Optical and transport properties of double perovskite strontium bismuth vanadate

Journal of Molecular Structure 1205 (2020) 127607

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Optical and transport properties of double perovskite strontium bismuth vanadate R.K. Parida a, D.K. Pattanayak b, Bhagyashree Mohanty a, B.N. Parida c, * a

Department of Physics, Faculty of Engineering and Technology (ITER), Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India Department of Physics, GIET University, Gunupur, Raygada, India c Department of Physics, Central Institute of Technology Kokrajhar, (Deemed to be University, MHRD, Govt. of India) BTAD, Assam, 783370, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 October 2019 Received in revised form 14 December 2019 Accepted 17 December 2019 Available online 18 December 2019

The present study reports the optical and transport properties of polycrystalline vanadium based double perovskite. The material preparation, formation, symmetry, morphology and phase identification were carried out by respective solid solution casting, X-ray diffraction (XRD), scanning electron micrograph (SEM) image, energy dispersive spectroscopy (EDS), FTIR and RAMAN technique. The optical properties of the material were investigated by UVeVis spectroscopy as well as photo luminescence (PL) spectra which reveal material may be useful for photo catalytic devices. For ferroelectric and transport behavior of the material were studied from dielectric, polarization and impedance measurement. It is observed from above measurement material have good room temperature dielectric constant (εr), polarization value (2Pr) and moderate transport properties with temperature for different device application. © 2019 Elsevier B.V. All rights reserved.

Keywords: XRD RAMAN FTIR Photo-catalytic Optical properties

1. Introduction The discovery of perovskite CaTiO3 mineral triggered material scientists to expedite the study of other perovskite minerals like BaTiO3, SrTiO3, PbTiO3, Pb(ZrTi)0.5O3 (PZT) etc., due to their tremendous ferroelectric, piezoelectric and pyroelectric behaviors. Above all PZT ceramics are mostly preferred due to their unique piezoelectricity properties useful in various electronic devices such as sensors, actuators and memory [1]. Toxicity and fatigue nature of Pb based material restrain the utilities [2] which enforced mineralogist to divert research towards lead free ceramics which can able to replace PZT [3,4]. In contrast to above, transitional metal oxide (TMO) based double perovskites are preferred due to their striking ferroelectricity with room temperature remnant polarization Pr ¼ 38lC/cm2 and have comparatively high Curie temperature Tc ¼ 320  C [5e7]. The other attractions to the researchers for TMO based double perovskite oxides (DPOs) are due to their tremendous physical and chemical properties. In view of the diverse utilities [8e10], like energy harvesting in LaNiMnO6 [9], ferroelectricity in Pb2Mn0.6Co0.4WO6 [11], Multiferroic behavior in Bi2NiMnO6 and Bi2FeCrO6 [12], superconducting phenomena of Sr2YRu0.95Cu0.05O6

* Corresponding author. E-mail addresses: [email protected], [email protected] (B.N. Parida). https://doi.org/10.1016/j.molstruc.2019.127607 0022-2860/© 2019 Elsevier B.V. All rights reserved.

[13], magneto resistance phenomena of Sr2FeMoO6 [14], dielectric resonator behavior of Ca2AlTaO6 and SrAlTaO6) [15], and photocatalysis phenomena of Cs2BiAgCl6 [16] are most important reports. Materials possessing ABO3 (perovskite) and A2B’B00 O6 (double perovskite) were thought of as they exhibit a broad range of multifunctional behavior such as ferroelectricity, antiferroelectricity, piezoelectricity, conducting and semiconducting, superconductivity, ferromagnetic and anti-ferromagnetic etc. Exhibits broad range of crystal structure systems [17e19]. Investigation of double perovskite oxide based compounds (DPOs) dates back to 1961 as reported by Longo and Ward [20]. DPOs are derived from ABO3 perovskite systems when the octahedral integrated Bcations are occupied by two different kinds of cations such as B0 and B00 with large difference in size or charge [21]. In addition to these B-cations in double perovskite possess chemical flexibility, compositional expanses and space spanning configuration. In view of the above DPOs are also reliant in chemical and physical aspects. Thus choice of B-site cation in DPOs is an essential significance for the chemical designing of structural, electrical, mechanical and optical characteristics. Ferroelectric perovskites which are application oriented novel materials are of substantial interest as they have been found befitting for non-volatile memory, multilayer capacitors, detectors, transducers, sensors and actuator applications [22,23]. In convoluted perovskite based ceramics, appraisal of the

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

2.2.2. Optical assessments FTIR and RAMAN spectra are recorded at room temperature to analyze the molecular vibration of the prepared sample both. The make for FTIR spectra was JASCO-FTIR/4100 Infrared spectrophotometer while the make for RAMAN spectra was NIR Raman spectrometer where Arþ laser was used as the source for excitation

20

30

40 50 60 Bragg's Angle (2 )

70

Fig. 1. Room temperature XRD pattern of Sr2BiVO6.

(315)

(220) (2 1 10)

(125) (300) (0 2 10)

(211) (119)

2.2.1. Structure and microstructure characterization The desirable formation of the compound and its structural system was monitored using the XRD pattern. Calcined powder was cold pressed and cylindrical pellets of 10 mm diameter and 0.5e2 mm thickness were developed at 4  106 N/m2 pressure by the help of a hydraulic press upon being mixed with a binder PVA (polyvinyl alcohol) to reduce its brittleness and the binder evaporates at high temperature sintering process. The sintered temperature was optimized at 725  C for 4 h after repeated firing. Pellets were then polished for parallel and smooth faces. The formation along with quality of the compound was confirmed from Powder Xray diffraction (XRD) technique. The XRD was carried out by using a diffractometer (RIGAKU, JAPAN, ULTIMA IV) with CuKa radiation (k ¼ 1.5404 Å). XRD was carried from 2q ¼ 20 to 2q ¼ 80 (where 2q ¼ Bragg’s angle) and the scanning rate being 2 per minute. Scanning Electron Microscope (SEM) (MODEL-JOEL JSM 5800) was used for micro-structural analysis of the sample.

(205)

2.2. Characterization techniques

The XRD technique of powdered sample is suitable to determine the crystal structure, lattice parameter, atomic position, Bravais lattice, particle size and space group. Fig. 1 reflects the XRD pattern of the investigated sample. It is observed several numbers of sharp peaks located at different Bragg angle which is diverged from that of the ingredients, concluding the formation of single phase new compounds. All the peaks are indexed in various crystal systems with the help of commercially available software ‘POWD’ it is found in monoclinic system have minimum deviation among observed and calculated value of inter-planer spacing. On the basis of above observation monoclinic crystal system is selected with respective lattice parameters a ¼ 7.71 Å, b ¼ 5.85 Å, c ¼ 29.06 Å. The average values of particle size (P) of the compounds are found to be 54 nm by using Scherrer’s equation [26]. As the diffraction peaks of all the samples are broad enough therefore, the average particle sizes are coming in nm order. Once again Bragg angle (2q) and Miller indices

(107) (202) (009)

The materials were purchased (M/s LOBA Chemie pvt. Ltd. India) with a purity of 99.99% AR grade and were weighed as per the desirable stoichiometry. The constituent powders are remarkably mixed using an agate-motor and pestle system under dry conditions for 2 h followed by methanol (wet) medium for another 3 h to obtain homogeneous mixture at room temperature. The mixture thus formed was calcined at a high temperature of 710  C (optimized) being placed in a high-pure alumina crucible for about 4 h in a high temperature muffle furnace. The sintering was done at an optimized temperature of 725  C.

3.1. Structural and micro-structural analysis

(110)

Bi2O3 þ V2O5 þ4SrCO3 ¼ 2 Sr2BiVO6

3. Characterizations

(601)

Sr2BiVO6 was prepared through solid-state reaction technique by using the raw materials SrCO3 (strontium carbonate), Bi2O3 (Bismuth III oxide)and V2O5 (vanadium pent oxide) in the powdered form. The raw materials are mixed in proper with which were mixed in stoichiometry proportions as per the following chemical equation.

(015)

2.1. Sample preparation

2.2.3. Dielectric measurements The electrical properties of the prepare sample were studied by a computer-controlled LCR meter (PSM LCR 4NL, Model: 1735, UK), with the help of a laboratory-designed sample holder, Achromel alumel thermo-couple and a digital mill voltmeter (KUSAM MECO 108). Pellet form of the sample was used for this purpose. Two opposite flat faces of the pellets were silver coated then dried at about 150  C for 4 h and then cooled to the room temperature before utilizing for measurement. The temperature of the pellets were varied from 30  C to 500  C and the frequency was varied from 1 kHz to 5 MHz.

(006) (311)

2. Experimental details

along with a diffraction grating of 1200 mm1 as the monochromator to compute the vibrational modes of the synthesized ceramic. Absorption spectra in the reflection geometry were examined using the UVeVisible (UV-VIS) absorption spectrometer in the provided range of 200e800 nm. The Luminescence dominion was measured by using a spectrophotofluorometer.

(111) (104)

relaxation behavior due to the defects in the compound formed during fabrication unexpectedly is studied from the modulus and impedance analysis. Information about conductivity, associated relaxation and deviation of the dielectric response is yielded through electric modulus analysis. Oxygen based sensors, oxide based fuel cell, oxygen separation devices etc. Can be fabricated by using doped Bismuth Vanadates [24,25]. This is the main thought for selecting double perovskite vanadates. Although plenty of works have been reported on DPO materials still lead free double perovskite is lacking in literature. In view of these we have emphasized on lead free double perovskite and explored its possible device application. We have selected the cost effective mixed oxide route to prepare the proposed sample Sr2BiVO6 and examined for possible device application.

Intensity (Arb. Unit)

2

80

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

of the strongest peaks in A-site partially substituted Srþ2 compound are 28.79 and (0 1 5) respectively which suggest crystal growth completed along (0 1 5) direction. In addition to POWD indexing, the peaks are tried to fit the full proof reitveld technique using Pseudo-Vogti function and found minimum goodness of fit (GOF)/(chi)2 is 2.63 which confirms the peaks are well matched with theoretical function. The theoretical fitting of experimental data of the studied sample is reflected in Fig. 2. The selected crystal system is monoclinic with respective parameters, space group, Bragg’s factor and atomic positions are compared in Table 1. To study density and electronic properties of the material SEM image of the sample is reflected in Fig. 3(a). It is observed small grains are uniformly distributed throughout the surface with well separated grain boundaries. It can be revealed that the charge carries may get a free path to increase the leakage current and the compound might be of lossy nature. As a result of which dielectric loss of sample as well as the ac conductivity may increase. The sintered samples are seen to possess some small voids with irregular shape and size. These voids are responsible for the hopping motion of the charge carriers. Intercept technique is used to calculate the average grain size and is found to be 1.553 mm. EDS analysis of the studied compounds is presented in Fig. 3(b). From the EDS it is confirmed that no foreign particles are present in the compound and also gives information on the percentage of the composition. Apart from XRD and SEM analysis, FTIR analysis is a vital tool to explore the molecular vibration in the sample. 3.2. Raman Spectroscopy Raman Spectroscopy is a non-destructive technique to identify the mode of vibrational frequency in the vanadate based compounds. This method of studying the spectra is complementary to IR absorption spectroscopy. For the measurements of the Raman scattering, the laser source was focused onto a diameter of 2.0 mm of the sample using a 10.0 mW of power and the scattered signal was acquired using a thermoelectrically cooled CCD camera as the detector. The strontium based vanadate double perovskite belonging to octahedral tilting system with monoclinic structure of space group P21/n. According to group theory P21/n space group possess 24 vibrational band which can be represented by G ¼ y1(Ag þ Bg) þ y2(2Ag þ 2Bg) þ y5(3Ag þ 3Bg) þ T (3Ag þ 3Bg) þ L (3Ag þ 3Bg). The experimental data detected in Fig. 4 only have eight vibrational bands observed at 265, 326, 451, 534, 619, 793, 834 and 1053 cm1 respectively. In the studied compound there are two formula units of Sr2BiVO6 per unit cell which reveals Raman spectra possess all the modes of components Ag and Bg. In single crystal frequency of Ag and Bg are very close for which polarization Raman spectra are necessary to resolve it. If similar situation is

3

arisen in polycrystalline sample i.e. in the studied sample, then it will behave as a single band of symmetric stretching y1, doublet for asymmetric stretching y2 and triplet for symmetric bending y5. The peak located at 834 cm1 in Raman spectra may be ascribed to oxygen symmetric stretching band y1. In the present sample octahedral VO6 and BiO6 possess symmetric stretching mode of vibration, where cations are stationary and oxygen atoms are moving about the BieOeV axis which result strong peaks are seen at 834 cm1 and 326 cm1 [27e29]. There exits weak intense peaks related to asymmetric oxygen stretching y2 but peak splitting cannot be detected without the help of polarization Raman spectra. The peak at 1053 cm1 in the studied compound is ascribed to SrVO4 scheelite phase which is also noticed in XRD data of the compound. Very weak AVO4 scheelite peaks are also noticed in the spectra of several other compounds [30,31]. The peak assigned at 326 cm1 can be ascribed as the internal oxygen bending vibration y5. As in the studied spectra Ag and Bg components are not properly resolved in normal Raman spectroscopy so it expected y5 mode is triplet degenerated. In few compounds [30] doublet or triplet degeneracy of peaks are clearly visualized as tolerance factor of the groups is unclear whether it is maximum or minimum. This bending mode of vibrations may be due to octahedral symmetric bending of OeSreO. The peak located at 265 cm1 can be ascribed as vibrational lattice mode of vibration. Generally in double perovskite P21/n space group possess six vibration modes three each from Ag and Bg component. In the investigated compound one vibrational mode of vibration is visualized and others are not seen, this is may be due to either lowest intense peaks or below the experimental limitation wave numbers [31]. There is an ambiguity for small peak located at 451 cm1 which can be assigned as either external vibrational or internal bending mode [30]. The translational lattice modes of vibrations are generally expected to appear in lower wave number range. As discussed above in double perovskite P21/n space group have both Ag as well as Bg components possessing triplet degeneracy of total six modes. There is also another ambiguity regarding the end of vibrational mode and starting of translational mode. To differentiate these two modes polarization Raman spectroscopy of the sample is essential. Ayala et al. [32]. reported in their study that vibrational lattice mode lies within 190e310 cm1 and below 190 cm1 related to translational lattice mode. In the present study neither it is approached polarization Raman spectra nor have data below 200 cm1 so it is difficult to assign any translational mode of vibration. The moderate and slightly broad peak located at 793 cm1 once again can be ascribed to stationary oxygen atoms are vibrating symmetrically about BieOeV axes [28]. The other weak peaks present are due to the VeO and SreO irrational modes about the main Raman mode of VeBi, VeSr, SreBi bonds (see Fig. 5). 3.3. FTIR analysis

Fig. 2. Illustrates the reitveld fitting of experimental data.

FTIR analysis is supplementary part to strengthen phase confirmation in the investigated compound. The room temperature FTIR data is illustrated in Fig. 5.In the studied wave number range, it is observed there are number of vibrations located at 3675, 1488, 1101, 965, 858, 665, 501 cm1 in the present sample. As per literature it is reported [33] ordinarily, lattice vibrations are occurred within the wave number range of 850 cm1e400 cm1. The strongest peak located at 665 cm1 can be ascribed to antisymmetric stretching mode of the cations VO6 and second strongest peak at 501 cm1 may be assigned to symmetric stretching BiO6 about the axis VeOeBi since octahedral BO6 belongs to higher concentration of charge carriers [34,35]. The weak intense peaks located at 684 cm1and 858 cm1 are may be due to symmetric and asymmetric bending mode of vibrations for OeVeO. The relatively

4

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

Table 1 Compares the different parameters and Atomic position of the compound. ssAtom

#

OX

SITE

X

Y

Z

SOF

H

ITF(B)

Bi Bi Bi Bi Sr Sr Sr Sr Sr Sr Sr Sr V V V V O O O O O O O O O O O O O O O O O O O O O O O O

1 2 3 4 1 2 3 4 5 6 7 8 1 2 3 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

3.0 3.0 3.0 3.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 5.0 5.0 5.0 5.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a

0.074 (2) 0.816 (2) 0.616 (2) 0.401 (2) 0.588 (2) 0.803 (2) 0.867 (2) 0.117 (2) 0.182 (3) 1.007 (3) 0.928 (3) 0.260 (3) 0.351 (2) 0.174 (2) 0.028 (3) 0.335 (2) 0.40545 0.16801 0.31289 0.52053 0.10038 0.06031 0.14523 0.39184 0.09583 -.19428 0.12143 0.08951 0.32443 0.37136 0.50226 0.14235 0.385 (3) 0.327 (4) 0.344 (4) 0.176 (4) 0.023 (4) 0.370 (4) 0.047 (3) 0.160 (3)

0.501 (3) -.007 (2) 0.004 (3) 0.488 (2) 0.026 (3) 0.484 (2) 0.044 (2) 0.477 (3) -.012 (2) 0.511 (3) 0.463 (2) -.027 (3) 1.056 (2) 0.027 (2) -.490 (2) 0.510 (2) 1.25862 1.14238 0.79945 1.02655 -.08889 -.08617 0.31922 -.03279 -.62214 -.46337 -.22372 -.65251 0.26055 0.73639 0.49261 0.55131 0.190 (4) 0.732 (5) 0.276 (4) 0.217 (5) 0.728 (5) 0.765 (5) 0.233 (4) 0.704 (4)

0.2010 (6) 0.1582 (6) 0.8975 (6) 0.4470 (4) 0.3747 (7) 0.8201 (7) 0.2781 (6) 0.7295 (6) 0.5190 (7) 0.07584 (7) 0.4737 (7) 0.1284 (7) 0.2388 (6) 0.0035 (6) 0.6000 (7) 0.8573 (7) 0.19935 0.26421 0.21142 0.28042 0.0527 -.04402 0.00419 0.00145 0.6512 0.59636 0.59811 0.55468 0.88836 0.8947 0.82161 0.82483 0.721 (1) 0.718 (1) 0.376 (1) 0.457 (1) 0.137 (1) 0.384 (1) 0.142 (1) 0.468 (1)

1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.38 1.38 1.38 1.38 1.38 1.38 1.38 1.38

(5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (9) (9) (9) (9) (9) (9) (9) (9)

GOF¼ (Chi) 2 ¼ 2.6315343. Rwp (%) ¼ 32.055767. Rexp(%) ¼ 12.181398. RB (%) ¼ 24.282022. Unit Cell: a ¼ 7.72 Å, b ¼ 5.84 Å, c ¼ 29.07 Å, a ¼ 90.0, b ¼ 94.27, q ¼ 90.0. Volume: V ¼ 1308.53 Å3. Space group: P 1 n 1. Crystal System: Monoclinic.

weak peaks at 965 cm1, 1101 cm1 may be ascribed to respective symmetric and asymmetric stretching vibration of OeSreO. The weakest bands at 1488 cm1 is may be assigned as defect vibration present in the material. The broad absorption band at 3675 cm1 is attributed to the stretching vibration due to presence of hydroxyl (OH) group that tends to obey the exclusion principle along with hygroscopic nature of the studied compound [36]. In the studied compound Table 2 compares the different modes of vibration for Raman and IR spectra. It is observed Raman active bands are inactive in IR spectra and vice-versa obeying mutual exclusive principle [37].

3.4. UVevis spectroscopy Ultravioletevisible (UVeVis) Spectroscopy has been used for determining band structures of semiconductors. It further amounts response of the sample to UV and visible range of electromagnetic radiations. The diffuse transmittance of the sample is given by

T¼

Number of photons transmitted by the sampleðIÞ Number of photons transmitted by the sampleðI0 Þ

The expression A ¼ - log10 (T) relates the absorbance (A) and transmittance (T) of the sample. The sample absorbs effectively with the absorbance attending maximum value in the visible region of the spectrum. Strong absorption band is seen between 300 nme400 nm in the Fig. 6 (a) corresponds to the absorption edge of vanadium. These bands are due to the transfer of charge from highest filled 2p orbital of O2 to the empty 3d orbital of vanadium [13]. Again in the absorption spectra under UV range a broad peak is observed which reveals sample absorb strongly in this range whereas the reverse effect is observed in the visible range. Further the Fig. 6 (a) shows the absorption cut-off wavelength to be 550 nm. Band gap of the proposed sample is calculated by using Mott and Davis [38].

a hv ¼ A ðhv  EgÞn

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

5

(a)

(b)

Fig. 3. (a) SEM monograph and (b) EDS spectra of the studied sample.

where, a ¼ absorption coefficient, hn ¼ photon energy, A ¼ constant, Eg ¼ optical energy and ‘n’ is an index associated with different electronic transitions. For direct allowed transition n ¼ ½, for direct forbidden transition n ¼ 3/2, for indirect allowed transition n ¼ 2 and for indirect forbidden transition n ¼ 3. The band gap energy of the prepared sample is calculated from (ahn) 2 vs. hn graph which is found to be about 2.47 eV by linear fitting the curve as represented in Fig. 6 (c). Fig. 6 (b) reflects the reflectance of the corresponding absorbance which confirms the supplementary idea about UVeVis spectra. The calculated energy band gap of the present sample is effectively useful for photo catalytic devices as like as few TiO2 based perovskite oxide compounds and compared in Table 3. The above UVeVis spectroscopy study confirms the material can be useful for photo catalytic activity. The proposed photo catalytic mechanism in the present sample can be explained as per the perovskite oxides. Usually the above activity in present double

perovskite sample is due to presence of octahedral VO6. As discussed above in the studied sample 2p-orbital of O2 act as valence band and empty 3d-orbital of Vþ5 reflects the conduction band. When ultraviolet light irradiated over the powder sample electrons are excited from valence band to conduction band which generates reactive electron-hole pairs to induce oxidation of organic molecules as a result holes are created at valence band. Generally, holes (hþ) are strong oxidant on the surface of the studied sample and able to oxidized the adsorbed H2O molecules or hydroxyl ions into OH radicals (H2O or OH þ hþ / $OH). On the other hand adsorb O2 reacts with electrons to form super oxide ions. These radicals and super oxide ions degrade and decompose the organic molecules [40e42]. 3.5. Photoluminescence spectroscopy According to molecular orbital theory [43] vanadium based

6

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

Absorbance(Arb. unit)

834 793

265

Abs UV Range

300-375 nm

Visible Range

534

(Arb. unit) % Reflectance

2500

Fig. 4. Room temperature Raman spectra of Sr2BiVO6.

Eg=2.47eV

50

( h )2

1053

1000 1500 -1 2000 Raman Shift (cm )

(c)

60 R

633 nm

500

633 nm

200 300 400 500 600 700 Wavelength (nm) (b)

619

451

Intensity (Arb. Unit)

326

(a)

40 30

data

20 10

300-375 nm UV range

Visible range

200 300 400 500 600 700 Wavelength (nm)

0 1

2

3

4 5 h (eV)

6

7

499 422 856 664 1099 961

1482

3675

Transmittance %age (Arb. Unit)

Fig. 6. Shows the UV-VIS spectra of the studied sample.

4000 3500 3000 2500 2000 1500 1000 -1 Wave number (cm )

500

Fig. 5. FTIR spectra (transmittance versus wave number) of the sample at a room temperature in the range of 4000e400 cm1.

compound are familiar for their efficient photoluminescence behavior. In the octahedral vanadate charge transfer between 2p to 3d orbitals of vanadium is responsible for excitation. Fig. 7 depicts the excitation and emission bands of vanadium based complex compound. As per the literature [44] the PL behavior in double perovskite Sr2BiVO6 is ascribed to (VO4)3- groups, which in turn a large Stokes shifts for the emission and density of states as well as projected

density of states for the excitation. It seems reasonable to ascribe the excitation to a charge transfer from oxygen (O) 2p orbital to central vanadium (V) 3d orbital inside the (VO4)3- groups due to the difference of density of state between the (V) 3d and bismuth (Bi) 6s-6p on the bottom of the conduction band and moreover the excitation of (VO4)3- is often found around 350 nm in the literature [44,45]. Additionally, the excitation process of Bi3þ do not involve a transition from the (Bi) 6s states situated at the top of the valence band to the lower energy (Bi) 6p states situated in the conduction band but to higher energy (Bi) 6p states which comforts of the “Vanadate” excitation. The room temperature photoluminescence spectra of annealed Sr2BiVO6 are illustrated in Fig. 7. The corresponding graph represents broad excitation band ranging from 360 to 500 nm, that is basically because of typical fef transition at the 3H4 ground state to the 4PJ (J ¼ 0, 1, 2 and 3) excited states of Sr2þ [44]. The probable sharp peaks are found at about 429 nm, 450 nm, 482 nm and 491 nm. The peaks at 450 and 482 nm depicted in Fig. 7 are due to magnetic dipole transition 5D0 / 7F1 of B-site atoms i.e. Bi and V in studied sample. This suggests that the B-site is a center-symmetric site [46], which is coincident with the structural data [47]. The peak at 429 nm is due to electric dipole transition 5D0 / 7F2 in the A-site ‘Sr’ atoms in the studied compound which is inversely becomes stronger than the magnetic dipole transition 5D0 / 7F1, which make us believe that, a structurally non-centrosymmetry site. Again according to Blasse theory [46] in order to have high energy transfer efficiency in the VeOeBi where V is a high-valence d0 ion

Table 2 Compares the RAMAN and FTIR Peaks for different modes of vibration. RAMAN Shift in cm1

Modes of Vibration & Ref.

FTIR Peaks in cm1

Modes of Vibration

265 326 451 534 619 793 834 1053

Librational Lattice mode Anti-symmetric bending vibration of BieOeV external librational or internal bending mode Irrational mode

501 665 684 858 965 1101 1488 3675

symmetric stretching anti-symmetric stretching symmetric and asymmetric bending mode of vibrations for OeVeO

symmetric stretching vibration of BieOeV oxygen symmetric stretching SrVO4 scheelite phase

symmetric and asymmetric stretching vibration of OeSreO defect vibration OH-hygroscopic nature

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

7

Table 3 Compare the energy band gap (Eg) as well as photo catalytic activity of the present compound with other perovskite based compounds. Eg in eV

Photo Catalytic activity

Reference

2.28 2.16 2.8 2.8 2.75 3.4 3.6 2.3e2.5

Present

[39]

Present May show the Photo Catalytic activity

[40]

P L Intensity (A. U.)

Name of the compounds CoTiO3 NiTiO3 FeTiO3 CdTiO3 PbTiO3 AgTaO3 KTaO3 AgVO3 LnBaCo2O5þd (Ln ¼ Eu, Gd, and Sm) Present compound Sr2BiVO6

2.47

450 nm

429nm=2.9eV FWHM=24 nm, 0.17 eV

482 nm 491 nm

360

380

400 420 440 460 Wavelength (nm)

480

500

Fig. 7. Room temperature photoluminescence spectra of the sample.

system, the angle VeOeBi should be kept near 180 so that the wave functions of Biþ3 and VO6 can be well overlapped through O2. The broadening of peaks towards the increasing wavelength of PL may be reasoned to the thermal fluctuation of interactions with the environment. As maximum of the intensified peaks are observed in the wavelength range of 400e500 nm so this ceramic can be used as a novel material for the blue LED based white LEDs [48]. The other small peaks which are observed in present study may be attributed due to oxygen vacancies and defects associated with BieSr related bonds behaving like defects due to compressive stress in the matrix of the studied sample.

3.6. Dielectric measurements 3.6.1. Frequency dependence of dielectrics Fig. 8 (a) depicts the variation of dielectric constant with frequency at different temperatures respectively. For dielectric materials, dielectric constant (εr) is the combination of different polarization like atomic polarization (Pa); ionic polarization (Pi), dipolar polarizations (Pd), space charge (Ps) responding at frequencies 1015, 1013, 1010 and 102 Hz respectively. Generally, at frequencies below ~102 Hz, all four polarizations add up to give the dielectric effect. Nevertheless, the space charge has no effect at frequency greater than 102 Hz. In the studied sample there is a significant change in the value of εr when frequency varies from lower frequencies to high values i.e. (above 103 Hz) confirms, space

charge polarization plays a significant role in the dielectric characteristics of the sample. From the experimental result it is confirmed that due to the doping of bismuth, strontium and vanadium in the double perovskite, a significant increase of space charge is observed. This may be due to two possible mechanisms. The 1st one is that the cations of bismuth and strontium are unstable in the compound because high temperature sintering creates oxygen vacancies which introduce oxygen ionic conduction to the system. The other one possibility is the coexistence of both tetra and pentavalent vanadium cations which would also create oxygen vacancies to increase the space charge polarization. As B-site cations are dominated by vanadium and bismuth the hopping effect takes place for which loss tangent and electrical conductivity increases appreciably. In the studied sample both εr and tand decrease with the increase of frequency [49]. It is also observed that εr has well pronounced dispersion at low frequency range for all temperatures with a higher value in comparison to higher frequency range where its value is independent of frequency due to absence of different polarization. Dielectric constant (εr) saturates for higher frequency at all temperatures and behaves linearly as it should for relaxors [38]. 3.6.2. Temperature dependence of dielectrics Fig. 8 (b) and (c) represents the graphs for εr and tand with the temperature at different frequencies. As seen from the figure both εr and tand are almost constant at low temperatures i.e., <300  C then increases with the rise in temperature. At high temperature range the increase of εr and tand may be due to the formation of thermally activated interfacial space charges [50,51]. As the frequency increases the carriers at the defects do not have time to rearrange with respect to the applied electric field [52,53] which leads to the existence of Nss (metal-nonmetal interface). At low frequency Nss follows the applied ac signal whereas it cannot follow the applied signal at high frequency (500 kHz). So at low frequencies εr and tand increases which depends on the time constant (t ¼ RC), but at high frequencies the interface state capacitance disappears and its contribution to εr is neglected [52,53]. The increase of dielectric constant (εr) with temperature is attributed to the change of ordering in electric dipoles. Strength of the relative dielectric (at frequency 1 kHz) is strong due to the domineering nature of the oxygen vacancy at upgraded temperatures. The continuous rise of εr with temperature in different frequencies ascribe to approaching towards the ferroelectric to paraelectric phase transition. The ferroelectric properties of the material have been verified in the polarization study section. Also the increase of tangent loss (tand) is significant towards higher temperature is ascribed to the thermally agitated charge carriers, some inherent defects (faults and dislocations) [54] or existence of any unknown impurity phase during the synthesis of the sample. Thus the ceramic exhibits a low or almost zero loss

8

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

2000

(a)

0

350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C Fit

r

1500 1000 500 1

2100

(b)

10 100 1000 Frequency (kHz) 4 (c)

1kHz 10kHz 100kHz

1800 1500

10000

3

1 kHz 10 kHz 100 kHz

2

r

tan

1200 900

1

600

0

300 0

100

200 300 o 400 Temperature( C)

500

0

100

200 300 400 o Temperature( C)

500

Fig. 8. Reflects (a) dielectric constant (εr) ~frequency at different temperature (b) εr ~ temperature and (c) tangent loss (tand) ~temperature at different frequencies of the studied sample.

tangents within the room temperature range i.e. up to 300 K. 3.6.3. Polarization study To confirm the ferroelectricity in the present sample room temperature polarization study carried out and reflected in Fig. 9. It is noticed there is a non-zero remnant polarization in the P-E hysteresis loop which confirms the presence of ferroelectricity in the studied compound. It is also observed there is round corner in the loop which may be due to presence of leakage property of the bulk ceramic. As discussed in earlier sections leakage property of

the material are mainly due to volatility of bismuth and valancy fluctuation from V4þ to V5þ [34]. Saturation in the loop is not observed significantly up to a maximum applied voltage which may be due to high loss due to low density of the studied sample. As compared to our previous report the present compound has significantly smaller value of 2Pr is 0.53 mC/cm2. The positive curvature seen in the P-E loop indicates the well-defined ferroelectric behavior by the compound which can be forwarded for device application. 3.7. Impedance spectroscopy The complex impedance mechanism is a very good technique to study transport properties of the material. The complex impedance (Z*) has real (Z0 ) and imaginary (Z00 ) components which are given by the following set of equations.

1.50 2

2Pr=0.53 C/cm

Z * ¼ Z’ þ jZ and Z 2 ¼ Z’2 þ Z}2

2

P ( C/cm )

0.75 0.00

Z0 ¼

-0.75 -1.50 -1.0

-0.5

0.0 E(kV/cm)

0.5

Fig. 9. Shows room temperature P-E loop of the studied sample.

1.0

R 1 þ ðutÞ2

and Z" ¼

uR 1 þ ðutÞ2

where, R, t are respective measured of resistance and relaxation time with t ¼ RC. Variation of Z0 vs. Z00 at different temperature is known as Nyquist plot is shown in Fig. 10 (a). This plot ordinarily helps to distinguish surface phenomena from bulk phenomena of the material [55]. It is noticed at lower temperature i.e., <400  C there is a single semicircular arc and above which it appears double semicircular arcs. It is also observed the centre of the arcs lies below Z0 axis confirms studied compound deviates from ideal Debye type

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

240

9

(a)

200 0

350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C Fit data

Z''(k )

160 120 80 40 0 0

120 160 200 240 Z'(k ) 200

(b)

150

(c) 0

375 C 0 400 C 0 425 C 0 450 C 0 475 C

150

0

375 C 0 400 C 0 425 C 0 450 C 0 475 C

100

Z'(k )

80

100

Z''(k )

200

40

50 0

50 0

1

10

100

Frequency(KHz)

1000

1

10

100

Frequency(kHz)

1000

Fig. 10. (a) Shows Niquesy plot (b) Variation of Z’~Frequency (c) Variation of Z’’~frequency at different temperature of the sample.

behavior. The intercept on Z0 axis determines the bulk as well as grain boundary resistance and capacitance respectively. The plot shows resistances are decreases with rise of temperature which confirms the material possess negative temperature coefficient of resistances (NTCR). As values of capacitances are very weak as compared to resistances it is difficult to determine these in the current technique. For this experimental data has been theoretical fitted with commercially available software ZSIMP-WIN version 2.0. It is observed the arcs 400  C well fitted with circuit model (CQR) and above which curves are well fitted in circuit model (CQR) (CR). In this technique it is found the values of R and C decreases with rise of temperature once again reflecting semiconducting nature of the sample. Fig. 10 (b) shows the frequency dependent variations of Z0 at selected temperatures. It is seen there is low frequency dispersion (<10 kHz) and high frequency invariant under different temperature in the studied sample. The low frequency dispersion may be due to proper correlation between rate of valancy fluctuation in Biþ3-Biþ4 and applied ac frequency which creates significant space charge polarization on the other hand at higher frequencies this correlation mismatches which causes insignificant space charge polarization near the grain boundaries. The low frequency dispersion causes NTCR behavior in the studied sample. Fig. 10 (c) represents the variation of Z00 with frequency at different temperatures. The value of Z00 is seen to increase rapidly with rise of frequency and attains maximum at a certain frequency known as relaxation frequency after which it decreases. The relaxation frequencies increases with rise of temperature is also noted this explain relaxation time for thermally activated charge carriers decreases. This relaxation procedure in dielectrics may be

ascribe to the immobile charges at lower temperatures and oxygen vacancies, defects at higher temperatures. Further shifting of the peaks of Z00 towards lower frequency with decrease in temperature shows that the relaxation in the ceramics is decreased [55]. It is also noted FWHM of the peaks increases with rise of temperature corresponds to disagreement from ideal Debye type behavior [35]. 3.8. Modulus analysis As discussed earlier in impedance section value of capacitances are insignificant as compared to resistances to determine. For this and to further confirmation about relaxation mechanism in the material modulus formalism is used [56]. The complex impedance (M*) has real (M0 ) and imaginary (M00 ) components which are given by the following set of equations.

M ¼ M0 þ jM" ¼ juCo Z M0 ¼  u C0 Z00 M00 ¼ u C0 Z0 pffiffiffiffiffiffiffi Where j ¼ 1, C0 ¼ (ε0A)/t and ε0 ¼ 8.85  1012 F/m, A ¼ Surface area of pellet, t ¼ thickness of the pellet. The variation of M’~M00 at different temperature is represented in Fig. 11 (a). It is noted graph follows semicircular arcs with center below the real axis, once again confirm non-Debye type as well as single phase response of the material [57]. On close observation it is observed that in the studied temperature range there are likely to be double semicircular arc. This once more confirms the presence

10

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607 0.004

(a)

o

350 C o 400 C o 450 C o 500 C

M''

0.003 0.002 0.001 0.000 0.000

0.001

0.002 M' 0.003

0.0012

(b)

0.0025

0.004

(c)

0

350 C 0 375 C 0 400 C 0 425 C 0 450 C 0 475 C 0 500 C

0.0010 o

0.0020 0.0015 0.0010 0.0005

0.0008

M''

M'

350 C o 375 C o 400 C o 425 C o 450 C o 475 C o 500 C

0.0006 0.0004 0.0002

0.0000 1

10000 0.0000

10 100 1000 Frequency (kHz)

1

10 100 1000 Frequency(kHz)

10000

Fig. 11. Shows (a) M’~M00 at different temperature (b) Variation of M’ ~Frequency and (c) M00 with frequency at selected temperatures of the sample.

grain as well as grain boundary effect. The intercept on M0 axis gives the values of grain as well as grain boundary capacitance and it decreases with rise of temperature infer the presence of NTCR behavior in the present material. Fig. 11(b) presents the disparity in M0 with respect to frequency at various temperature in the studied compound. As M0 approaches zero value at low frequency confirming insignificant contribution of electrode polarization. At high frequency range, M0 gets dispersed showing the presence of conduction due to short range charge carriers [55]. This clearly indicates that the restoring force for flow of charge under the influence of steady electric field is very small. At very high frequency, saturation of M0 values with respect to frequency and temperature is due to leakage in the material developing from the short range mobility of charge carriers. Fig. 11 (c) shows the changes of M00 at different temperatures with frequency by the studied sample. In the same manner M00 in complex modulus plot increases with the increase of frequency and attains a maximum (M00 max) value which confirms the presence of relaxation in the sample. With the rise in temperature the peaks M00 max displace towards higher frequency range suggesting the temperature dependent relaxation in the compound. The temperature

50

(a)

4. Conclusion The polycrystalline revived double perovskite vanadate is prepared by usual solid solution casting method. The structure, phase identification and purity of the sample are carried out by respective XRD, RAMAN, FTIR and EDS analysis. The optical properties of the material is carried out by utilizing UVeVis and PL spectra analysis which confirms it can be useful for photo catalytic devices. The temperature dependent study of εr and room temperature polarization study reveals it is ferroelectric in nature. The room temperature εr value of the material confirms it can be useful for various energy storage devices. The transport property of the material is studied by utilizing impedance spectroscopy technique. It is observed the different types of charge carriers are responsible for conduction and relaxation process. The basic charge carriers responsible are hopping type motion of electrons, space charge polarization and short range mobility of charge carriers. It is also noticed material possess NTCR behavior which confirms the semiconducting property.

0.0010

-9

40

(b)

20

Z''

M'' o

450 C

10

o

450 C

0.0004

-10

m

0.0006

M''

Z''(k )

0.0008 30

ln z & ln

0.0030

dependence of relaxation in the material suggests a leakage caused by hopping phenomenon is due to thermally activated carriers. Asymmetric broadening of peaks in Fig. 11 (b) resolves the nonDebye-type behavior of conduction mechanism in the material [58]. The combine plot of M00 and Z00 with frequency at a selected temperature reveals the presence of the smallest capacitance and the largest resistance [56] in the material. This plot also helps to interpret relaxation occurring due to the long or short range mobility of charge carriers. It is observed in Fig. 12 (a) there is a mismatch in peaks of Z00 and M00 confirms material possess short range mobility of charge carriers as well as non-Debye type behavior [44,45]. The variation of relaxation time calculated from Z00 and M00 spectra with reciprocal of temperature illustrated in Fig. 12 (b). According to transition state theory the activation energy is the difference in energy content between atoms or molecules in an activated or transition state configuration and the corresponding atoms and molecules in their initial configuration. The activation energy calculated from Z00 versus frequency represents localized conduction, whereas the activation energy calculated from modulus spectra represents non-localized conduction [60]. In the present compounds, activation energy calculated from both the spectra are nearly same which suggests that in the studied material, charge carrier taking part in both localized and non-localized conductions are similar in nature.

-11 data(EaM=0.63eV) data(EaZ=0.62eV) Fit Linear

0.0002 -12

0 1

10 100 1000 Frequency(kHz)

0.0000 10000

1.3

1.4 1.5 3 -1 10 /T(K )

1.6

Fig. 12. (a) Combine plot of M" & Z00 varying with respect to frequency (b) shows the variation of relaxation time with reciprocal of temperature of the sample.

R.K. Parida et al. / Journal of Molecular Structure 1205 (2020) 127607

Contribution of authors R K Parida: Dr. Parida looks after the few experimental study of the compound and their explanations like XRD, SEM, FTIR and PL. D K Pattanayak: Dr. Pattanayak synthesized the sample and carried out some other experimental study like electrical measurements and their calculations. Bhagyashree Mohanty: M/s Mohanty performed few theoretical explanation of Impedance spectroscopy. B N Parida: Dr. Parida performed the defining problem as well as over all discussion of the manuscript. References [1] S.K. Pandey, O.P. Thakur, D.K. Bhattacharya, C. Prakash, R. Chatterjee, J. Alloy. Comp. 468 (2009) 356. [2] X.M. Chen, G.D. Hu, J.C. Wang, L. Cheng, C.H. Yang, W.B. Wu, J. Alloy. Comp. 509 (2011) 431e434. [3] S.O. Leontsev, R.E. Eitel, Sci. Technol. Adv. Mater. 11 (2010), 044302. [4] W. Li, J. Hao, W. Bai, Z. Xu, R. Chu, J. Zhai, J. Alloy. Comp. 531 (2012) 46e49. [5] K.A. Razak, C.J. Yip, S. Sreekantan, J. Alloy. Comp. 509 (2011) 2936. [6] A. Zaraq, B. Orayech, A. Faik, J. Igartua, A. Jouanneaux, A. Bouari, Polyhedron 110 (2016) 119. [7] C. Lan, S. Zhao, T. Xu, J. Ma, S. Hayase, T. Ma, J. Alloy. Comp. 655 (2016) 208. [8] B.N. Parida, R. Padhee, D. Suara, A. Mishra, R.N.P. Choudhary, J. Mater. Sci. Mater. Electron. 27 (2016) 9015. [9] D.E. Bugaris, J.P. Hodges, A. Huq, H.C. Zur-Loye, J. Solid State Chem. 184 (2011) 2293. [10] N. Xiao, J. Shen, T. Xiao, B. Wu, X. Luo, L. Li, Z. Wang, X. Zhou, Mater. Res. Bull. 70 (2015) 684. [11] Z. Zhong, W. Ding, W. Hou, Y. Chen, Chem. Mater. 13 (2001) 538. [12] M. Liegeois-Duyckaerts, P. Tarte, Acta Part A Mol. Spectrosc. 30 (1974) 1771. [13] M.T. Anderson, K.B. Greenwood, G.A. Taylor, K.R. Poeppelmeier, Prog. Solid State Chem. 22 (1993) 197. [14] J. Longo, R. Ward, J. Am. Chem. Soc. 83 (1961) 2816. [15] Y. Kim, P.M. Woodward, J. Solid State Chem. 180 (2007) 2798. [16] N. Ramadass, Mater. Sci. Eng. 36 (1978) 231. [17] G.H.J. Haertling, Am. Ceram. Soc. 82 (1999) 797. [18] F. Abraham, J.C. Boivin, G. Mairesse, G. Nowogrocki, Solid State Ion. 40 (1990) 934. [19] J.C. Boivin, G. Mairesse, Chem. Mater. 10 (1998) 2870. [20] L. Chen, K.J. Chen, S. Hu, R.S. Liu, J. Mater. Chem. 21 (2011) 3677. [21] D.K. Pattanayak, R.K. Parida, Nimai C. Nayak, A.B. Panda, B.N. Parida, J. Mater. Sci. Mater. Electron. 29 (2018) 6215. [22] F. Abraham, J.C. Boivin, G. Mairesse, G. Nowogrocki, Solid State Ion. 40 (1990) 934. [23] J.C. Boivin, G. Mairesse, Chem. Mater. 10 (1998) 2870. [24] Y.K. Taninouchi, T. Uda, T. Ichitsubo, Solid State Ion. 181 (2010) 719. [25] C.N.W. Darlington, J. Phys. Condens. Matter 3 (1991) 4173. [26] E. W. POWD, An Interactive Powder Diffraction Data Interpretation and Indexing Program, Ver 2.1, School of Physical Science, Finders University of South Australia, Bedford Park, S.A. 5042, Australia. [27] J. Goubeau, “H. Siebert, Applications of vibrational spectroscopy in inorganic

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