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NANOSYSTEMS: Jamdade S.G., et al. Nanosystems:
PHYSICS, CHEMISTRY, MATHEMATICS Phys. Chem. Math., 2023,14 (2), 254-263.
http://nanojournal.ifmo.ru
Original article DOI 10.17586/2220-8054-2023-14-2-254-263
Structural and magnetic study of Tb3+ doped zinc ferrite by sol-gel auto-combustion technique
Shrinivas G. Jamdade1, Popat S. Tambade2, Sopan M. Rathod3 1 Department of Physics, Nowrosjee Wadia College, Pune, India
2Department of Physics, Prof. Ramkrishna More Arts, Commerce and Science College, Akurdi, Pune, India 3Department of Physics, Abasaheb Garware College, Pune, India
Corresponding author: Shrinivas G. Jamdade, hv_jamdade@yahoo.com
Abstract In this study, the influence of Tb3+ substitution in zinc ferrite is reported. ZnTbxFe(2-x)O4 (x = 0, 0.025,0.05, 0.075, 0.1,0.125, and 0.15) were prepared using sol-gel auto-combustion technique. All the samples were sintered at 400 °C for 4 hours. The structure has been studied using XRD, FTIR, UV-visible, and VSM. X-ray diffraction evaluation demonstrates formation of spinel ferrite with nano size distribution. Vibrating sample magnetometer was used to study the magnetic properties of the samples. It was found that as terbium content increases, the coercive field decreases while the saturation magnetization increases. The Tb3+ doped nano-crystalline zinc ferrites show ferrimagnetic behavior. FTIR analysis show the presence of two expected bands attributed to tetrahedral and octahedral metal oxygen vibrations at 320 and 450 cm-1.
Keywords sol-gel auto-combustion technique, zinc ferrite, terbium additive, cubic spinel structure, nano structure
For citation Jamdade S.G., Tambade P.S., Rathod S.M. Structural and magnetic study of Tb3+ doped zinc ferrite by sol-gel auto-combustion technique. Nanosystems: Phys. Chem. Math., 2023,14 (2), 254-263.
1. Introduction
Nano crystalline spinel ferrites were studied by many researchers in recent years. The most common formula for the spinel ferrite is given as AB2O4 where A2+ ions occupy tetrahedral with B3+ ions at the octahedral sites. Cation distribution in the tetrahedral and octahedral sites changes the properties of spinel ferrites. It has been studied that the properties of ferrites were changed with the help of the doping and substitution of a foreign ion, and introduction of a metal cation with higher electronegativity. Rare-earth elements (RE) are commonly used additives for improving the spinel ferrite properties [1,2]. In case of spinel ferrites, the conduction mechanism and magnetic properties are mostly due to the spin coupling of 3d electrons which arises from the Fe-Fe interaction. But, when spinel ferrite Fe3+ ions are partially replaced by rare-earth ions of 4f elements, the RE-FE interaction appears and results in 3d-4f coupling. This 3d-4f coupling in ferrites results in variation in the electric and magnetic properties due to the orbital nature of the unpaired 4f electrons of RE ions [3,4].
Among various spinel ferrites, zinc ferrite has a wide range of applications such as photoconductive materials, information storage, sensors, electronic devices, and in high frequency applications. ZnFe2O4 magnetization studies show the presence of a paramagnetic phase at room temperature and a weak anti-ferromagnetic property below its Neel temperature [5-7]. Recent studies on nano-crystalline ZnFe2O4 ferrite also mentioned the presence of ferrimagnetic ordering at room temperature. These changes in magnetic ordering are owing to the cation's redistribution between tetrahedral and octahedral sites. Thus, the properties shown by ZnFe2O4 ferrites are changed with the crystallite size and inversion of cations on the sites [8-10].
Various methods were developed to have different nano-crystalline ZnFe2 O4 spinel ferrites like microwave based hydrothermal, decomposition of double layered hydroxide precursor, hydrothermal synthesis, urea combustion, reverse micelle method, dip-coating, solvo-thermal analysis, and co-precipitation [11-14]. The sol-gel auto combustion method is preferred because of its high degree of compositional homogeneity, low processing temperature, and low cost [15-21]. In the present study, the aim is to see the influence of rare-earth ion Tb3+ on structural and magnetic properties of Zn ferrites. ZnTbxFe(2-x)O4 nano ferrite is synthesized by sol-gel auto combustion method followed by its heat treatment and then its characterization is done by XRD, UV-visible, FTIR, and VSM tools. One can find several papers concerning to different properties of doped nano-crystalline zinc ferrites [23-25].
© Jamdade S.G., Tambade P.S., Rathod S.M., 2023
2. Synthesis details
Terbium substituted zinc nano crystalline ferrite (ZnTbxFe(2-x)O4) (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125, and 0.15 represented by JX1, JX2, JX3, JX4, JX5, JX6, and JX7, respectively, in all figures) were prepared by sol-gel autocombustion method. The high purity AR grade ferrite nitrate ((Fe(NO3)2>9H2O), zinc nitrate ((Zn(NO3)2>6H2O), terbium nitrate ((Tb(NO3)2>5H2O), ammonium hydroxide solution (NH4OH) were used. The citric acid (C6H8O7, H2O) was used as a fuel. The nitrates and citric acid in stochiometric proportion are dissolved in 100 ml distilled water, which is stirred till homogenous solution is obtained. Drop by drop ammonium hydroxide solution is added to maintain pH = 7 during the stirring process. Then the mixed solution was kept on hot plate at 80 °C for 2 to 3 hours to obtain the sol of it. After half an hour this sol becomes a viscous gel. Then, we get fine powder of ferrite nanoparticles after auto-combustion happens. The powder was sintered at 400 °C for 4 hours in the muffle furnace.
3. Results and discussion
3.1. X-ray diffraction (XRD) analysis
XRD analysis of prepared ZnTbxFe(2-x)O4 , where JX1 is x = 0, JX2 is x = 0.025, JX3 is x = 0.05, JX4 is x = 0.075, JX5 is x = 0.1, JX6 is x = 0.125, and JX7 is x = 0.15 nano-particles sintered at 400 °C for 4 hours, were performed using X-ray diffractometer with CuKa radiation of wavelength 1.5405 A. XRD Fig. 1 for pure and terbium doped zinc ferrite powder sample shows six major diffractions peaks in the range of 20 equal to 29 - 30, 35 - 36, 42 - 43, 53 - 54, 56 - 57, and 62 - 63 corresponding to planes (220), (311), (400), (422), (511), and (440), respectively. These peaks are found to be associated with cubic spinel structure (JCPDS card number 22-1012). The robust reflection comes from the (311) plane denoting the spinel structure. The moderate peak intensity indicates low degree of crys-tallinity of prepared ferrite samples while the broadening of peaks indicates the nanometer size of the crystallite. Many researchers have mentioned orthorhombic phase (TbFeO3) formation in the rare-earth doped ferrites even at a low doping concentration. In our case, we observed the secondary phase as a small reflection peak at 20 = 31.78° as TbFeO3 phase while Fe2O3 phase is seen at 20 = 36.66° for the reflection plane (121) and (110), respectively [2].
Fig. 1. XRD pattern of ZnTbxFe(2-x)O4 powder
Using XRD data, lattice constant values of ferrite samples were calculated by Bragg's equation
a = d • Vh2 + k2 + l2, (1)
where d is interplanar distance and h, k, l are the miller indices. Normally the lattice constant value increases with addition of rare-earth doping owing to the difference in radii of rare-earth ions and Fe3+ ions. However, a slight decrease is observed for the sample JX4 with x = 0.075. This suggests the possibility of some Tb3+ ions occupying the tetrahedral sites up to this terbium concentration, which results in the contraction of the unit cell. For x > 0.075, lattice constant is increasing owing to the expansion of the unit cell caused by the Tb3+ substitution. As the Tb3+ ions, having large ionic radii (0.923 A), enters into the octahedral (B) site in place of Fe3+ ions having smaller radii (0.67 A) results into internal stress to make the lattice distorted and unit cell expansion. Similar results have been reported by many researchers, this is attributed to the nonlinear variation of lattice constant owing to the distribution of cations at A and B sites [3].
The crystallite size was calculated using Debye-Scherrer formula,
D = 0.89 * (2)
p cos 0
The crystallite size was calculated for all the compositions using the high intensity (311) peak. The obtained crystallite size of the samples is within the nano region. The crystal size varies from 5 to 7 nm. The preparation condition followed here probably gives rise to different rates of ferrite formation for different concentrations of Tb3+ favouring the variation in crystallite size. The crystallite size was observed to increase up to x = 0.05 by increasing Tb3+ concentration. Thereafter crystallite size is found to decrease with terbium content. Excess of Tb3+ ions on or near the grain boundaries which impedes the grain boundary mobility may be the reason for this decrease. The average crystallite size (D) and lattice strain (e) also was calculated using Williamson-Hall (W-H) equation which is given as
P cos 0 1 e sin 0
V = D + — (3)
Here e is the lattice strain, D is the crystallite size, A is the wavelength of X-ray used, P is the Full Width at Half Maximum and 0 is Bragg's angle. From above equation (3) the plot of p cos 0 versus sin 0 will be in the form of a straight-line equation giving the slope as lattice strain and from the intercept we can calculate the crystallite size. Non-zero slopes of the W-H plot are indicative of inhomogeneous (strained) growth of the unit cells. These values are in good agreement with the value obtained from the Debye-Scherrer formula. The crystallite size obtained from Debye-Scherrer's formula and W-H plot are tabulated in Table 1. The uncertainties in all values mentioned in Table 1 are ±0.5 %.
In the present case, the lattice strain was seen to decrease with Tb3+ doping up to x = 0.05 and then increases for x > 0.05. The variation in lattice strain parameter also may be due to presence of impurity phases and incomplete replacement of Fe3+ cations by Tb3+ cations. The dislocation density is given as pd = 1/D2, where D is the crystallite size of the sample. In the present case, the dislocation density increases up to x = 0.1 except x = 0.05 and then reduces with Tb3+ ion concentration as the crystallite size of all the samples is small [3,4]. Thus, the size of nano crystals (D) and the lattice parameter (a) non monotonically increases while the lattice strain and the dislocation density non-linearly decrease with doping of Tb3+ in the Zinc ferrite sample.
X-ray density were calculated using the equation
8M (4)
s No3'
Here 8 represents the number of atoms in the unit cell of the spinel lattice, M is the molecular weight of TbZn ferrite samples, N is Avogadro number, and o is lattice constant. The values of X-ray density of the synthesized sample vary from 5.3189 to 5.688 g/cm3 (Fig. 2). The increase in X-ray density of samples is due to increase in the molar mass of the doped Tb3+ ions (158.92 g/mole) as compared to Fe3+ (55.84 g/mole), and Zn2+ (65.41 g/mole) ion. Fig. 2 depicts the change in the value of X-ray density with terbium substitution.
The distance between magnetic ions (hopping length) in site A (tetrahedral) and B site (octahedral) where calculated by using the following relations
La = MA - MA = ^^ j o, (5)
LB = MB - MB = ^ ^Pj a, (6)
LAB = MA - MB = ", (7)
where o is the lattice constant.
The determination of the inter-atomic distance is an amenable method to give one a description of the crystallographic structure and the magnetic properties, where MA and MB refer to the cations at the center of the tetrahedral (A) and octahedral (B) sites, respectively [5]. The variations in the hopping length for tetrahedral site (LA) and octahedral site (LB) were shown in Fig. 2. It is clear that the distance between the magnetic ions increases as the Tb3+ content increases except for JX3, x = 0.1 sample.
3.2. UV-visible spectroscopy analysis
Figure 3 illustrates the DRS (absorption mode) optical absorption spectra for ZnTbxFe(2-x)O4 (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125, and 0.15) nanoparticles in the UV-visible range for the prepared samples at room temperature sintered at 400 °C. There are three bands of absorption found at 280, 350 and 420 nm in correlation with the XRD conclusion. This absorption builds upon the constitution and measure of heat content of the test samples as given in the Figs. 3 and 4.
Three kinds of electronic shifts take place in the optical absorption of Fe3+ samples. As is disclosed from Fig. 3 that for x = 0.1 composition sample, the electronic transition for charge carries over lies in the wavelength range of 450
FIG. 2. W-H Plots, X-ray Density verses 20, M-H loops, LA verses 20, LB verses 20, and LAB verses 20 forZnTbxFe(2-x)O4 powder
to 500 nm which is in the optical region while the ligand field change over occurs in the interval of wavelength 750 to 780 nm also in the optical region.
The band gap energy is calculated using the formula
E = h7 = hC. (8)
The band gap energy decreases from 2.579 to 2.103 eV for the samples which are sintered at 400 °C. The outcome shows that composition and annealing measure of the heat contained in the samples put a major impact on the optical characteristics [6-8].
3.3. FTIR analysis
The FTIR spectra of the investigated ZnTbxFe(2-x)O4 samples are shown in Fig. 5. In the wave number range of 1600 to 200 cm-1 two main metal oxygen bands are seen in the infrared spectra of spinel ferrites The higher band 71, generally observed in the range of 520 to 530 cm-1, is caused by the stretching vibrations of the tetrahedral metal oxygen bond and the lowest band y2, usually observed in the range 400 to 420 cm-1, is caused by metal oxygen vibration in the octahedral sites. IR absorption bands mainly appear owing to the oxygen ions vibrations with cations at different
Fig. 3. Absorbance spectra, and (ah7)2 versus photon energy (h7) for ZnTbxFe(2-x)O4 powder
Fig. 4. (ah7)2 versus photon energy (h7) for ZnTbxFe(2-x)O4 powder
frequencies. But the spinel structure exhibits two IR vibrational bands at 71 = 546.86 cm-1 and at 72 = 449.42 cm-1 corresponding to familiar intrinsic vibrations of tetrahedral site and octahedral site.
The vibrational frequencies of the IR bands 71 and 72 are such that the values of 71 slightly increase but 72 shifts to the lower frequency side with the increase of terbium content. It is known that increase in the site radius reduces the fundamental frequency and therefore the centered frequency should shift towards the lower frequency side. The shift in 71 may be due to the perturbation occurring in the Fe3+-O2- bonds by the substitution of Tb3+ ions. Increase in site radius may be expected due to the replacement of smaller Fe3+ ions by larger Tb3+ ions in the octahedral sites. Decrease in 72 and increase in 71 observed for the sample with x = 0.1 may be due to the formation of TbFeO3 phase [9-11].
3.4. VSM analysis
Figure 2 shows magnetic hysteresis loop of samples at room temperature. The saturation magnetization (MS), co-ercivity (Hc), and magnetic remanence (Mr) of all the samples are given in Table 2. It is clear from the value that MS is decreasing with increase in Tb3+ concentration owing to large Tb3+ ion radii (0.93 A) than ionic radii of Fe3+ ions (0.695 A) preferably occupying octahedral (B sites). The magnetic moments of rare-earth ions generally originate from
500 1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm"1)
FIG. 5. FTIR spectra for ZnTbxFe(2-x)Ü4 powder
localized 4f electrons and these are characterized by lower magnetic ordering temperatures. Therefore, their magnetic dipolar orientations exhibit disordered form at room temperature and hence are paramagnetic and contribute very little to the magnetization of doped ferrites at room temperature. As the terbium concentration increases, B sub-lattice magnetization decreases. Further substitution of Fe3+ magnetic ions by paramagnetic Tb3+ ions in B site deteriorates AB super-exchange interaction. Thus, the ferrimagnetic ordering of Zn ferrite is disturbed by the addition of Tb3+ ions and hence MS decreases.
The high magnetization at low doping is attributed to the ratio of Fe3+ to Fe2+ being maximum at x = 0 composition. The saturation magnetization monotonically decreases with increasing Tb3+ concentration. Value of saturation magnetization depends on grain size and preparation temperature. The decrease in saturation magnetization is owing to the magnetic disorder on B sites caused by the presence of paramagnetic terbium ion on B site. It has been observed that the spin canting is caused by the doping of rare earth ions. Since Tb is a rare earth metal ion, and it causes the transformation of co-linear ferrimagnetic order into non-colinear arrangement of spins on B sites, hence saturation magnetization is reduced. Also, no more Fe3+ ions are transferred to the octahedral sites which lead to the decrease of saturation magnetization. The decrease of saturation magnetization for higher concentration of Tb3+ ions may also be attributed to the secondary TbFeO3 phase which has a low value of magnetization.
The coercivity value slowly increases with increase of Tb3+ content till it reaches maximum value at x = 0.1 composition and then decreases. The increase in coercivity at x = 0.1 may be attributed to an enhancement of the magneto crystalline anisotropy and reduction in the grain size. The coercivity is enhanced due to reduction of particle size until single domain particles are reached and then decreases as the super paramagnetic limit is approached. For x > 0.1, the coercivity decreases which may be due to the decrease in the magnetic anisotropy. The large and small grains would reduce the coercivity due to the emergence of super para-magnetism. The magnetic anisotropy is the accumulative contribution of cations in A site and B site. Therefore, magnetic LS coupling at lattice sites is directly related to the magnetic property such as super para-magnetism. The increase in coercivity may also be related to the appearance of secondary phases TbFeO3 on or near the grain boundaries which impede the motion of domain walls.
The reduction in Mr may be explained on the basis of magnetic dilution. The Zn ion is a divalent cation while terbium is trivalent, when a divalent ion replaced by a trivalent ion then some of the Fe3+ ions are converted to Fe2+ ions to maintain the electro-neutrality which results in the reduction of magnetic moment as Fe3+ has magnetic moment of 5^B while that of Fe2+ has 4^B. The super exchange interaction decreases as Fe2+-O2--Fe3+ exchange interaction is weaker than Fe2+-O2--Fe3+ which is responsible for the reduction in Mr.
The saturation magnetization (MS), coercivity (Hc), remanence (Mr) and Statured Magnetic Field (HM) values are found to be in the range of 10.53 - 3.39 emu/g, 0.20 - 1.09 Oe, 2.95 - 0.30 emu/g, and 5.985 - 5.90 Oe, respectively. These values demonstrate the ultra-soft ferrimagnetic nature of different ZnTbxFe(2-x)O4 ferrites at room temperature. The values of the magnetic parameters are given in Table 2. The uncertainties in all values mentioned in Table 2 are ±0.05 %.
table 1. Diffraction Angle (28), d Interplaner Spacing (d), Full Width at Half Maxima (ß), Lattice Constants (a), Volume of Cell (Vcell), X-ray Density (DX), Crystallite Size (D), and Bandgap (eV) with substitution for /nTbxI e,-, , ,() i
X 26 d FWHM C0) Lattice Parameter (a) Â Vcell (Â3) X-ray Density (DX) in gmcm D (nm) (DS Method) D (nm) (WH Method) Strain (£) D-S Method Strain 00 W-H Method Dislocation Density (.S) Bandgap (eV)
0 35.22 2.5460 0.2450 8.4440 602.0727 5.319 5.871 5.3984 0.1587 0.15028 0.03010 2.08
0.025 35.16 2.5502 0.2249 8.4580 605.0623 5.349 6.395 5.9347 0.1503 0.11354 0.03010 2.1
0.05 35.08 2.5558 0.2082 8.4767 609.0801 5.370 6.905 5.9203 0.1391 0.08855 0.02265 2.103
0.075 35.38 2.5348 0.2511 8.4071 594.1984 5.562 5.731 5.92979 0.1603 0.18726 0.03158 2.065
0.1 35.32 2.5390 0.2714 8.4209 597.1347 5.592 5.302 5.48546 0.1673 0.14514 0.03315 2.1
0.125 35.22 2.5460 0.2667 8.4440 602.0727 5.603 5.393 5.24548 0.1655 0.17397 0.03298 2.08
0.15 35.28 2.5418 0.2550 8.4301 599.1032 5.688 5.642 5.2620 0.1577 0.17597 0.02984 2.04
Table 2. Summary of Coercivity (He), Saturation Magnetization (Ms), Retentivity (Mr), Squareness Ratio (SR), Statured Magnetic Field (HM), m peak (Peak Relative Permeability), Molecular Weight of Composition, Magneton Number nB (Calculated), Magneton Number nB (Observed), and Anisotropic Constant (K) with substitution for /n-l'h , I Cr, .,,Oi.
Co
s-
r;
a
X Hc (Oe) Ms (emu/g) Mr (emu/g) SR (Mr/Ms) HM /x peak (Peak Relative Permeability) Molecular Weight of Composition Magneton Number nB (Calculated) Magneton Number nB (Observed) Anisotropic Constant
0 0.25 10.533 2.95 0.280072 5.967 1404.70847 241.066 0.454637 0.671039 2.7429688
0.025 0.3 7.1799 1.1 0.153205 5.925 964.31783 243.646 0.313223 0.671055 2.2437188
0.05 0.3 5.1389 0.65 0.126486 5.985 683.27600 246.223 0.226556 0.673039 1.6059063
0.075 0.2 6.032 1.7 0.281830 5.93 809.46261 248.8008 0.268713 0.668366 1.2566667
0.1 1.09 3.392 0.4 0.117924 5.9 457.50304 251.3778 0.152672 0.669066 3.8513333
0.125 0.41 3.76 0.3 0.079787 5.96 502.03239 253.9558 0.170971 0.670724 1.6058333
0.15 0.69 3.44 0.6 0.174418 5.976 458.07649 256.5315 0.158006 0.669748 2.4725
S"
o
The experimental magnetic moment (nB) (observed) was calculated using the relation
MMs
nB = W * (9)
Table 2 shows nB values. The squareness ratio, R = Mr /MS, for all the samples were calculated from MS and Mr data which indicates ferrimagnetic behavior of synthesized samples. Usually, the nanoparticles are considered to be in the multi-magnetic domain for R > 0.5 and in single magnetic domain when R < 0.5. Thus, all nanoparticles are within a single magnetic domain. It is worth noting that squareness ratio values are below than 0.5 signify the presence of strong surface spin disordering.
3.5. SEM analysis
Scanning electron microscopy was done to reveal the microstructure of ferrite and the morphology of the nanoparticles. The SEM images (Fig. 6) show a non-uniform particle distribution. All the samples show agglomeration of particles with nearly spherical kind of morphology. The agglomeration of the particles leads to the present structure. It is due to the effect of reaction time and sintering temperature.
(a) (b)
Fig. 6. SEM images of ZnTbxFe(2-x)O4 powder for x = 0.025 (a) and 0.075 (b)
3.6. EDX analysis
EDX has been performed in order to study the composition of the samples. EDX study exhibits the elemental percentage of each element expected to be present in the ferrite sample. The height of the peaks in the EDX graphs (Fig. 7) represents the proportion of each element in the finally sintered ferrite sample. With the increase of Tb concentration, the graph exhibits an increase in height of the Tb peaks. The atomic ratio of Zn:Tb:Fe is about 1:1:4 indicating that the chemical formula of the prepared specimen is consistent with the experimental stoichiometry. Also, all the elements are seen to be present in their respective compositions (Table 3). Terbium concentration was found to be in the grain boundary rather than that in grains. This indicates low solubility of terbium in the spinel structures. This indicates that only a small amount of terbium may be actually getting substituted inside the spinel lattice.
Table 3. Elementwise weight % and atomic % of different composition of ZnTbxFe2-xO4 ferrite
Element Weight % Weight % Weight % Atomic % Atomic % Atomic %
x 0.0 0.05 0.125 0.0 0.05 0.125
FeK 65.29 52.23 51.85 69 57.52 58.98
ZnL 34.05 43.32 38.05 30.75 40.76 36.98
Tb L 0.66 4.45 10.11 0.24 1.72 4.04
(b)
FIG. 7. EDX images of ZnTbxFe(2-x)O4 powder for x = 0.025 (a) and 0.075 (b)
4. Conclusion
Fine particles of ZnTbxFe(2-x)O4 were prepared by sol-gel auto-combustion technique. The particle size steadily increases with increase in terbium content. The crystallite size lies in the range of 5 to 7 nm. XRD shows partial immersion of Tb3+ ions in the spinel lattice and other ions are reversed into secondary phases on grain boundaries. Structural distortions are seen in the lattice. It is revealed that with an increase in terbium amount, the coercive field decreases significantly after maximum value while the saturation magnetization and permanent magnetization also decreases. This may be due to particle size effect, magnetic dilution, spin canting phenomena, and hence reduction of super exchange interaction. The results revealed that terbium cation could reduce magneto crystalline anisotropy of zinc ferrite and consequently altered the magnetic phase to a soft one.
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Submitted 1 December 2022; revised 29 January 2023; accepted 25 February 2023
Information about the authors:
Shrinivas G. Jamdade - Department of Physics, Nowrosjee Wadia College, Pune, India; ORCID 0000-0002-9393-5619; hv_jamdade@yahoo.com
Popat S. Tambade - Department of Physics, Prof. Ramkrishna More Arts, Commerce and Science College, Akurdi, Pune, India; ORCID 0000-0002-2108-7067; pstam3@rediffmail.com
Sopan M. Rathod - Department of Physics, Abasaheb Garware College, Pune, India; ORCID 0000-0003-2357-9791; smragc@rediffmail.com
Conflict of interest: the authors declare no conflict of interest.
