg2YVO6 and Sr2YVO6 samples accompanying the SEM data are shown in Fig. 4. These verify that the samples contain the same elements as the raw materials. Furthermore, the EDX data also shows that there is a correlation between the element proportions in the final products with that of the starting (raw) materials. The difference between these two sets of elemental ratios is very small, and may be a result of residual carbon in the samples after the reactions, especially in the Mg2YVO6 specimen, as defects; indeed, a few defects in the XRD patterns and the raised fragments in the SEM images support this hypothesis. Fourier-transform infrared spectroscopic analysis Fourier-transform infrared spectroscopy was used in order to verify the synthesis of the prepared Mg2YVO6 and Sr2YVO6 samples. The FTIR spectra for the samples are shown in Fig. 5. The two samples, Mg2YVO6 and Sr2YVO6, have simple, very similar spectra, following the typical pattern of perovskite material FTIR spectra. However, a slight broadening of all bands without shift was observed in case of Mg2YVO6. The FTIR spectra of the samples shows two strong and well-defined absorption bands typical of perovskite materials. The O–V–O stretching vibration mode appears at 448 cm−1, and the mode near 470 cm−1 corresponds to the B2g vibrations of the VO4−3 ion. In addition, the (Eg) mode is broadened, having a doublet structure of which the second component is around 560 cm−1 [24]. Two main absorptions peaks were clearly split and that probably as a consequence of the (V– O6) octahedral series of double perovskite series. The strong high-energy band centered around 640 cm−1 may be assigned to the anti-symmetric stretching vibration of VO6 [29,30]. The MgO stretching vibrations in the range from 600 to 800 cm−1 partially overlap with V–O stretching vibrations. Another interesting point raised by these spectra is the presence of a high-intensity band near the peaks at 820 and 880 cm−1 that assigned to the symmetric stretching vibrations of the VO6 octahedron [30,31]. Some of the weak peaks found in the FTIR spectra may perhaps be attributed to impurities of the raw materials. The band
Conclusion A2YVO6 (A = Mg, Sr) compounds were successfully prepared using a solid-state route. Characterization and optical property investigation were undertaken using X-ray diffraction analysis as well as FTIR and UV–Vis spectroscopy. Mg2YVO6 and Sr2YVO6 were demonstrated to have monoclinic crystal structures, belonging to the (P2/m) space group, and the crystallite sizes in the samples was found to decrease from 56.320 to 40.480 nm upon increasing the ionic radius at the A site. SEM data revealed that the samples had a microstructure consisting of homogenous crystallites and the EDX data showed that was obvious a correlation between the proportions of the elements in the final product and that of the raw material elements. FTIR absorption spectroscopy was used as a method for monitoring the formation of the perovskite phase. The spectra of the two samples were found to be similar and the formation of the A2YVO6 (A = Mg, Sr) double perovskite oxide was confirmed. The UV spectra results indicated that the samples have intense absorbance peaks at 365 and 370 nm, with associated band-gap energies of 2.9 and 2.48 eV for Mg and Sr, respectively. The samples 5
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were demonstrated to possess insulating characteristics. The spectra of the Mg2YVO6 and Sr2YVO6 samples revealed that the different element affects the size of the optical band gap.
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