Effect of Fe doping on the structural, morphological, optical, magnetic and dielectric properties of BaSnO3

Doping at the possible A or B site in the ABO3 perovskite can alter the structural, morphological and optical properties of the compounds. Perovskite alkaline earth stannate BaSnO3 doped with Fe resulted in BaSn1-xFexO3 compounds having different concentrations (x = 0.00, 0.01, 0.02, 0.03, 0.05 and 0.07) were prepared by solid-state method. The Rietveld analyzes were performed to determine the phase purity and the changes in structural parameters. Morphological analysis identifies the formation of hexagonal nanorods with doping. The non-linear optical property of the prepared samples shows optical power limiting behavior. Magnetic property signifies the existence of ferromagnetic order and EPR studies reveal the possible ferromagnetic ordering of Fe-doped samples. Dielectric loss decreases with Fe doping and has a gradual decrease in the higher frequency regime and has applications in high frequency devices.


Introduction
Alkaline earth stannates belong to the perovskitetype oxides having the general formula ASnO 3 (A = Ba, Sr, Ca) have attracted considerable attention nowadays due to their interesting physical properties such as high optical transparency, photocatalytic activity, ferroelectricity, superconductivity, and magnetism [1][2][3][4]. Among various perovskites, BaSnO 3 is a semiconductor with a cubic structure having a wide bandgap of 3.4 eV [5,6]. It can remain stable even at high temperatures [7]. BaSnO 3 has been found applications in thermally stable capacitors, dye-sensitized solar cells, photocatalyst, humidity sensors and gas sensors [8][9][10]. Appropriate doping in BaSnO 3 leads to interesting magnetic, semiconducting and ferroelectric properties [11]. The partial substitution at the cationic and the anionic sites leads to a change in the properties while preserving the perovskite structure [12]. The oxygen vacancies are common in these structures which in turn affect the physio-chemical properties of materials [13]. The dopant atoms at Ba 2? and Sn 4? cationic sites will enhance in the physical properties and also cause a change in the octahedral tilting of the perovskite structure. Upadyay et al., doped La on BaSnO 3 at Ba site and found that there is a decrease in the lattice parameter and grain size with increase in lanthanum doping concentration [14]. They attributed the conduction and dielectric relaxation to hopping of electrons among Sn 2? or Sn 3? and Sn 4? ions [14]. High electrical mobility of 320 cm 2 (Vs) -1 at room temperature and superior thermal stability at high temperatures is reported in La-doped BaSnO 3 [15]. The reduction in the bandgap of Co-doped BaSnO 3 is ascribed due to the crystallite size variation and defects such as oxygen vacancies [16]. Due to the capability of doping in A and B sites, high electron mobility and oxygen stability, BaSnO 3 has large potential applications as a system to combine the versatile perovskite structure and the semiconductor technologies [17]. Moura et al., reported that Fe doping decreased the bandgap energy of BaSnO 3 and favored the photocatalytic reaction due to the formation of intermediate levels inside the bandgap [18]. The observation of ferromagnetism in Mn and Fe-doped BaSnO 3 is an important leap in the literature that opened up the path for BaSnO 3 dilute magnetic semiconductor (DMS) applications [11,22]. DMS are formed when a small quantity of magnetic ions are doped with semiconductors. The utilization of spin and charges of electrons in the DMS materials have attracted in the application of spintronic devices [11]. The magnetic properties of Cr-doped BaSnO 3 nanostructures synthesized by chemical precipitation method has showed a change from diamagnetic to ferromagnetic behavior [23]. Manju et al., reported the effect of Fe/ Co ions on the structural and magnetic properties of BaSnO 3 nanoparticles and found that the ferromagnetic property varies gradually with increase in substitution of Co content. [24]. James et al., reported that the presence of oxygen vacancies, structural deformation and surface morphology of Fe-doped BaSnO 3 have improved the magnetic properties of BaSnO 3 [19]. Licheron 3 and found that the remnant polarization and the coercive field decreases with increase of La in the lattice [21]. To the best of our knowledge, detailed structural analysis using the Rietveld analysis of XRD, and measurement of nonlinear optical absorption and dielectric properties, are not reported so far for Fe-doped BaSnO 3 .
There are different methods for the preparation of BaSnO 3 . Many preparation methods were reported to obtain nanoparticles of BaSnO 3 with cubic perovskite structure such as sol-gel method [25], hydrothermal method [26] and modified combustion technique [27]. In the present work, BaSn 1-x Fe x O 3 (x = 0, 0.01, 0.02, 0.03, 0.05 and 0.07) compounds were prepared by the solid-state reaction method, and the structural, morphological, optical, electronic and their magnetic properties have been investigated in detail.

Experimental details
The powder samples of pure BaSnO 3 and doped BaSn 1-x Fe x O 3 (x = 1,2,3,5 and 7 mol % concentration) were synthesized by solid-state reaction method. Solid-state reaction is a mechanochemical reaction that occurs between powders in the solid state. Its main advantages are simple and having a relatively lower cost. In this method solvents are not needed in the reaction and hence waste disposal does not occur. This is of considerable importance in the rapidly emerging field of Green Chemistry which has resulted in major changes in the way synthetic chemists develop processes and products. Also, products do not require extensive purification to remove traces of solvent and impurities. BaCO 3 , SnO 2 and Fe 2 O 3 powders (Sigma, purity-99.99%) were taken in stoichiometric ratio and is mixed using an agate mortar and pestle. The mixture is then grinded well, using acetone as the mixing media. The powder thus grinded was taken in a platinum crucible and is heated at a temperature of 1250°C in a programmable furnace for 6 h at a rate 5°C min -1 and was allowed to cool down naturally to room temperature. To ensure the complete reaction, these mixtures were again milled and further heated at a temperature of 1250°C for 6 h. The prepared Fedoped BaSnO 3 samples with x = 0.00, 0.01, 0.02, 0.03, 0.05 and 0.07 are denoted as BF0, BF1, BF2, BF3, BF5 and BF7, respectively. The synthesized pure BaSnO 3 and Fe-doped BaSnO 3 samples were analyzed by different characterization techniques. The crystalline purity of the prepared samples was characterized by the Bruker Advance 8 XRD diffractometer. The XRD pattern was recorded using Cu-Ka L radiation of wavelength 1.5406 Å and at a scan speed of 2°min -1 in the 2h range 20-80°with a step size of 0.04°. FTIR spectra were recorded by Perkin Elmer FT-IR/FIR Frontier Spectrometer in the frequency range of 400-4000 cm -1 . Micro-Raman spectra of the synthesized powders were recorded with excitation radiation of a wavelength of 633 nm from a helium ion laser using Labram-HR 800 spectrometer (Horiba Jobin Yvon) equipped with synapse CCD camera system maintained at -70°C having a spectral resolution of about 1 cm -1 . Nova Nano SEM-450 Field Emission Scanning Electron Microscope (FEI-USA) was used for analyzing the surface morphology of the synthesized samples. Perkin Elmier Lambda 950 spectrophotometer was used for capturing the UV-Visible spectrum in the wavelength range of 2300-250 nm. Open aperture Z-scan experiments were done using a frequency doubled Q-switched Nd:YAG laser of wavelength 532 nm for measuring the third order nonlinear absorption coefficient. Magnetic properties were studied using a Lakeshore 7410S Vibrating Sample Magnetometer (VSM) of sensitivity 10 -7 emu and magnetic field resolution of ± 0.001%. Electron spin resonance studies were performed at X-band, 9.5 GHz (JEOL, Model:FA200). The dielectric properties of the pure and Fe-doped compounds were studied using HIOKI3532-50LCR Hi TESTER.  [22]. The sharp and intense nature of the diffraction peaks shows the high-quality polycrystalline nature of the prepared samples. It can be seen that the most intense (110) peak suffers a progressive shift toward higher diffraction angle with increase in Fe doping concentration (BF1 to BF7) in the BaSnO 3 lattice (Fig. 1b). The slight shift in the peak position with increase in doping concentration Fe may be due to the difference in ionic radius of the host (Sn 4? ) ion and dopant (Fe 3? ) ion. The ionic radius of Sn 4? ion (0.69 Å ) is larger than that of Fe 3? (0.55 Å ) ion with low-spin state or Fe 3? (0.645 Å ) ion with high-spin state and can result in the distortion of host lattice [28]. The small decrease in lattice parameters with increase in Fe concentration follows Vegard law. Further, the presence of any additional peaks correspond to secondary phases are not visible in the XRD patterns and it gives an evidence for the partial substitution of Fe ions in the host lattice. The peak broadening and decrease in crystallite size with doping concentration also confirms that the dopant Fe ion involves in the growth mechanism of the synthesized compounds. The Rietveld analysis of XRD was done to understand the changes in structural parameters with Fe doping. The refinement of powder XRD patterns of BaSn 1-x Fe x O 3 samples were carried out with the GSAS II software [29]. The background was fitted using shifted Chebyshev function and the peak profile parameters were refined using pseudo-Voigt functions. The fitting can be judged using R p , R wp , R exp and v 2 parameters where R wp is the index that gives the idea of converging nature of refinement, R exp is the expected statistical value for R wp , R p is related to the crystalline structure and v 2 = R wp /R exp . A good fit corresponds to v 2 * 1 and R wp \ 5%. The Rietveld analysis of the pure and Fe-doped BaSnO 3 samples are shown in Fig. 2. The structural parameters obtained from the fit are given in Table 1. From the refined values, we can see that the lattice parameter of BaSn 1-x Fe x O 3 shows a decrease with substitution of Fe ions when x changes from 0 to 0.07. This clearly indicates that the lattice constant shrinks with increase concentration of Fe and are caused by the difference in ionic radii of Sn and Fe ions ( Table 2). The average crystallite size in the prepared samples is calculated using Scherer formulae [30] given as.
where k is a constant, k is the wavelength of the Xray, b hkl is the full width at half maximum (FWHM) and h hkl is the diffraction angle. The lattice parameter is found to be decrease with an increase in Fe doping concentration and their values for different samples are shown in Table1. The slight shift in the 2h angle of doped Fe samples toward the higher 2h region with   respect to that of the pure sample produces a decrease in the value of lattice parameter. The variation in the intensity of the XRD peak and average crystallite size of the doped samples supports the Fe incorporation of the prepared samples. The variations in the lattice parameter and bond length can be due to the difference in the ionic radius of the exchanged cations and defects caused by the dopants [31,32]. Doping of an element can cause lattice deformation and introduce strain in the lattice of the compound. Williamson and Hall method can be used for finding the crystallite size and strain caused by the dopants in the host lattice [33]. Williamson and Hall relation is given by.
where D 0 is the average crystallite size in nanometers, g is the microstrain.The slope of the Williamson-Hall (W-H) plot gives the strain and from the Y intercept, we can calculate the average crystallite size. From Fig. 3 it is found that all the prepared samples have tensile strain and this can be due to the incorporation of the dopant ions in the host lattice. The lattice strain of the pure and the Fe-doped BaSnO 3 calculated from the W-H plot is shown in Table 1. The value of the strain in the compounds is found to be decreased with an increase in dopant concentration. Thus the XRD analysis suggests the incorporation of dopant ion in the host Ba-Sn-O lattice.

Raman analysis
Micro-Raman analysis is a sensitive tool to determine the structural changes in pure and doped samples. The partial substitution or replacement of atoms A or B cations in an ideal perovskite ABO 3 (cubic) lowers the symmetry of the structure and this can be clearly evidenced from the Raman spectroscopy [34]. Fig. 4 shows the micro-Raman spectra of BaSnO 3 and Fedoped BaSnO 3 samples. In general, the BaSnO 3 having an ideal cubic structure with space group Pm3m did not show first-order Raman scattering because of its centrosymmetric crystal structure [27]. The Raman bands observed in Raman spectra of these samples can be due to the presence of defects or dopants. The undoped sample exhibit the Raman modes at 140, 193,332, 410, 561, 712, 835 and 1056 cm -1 .These

FTIR analysis
The FTIR spectra of the pure BaSnO 3 and Fe-doped BaSnO 3 samples measured in the range 4000-400 cm -1 are shown in Fig. 5. The IR spectra below 1000 cm -1 correspond to Sn-O bonds or the deformation modes of Sn-O bonds in SnO 6 octahedral. The peak at 624 cm -1 may be assigned to asymmetric stretching of Sn-O bonds [32] and the peak at 490 cm -1 may be due to Sn-O vibration at the octahedral sites. The peak position at 852 and 1476 cm -1 shows C-O stretching vibrations due to the moisture absorbed from atmospheres by the material and the asymmetric stretching vibration of Sn-OH bonds, respectively [36]. The slight variation in the position of the Sn-O band may be due to the incorporation of Fe ions in the doped samples.

Morphological analysis
Field emission scanning electron microscopy (FESEM) was used for analyzing the surface morphology of the prepared compounds. Figure 6 shows the FESEM images of pure BaSnO 3 and Fe-doped BaSnO 3 compounds. From the images, it is observed that the pure BaSnO 3 particles (BF0) were of cuboidal shape with irregular size having a high degree of agglomeration. Also, from the FESEM images, it is observed that on doping Fe with different concentrations the shape of these particles changed into nanorods. This may be due to the involvement of Fe ions in the growth mechanism and will alter the surface morphology. The size and shape of compounds depend on nucleation and grain growth processes. In general, the particles with higher surface energies tend to grow faster than those with lower surface energies which results in non-uniform growth and the particles tend to minimize their surface-free energy by growing into larger particles [37]. Thus the poly-dispersed particles combine together to form hexagonal rod-like structure [32]. The equilibrium shape of a crystal corresponds to the minimization of the total surface energy, which varies with the orientation of the crystal [38]. The formation of rod-shaped particles is more pronounced for  Figure 7 shows the EDS mapping and spectra of Fe-doped BaSnO 3 (BF3) sample and it confirms the presence of the constituent elements in the prepared compounds. The TEM images (Fig. 8a, c) and the SAED pattern (Fig. 8b, d) of pure and Fe-doped samples shown, respectively, support the FESEM analysis. The formation of a rod-shaped particle of the Fe-doped sample can be confirmed from the TEM analysis.

XPS analysis
The oxidation state and surface chemical composition of elements present in the prepared pure BaSnO 3 and Fe-doped BaSnO 3 compound were analyzed by X-ray photoemission spectroscopy (XPS). The XPS survey spectrum of pure BaSnO 3 compound is shown in  Fig. 9a and it shows peaks correspond to elements Ba, Sn,O and C. The C1s peak can be due to the presence of contamination on the surface of the synthesized compound. The fitted and core-level XPS spectra of Ba with doublet peaks having binding energy 779.6 eV and 794.9 eV corresponding to Ba3d 5/2 and Ba3d 3/2 states, respectively, are shown in Fig. 9b. The separation between the peak Ba3d 5/2 and Ba3d 3/2 states representing the binding energy 15.3 eV confirms that the valence state of barium is in Ba 2? state [39,40]. Figure 9c shows the core-level splitting of the Sn3d spectrum having the spin-orbital splitting, for the Sn (3d) state with 3d 5/2 and 3d 3/2 states having doublet peaks with binding energy values 485.4 eV and 493.8 eV, respectively, with a peak separation of 8.4 eV. The separation of Sn spinorbit doublet corresponds to the Sn 4? state [41,42]. The deconvoluted and core-level O1s spectrum of the pure sample is shown in Fig. 9d. The deconvoluted O1s have three peaks corresponding to binding energies 528.78, 530.09 and 530.64 eV, respectively. The lower binding energy corresponds to the lattice oxygen species (O 2-) [43]. The intermediate and the higher binding energy values correspond to the adsorption oxygen species and the oxygen vacancy present in the compound, respectively [43].
The XPS survey spectrum of 3 mol% Fe-doped BaSnO 3 compound are shown in Fig. 10a and shows the peak corresponding to constituent elements Ba, Sn, O and Fe. The Fe-doped BaSnO 3 has the spinorbit doublet splitting of element Ba with Ba3d 5/2 and Ba3d 3/2 states (Fig. 10b) with the binding energy value779.4 and 794.7 eV and peak separation of 15.3 eV. This binding energy value corresponds to Ba 2? state [39]. The spin-orbit doublet of Sn3d 3/2 and Sn3d 5/2 states (Fig. 10c) with binding energy values at 485.46 eV and 493.76 eV presents a binding energy peak separation of 8.3 eV [41]. The XPS spectrum of O1s peak can be fitted and deconvoluted into three peaks centered at 529.1 eV, 530.94 eV and 531.15 eV (Fig. 10d). The lowest binding energy (529.16 eV) peak is associated with lattice oxygen. The peak corresponding to binding energy at 530.94 eV corresponds to the adsorption oxygen species (O 2 2-) and  Figure 11 (a) shows the absorbance spectra of the pure and Fe-doped BaSnO 3 samples in the wavelength region 250-2500 nm. The shift of optical absorption to the longer wavelength region due to Fe doping can be clearly evidenced from the absorbance spectra and it is due to the possible electronic transitions between dopant ions and host ions. The absorption peak of the samples can be due to the charge transfer between the oxygen and metal in SnO 6 octahedral groups. The optical band gap (E g ) values of pure and doped samples were determined using the Tauc plot equation [39] given as.

UV-Visible analysis
Where a is the absorption coefficient, h is the Planck's constant, m is the frequency of radiation and A is the band edge sharpness. For all the samples the straight line Tauc plot is obtained for the case of exponent n = which corresponds to direct allowed transition [46]. By extrapolating the plot between (ahm) 2 versus  Fig. 11b. The band gap energy value for pure BaSnO 3 was found to be 3.15 eV which is in agreement with the previously reported value [22]. It is found that the band gap energy value decreases systematically with increase in Fe doping concentration. The variation of bandgap energy with Fe dopant concentration is shown in Fig. 11c. There will be a formation of new intermediate energy levels due to the dopant atoms in the host lattice. Similar behavior of redshift in the bandgap of the Cr-and Mn-doped BaSnO 3 nanostructures is reported [20,47]. Thus the optical absorption spectrum clearly gives an evidence for the interaction between the host BaSnO 3 lattice and the doped Fe atoms. The nonlinear absorption coefficient can be found out by.
Where a 0 is the linear absorption coefficient at the excitation wavelength, 'I' is the input laser intensity, I s is the saturation intensity and b is the third order RSA coefficient. The corresponding differential equation describing the nonlinear propagation of light through the sample is given by.
which can be fitted numerically to the measured data to find I s and b. Here, z' is the propagation distance within the sample [47]. The normalized transmittance plotted as a function of input intensity ( Fig. 12(b)) reveals the RSA behavior of the samples. Values of b and I s , calculated from the numerical best fits to the experimental data, are given in Table 3. It is interesting to note that as the Fe doping concentration is increased the reverse saturable absorption coefficient increases systematically. The increase of b values with Fe dopant concentration may be due to the decrease in the variation of bandgap energy of the samples. The obtained b values are fairly high, of the order of 10 -11 mW -1 . Also it is to be noted that as the Fe doping concentration is increased the value of saturation intensity decreases. The highest value of b (1.1 9 10 -10 m/W) and the lowest value of saturation intensity (0.3 9 10 13 W/m 2 ) is obtained for BF7 samples. The graph showing the variation of RSA with Fe doping concentration is shown in Fig. 12c. The strong RSA nature of these samples makes them ideal candidates for optical power limiting applications in which it blocks the high-intensity light while being transparent to the lower intensity beams. They are very critical in protecting various optoelectronic detectors and eye from intense laser beams. A similar trend of increase of b values and decrease of I s value with increase in doping concentration is obtained for Mn doped samples [47].

Photoluminescence analysis
PL is a sensitive technique used for the analysis of defect states and the disorder cluster parameters present in the prepared samples. Figure 13 shows the room temperature PL spectra of pure and Fe-doped BaSnO 3 compounds when excited with a radiation of wavelength of 350 nm. Doping of iron creates defects in BaSnO 3 system. Pure and Fe-doped BaSnO 3 compounds exhibit emission at 395 nm (3.1 eV) which corresponds to the band edge. UV-Visible spectra show a systematic decrease of band gap energy with increase in Fe doping concentration. In the PL spectra also these peaks show a red shift. The blue-green emission peak with 450 nm and 465 nm can be attributed to the transitions within the defect centers,

Magnetization studies
The field-dependent magnetization of pure and Fedoped BaSnO 3 compounds is shown in Fig. 14. As reported, undoped sample (BF0) shows diamagnetic nature [29] and with an increase in Fe doping concentration there arises a positive value of magnetic moments. The magnetic moment monotonically varies with Fe concentration. The inset of Fig. 14 shows the well-defined hysteresis loop of Fe-doped BaSnO 3 compounds with coercivity ranges from 73 to 235 Oe which indicate the existence of ferromagnetic domains in the Fe-doped samples. The possible reasons for the FM ordering at room temperature has been explained by various authors [11,39]. It is reported that the spin of the electron trapped in the oxygen vacancy interacts with each spin of Fe ions within its orbital and results in Fe-Vo-Fe configuration which leads to ferromagnetic order between the two Fe ions. Also, the observed magnetic moments can be due to F-centre exchange (FCE) mechanism of Fe-V 0 -Fe in which direct ferromagnetic coupling of metal ions takes place through an oxygen vacancy [48]. In oxide-based DMS systems the magnetism occurs predominantly due to the exchange interactions via oxygen or vacancies. This has been observed in various oxide-based DMS systems. In the present case the presence of oxygen vacancies were confirmed from the XPS and EPR analysis. The room temperature ferromagnetism in Mn-doped BaSnO 3 compounds was explained on the basis of F-centre exchange mechanism where the magnetic moments of the dopant are coupled ferromagnetically by the interaction between the spins of F-centre and that of magnetic dopants [11]. Pratiba et al., reported the origin of ferromagnetic behavior in Fe-doped BaSnO 3 is through electrons trapped at defects such as the oxygen vacancies (F-center) form a bound magnetic polaron [49]. Coey et al., [50] reported the bound magnetic polaron (BMP) model for the observed ferromagnetism in Fe-doped CaSnO 3 and for magnetic ordering in insulating systems. When the concentration of Fe increases, oxygen vacancies are expected to arise near Fe sites and d electrons of Fe are supposed to interact through FCE [47,51]. The spin of localized defects (V o ) align with the nearby Fe ions and hence long range ferromagnetic interactions occur. Thus, Fe spins may align parallel along the field which leads to ferromagnetic coupling and increased magnetization behavior in the prepared compounds. The unique property of utilizing the spin of the electron which is responsible for the short ferromagnetic order in the prepared samples has found useful application as DMS materials.

EPR studies
Electron Paramagnetic Resonance (EPR) studies were used to study the magnetic microstructure and the presence of unpaired electrons. Figure 15 shows the EPR spectra of pure and Fe-doped BaSnO 3 samples recorded at room temperature. From the figure, it is observed that pure and all Fe-doped BaSnO 3 compounds exhibits resonance signals. It is known that the pure BaSnO 3 compound is found to be EPR inactive due to the absence of unpaired electrons [52]. But in the present work, EPR is active for pure BaSnO 3 compound which reveal the presence of F-centers created by the electrons trapped at (oxygen) anion vacancies [53]. Since the samples have prepared by high temperature solid-state synthesis route, that can lead to the formation oxygen vacancies. The oxygen vacancies act as double electron donors having charge carriers and the spins can polarize through exchange interactions. The sharp resonance signal that appeared with g * 2.0 may be attributed to the oxygen vacancy electron center [32,54]. In the case of Fe-doped samples, the intensity of F-center signal is higher compared with that of pure sample and the g value is found to be * 1.99. It is reported that the g value of isolated Fe 3? ions is very close to that of a free electron [55]. Hence, the observed signals at * 338 Oe indicate the presence of isolated Fe 3? ions. It is also noted that there appears broad and less intensity signals at higher field region and it can be due to the ferromagnetic coupling of Fe 3? pairs [22]. The broad ferromagnetic resonance signal with g value of 1.95 is caused by the exchange interactions between Fe 3? ions [56]. The EPR studies reveal the possible ferromagnetic ordering of Fedoped samples.

Dielectric analysis
Dielectric properties of a material such as dielectric constant, dielectric loss will depend on different parameters such as frequency of the applied field, temperature, structure and particle size, is an important property for designing optoelectronic devices. Figure 16 shows the dielectric properties of pure and Fe-doped BaSnO 3 in a wide range of frequencies taken at room temperature. The dielectric constant (Fig. 16a) is found to be decreasing with increasing frequency for all the pure and Fe-doped samples. The dielectric constant is maximum for low frequency and is minimum for high frequency and this behavior can be due to Maxwell-Wagner model polarization [57] in which material is composed of conducting grains separated by poor conducting grain walls. At low frequency, grain boundary is more effective and at high frequency grains are effective. For higher frequency, due to accumulated charge carriers within the grain boundary, it cannot follow the applied field, which decreases the dielectric constant [58]. It is reported that the high values of dielectric constant at low frequencies can be due to oxygen vacancies, grain boundaries effect, interfacial dislocations, charged defects, and space charge polarization due to heterogeneous dielectric structure [59]. The decrease in dielectric constant with Fe doping may be due to the decrease in polarizability caused by the neutralization of Fe ions with oxygen ions [60]. Similar behavior is reported by Chang et al. [61]. Figure 16b shows the variation of dielectric loss (tan d) values of prepared pure and Fe-doped BaSnO 3 as a function of frequency. The energy loss in the system with the applied field is represented by loss tangent (tan d). From the Fig. 16b, we can see that dielectric loss values decrease with the increase of frequency for pure and doped samples which may be due to the space charge polarization. The pure BaSnO 3 sample has maximum loss which decreases with Fe doping and has a gradual decrease in the higher frequency regime. Figure 17 shows the variation of conductivity with the frequency of pure and Fe-doped BaSnO 3 . The ac electrical conductivity of the synthesized pure and Fe compounds is calculated using the relation.
where e 0 and e' are the permittivity of free space and material, respectively, and x is the frequency of the applied field. The conductivity of all the samples is found to be increasing with the increase of frequency and this may be due to the hopping between the charge carriers. For a particular frequency, conductivity is found to decrease with the increase in Fe concentration. This may be attributed to the fact that the dopant introduces defects (interstitials atoms and oxygen vacancies) in the system. These defects tend to segregate at the grain boundaries due to diffusion process resulting from sintering and cooling processes. Thus, doping increases the defect ions which facilitates the formation of grain boundary defect barrier leading to blockage to the flow of charge carriers. This in turn decreases the conductivity of the system on doping. The increase in Fe concentration produces defects at the grain boundaries of host system which decreases the conductivity with the increase in Fe concentration. Similar behavior is reported in the case of Co-doped ZnO [62]. The grain boundary defect barrier leads to the blockage to the flow of charge carriers and results in decreases in conductivity with Fe dopant concentration [63]. The Fe dopant in the host lattice ceases the hopping mechanism resulting in the decrease of electrical conductivity with an increase in Fe concentration. Also, the DC electrical resistivity of the Fe-doped compounds is found to increase with doping concentration and is in the range of 10 7 Xm.

Conclusion
Perovskite pure and Fe-doped BaSnO 3 compounds were synthesized by the solid-state method. XRD patterns reveal that the pure and the doped samples have the polycrystalline cubic phase. Rietveld analysis gives information about the change in the lattice parameter and the Sn-O bond lengths of Fe-doped samples. The structural identification of the prepared compounds was done by micro-Raman and FTIR spectra. The morphological behavior shows that there is a change in the shape of particles from cuboidal to rod shape. The elemental analysis of the prepared compounds was done by EDS and XPS analysis. The presence of oxygen vacancies was confirmed from the XPS and EPR analysis. The observed enhancement in the feeble magnetic moments with Fe-doped BaSnO 3 compounds is beneficial in spintronic device applications. The nonlinear optical absorption exhibited by the synthesized compounds can be used for optical power limiting applications. The observed decrease in the dielectric loss with Fe-doped samples has useful applications in optoelectronics and in high frequency devices.

Disclosures
Conflict of interest The authors declare that they have no known competing financial interests that could have appeared to influence the work reported in this paper. Overall, there is no conflict of interest.