Structural investigations, band gap narrowing and magnetic characteristics of multiferroic [Pb(Fe0.5Nb0.5)O3]0.4 - [(Ca0.2Sr0.8)TiO3]0.6

doi:10.21203/rs.3.rs-1460525/v1

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Research Article

Structural investigations, band gap narrowing and magnetic characteristics of multiferroic Pb(Fe0.5Nb0.5)O30.4 - (Ca0.2Sr0.8)TiO30.6

https://doi.org/10.21203/rs.3.rs-1460525/v1

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posted 22 Mar, 2022

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We report a detailed and systematic investigation of the multifunctional properties of calcium strontium titanate [(Ca_0.2Sr_0.8)TiO₃] substituted lead iron niobate [Pb(Fe_0.5Nb_0.5)O₃] ceramic oxide manufactured by the effective high temperature solid state reaction method. Rietveld studies reveal the single phase formation of compound with a pseudocubic structure. The Williamson-Hall (W-H) analysis predicts the presence of tensile strain. Different molecular vibrations were detected using the Fourier transform infrared (FTIR) analysis. The local structures and optical modes were analyzed using Raman spectroscopy technique. The microstructural analysis had been done by field emission gun scanning electron microscope (FESEM). Energy dispersive X-ray spectroscopy (EDS) analysis was performed to check the purity. The detailed chemical structure and oxidation states were analyzed by incorporating the efficient X-ray photoelectron spectroscopy (XPS) technique. In addition to the valence identifications, the presence of oxygen vacancies was evident from the XPS analysis. The optical band gap and Urbach energy were analyzed by UV-Visible spectroscopy analysis. Effect of structural distortions, oxygen vacancies and electro-negativity on band gap were discussed. The photocatalytic response was studied by using Mulliken’s model. The electrical properties were analyzed in the frameworks of dielectric constant, tan δ, complex impedance spectroscopy and conductivity studies. The Jonscher’s law analysis revealed the presence of correlated barrier hopping (CBH) conduction mechanism. The binding energy (W_m), minimum hopping length (R_min) and density of states near Fermi level (N(E_F)) were analyzed. The ferroelectricity was studied by room temperature P-E loop. The magnetic study was performed by means of the room temperature M-H loop. The detailed magnetic structure of Fe nuclei was analyzed by incorporating the room temperature ⁵⁷Fe Mössbauer spectroscopy.

XRD

Raman

XPS

UV-Visible

Mössbauer spectroscopy

Multiferroic

Multiferroic ceramics are of utmost importance because of their extensive applications to fabricate the next generation multilayer capacitors, actuators, transducers, memory and media devices. Lead iron niobate (PFN) is an essential member of the multiferroic community from decades because of its high dielectric permittivity, low leakage currents, high break down strength and most importantly due to its magneto-electric characteristics [1]. PFN materials posses a typical perovskite structure with formulation A(B’B”)O₃, where B’ and B” are respectively the low and high valence cations like Fe³⁺ and Nb⁵⁺ [2]. The electric ordering is generated by the Pb²⁺ and Nb⁵⁺ species whereas the magnetic property is favoured by the Fe³⁺ species in B-site. PFN compounds have also been widely studied because of their optical and solar energy harvesting applications due to their suitable low band gaps, but further reduction in band gap is highly desirable for its better carrier separations [3]. To optimize the functional properties of PFN ceramics and for its extensive uses in advance multifunctional devices, it is extremely vital to minimize / remove the secondary impure pyrochlore phase. A detailed literature survey reveals that PFN compounds are often accompanied with two most commonly encountered impure pyrochlore phases like Pb₂Nb₂O₇ and Pb₃Nb₄O₁₃, which serves as a barrier for its efficient applications in sensitive devices [1–2, 4–6]. In addition to this, considering its uses in high temperature dielectric capacitors, it is also necessary to minimize the loss tangent, especially at high temperatures (above 300⁰C).

Calcium strontium titanate (CST) is a novel eco friendly cost effective dielectric ceramic investigated for its energy storage and high temperature stability applications [7]. In addition, the high breakdown strength and better charge-discharge efficiency makes CST an attractive candidate for its extensive adaptations in the electronic industry [8–9]. Zhang et al performed a detailed study about the energy storage density properties and dielectric behaviour of CST materials up to 400⁰C [10], but they do not found any dielectric anomaly up to such a high temperature. CST ceramics are synthesized by high temperature solid state route having calcinations temperatures around 1200⁰C and even more than that [7–8, 11]. Notably, CST ceramics have been reported as a single phase material from the literature [8, 10–11]. A detailed literature survey suggests that not much of work on different compositions of CST materials has been done, especially highlighting their optical properties and detailed electronic structure, although CaTiO₃ and SrTiO₃ have been widely studied. Actually, to our knowledge, only a few papers reporting about its electrical properties are found in literature.

In this research work, an attempt has been made to fabricate a new compound of CST substituted PFN using the inexpensive and effective high temperature solid state reaction route. Here, [Pb(Fe_0.5Nb_0.5)O₃] has been modified with [(Ca_0.2Sr_0.8)TiO₃] with the formulation 0.4 [Pb(Fe_0.5Nb_0.5)O₃] − 0.6 [(Ca_0.2Sr_0.8)TiO₃] (abbreviated as PFN-CST) resulting in [(Pb_0.4Ca_0.12Sr_0.48) (Fe_0.2Nb_0.2Ti_0.6) O₃]. This combination is chosen because (i) CST has high temperature stability, (ii) it is a cost effective and eco friendly material, (iii) to reduce / eliminate the pyrochlore phase of PFN, (iv) not much of work has been done on the different physical properties of CST and (v) to our knowledge, the selected PFN-CST combination has not been reported. Here, PFN content has been reduced by 60 percent with CST substitution, which is highly desirable and a detailed investigation of structural, micro-structural, optical, photocatalytic, dielectric, conductivity, impedance, ferroelectric and magnetic characteristics has been carried out using optimum experimental parameters with the help of the existing theoretical models.

2.1 Sample preparation

The proposed PFN-CST ceramic has been prepared using the high temperature solid state reaction route from the starting materials PbO (Loba Chemie), Fe₂O₃ (CDH), Nb₂O₅ (Loba Chemie), CaCO₃ (Fivor Reagents), SrCO₃ (Himedia), TiO₂ (Loba chemie) with purity ≥ 99% in a suitable stoichiometric proportion. First, the raw materials were properly mixed in dry mixing (in air) for about 2–3 hours and thereafter, in wet mixing (in methanol) for about 5–7 hours. Calcinations were done at different temperatures for different durations and finally calcined at an optimal temperature of 1225⁰C for 4 hours in a closed alumina vessel. The calcined lump was repeatedly grinded to obtain the fine powder of the mixture. Poly vinyl alcohol (PVA) was added with the fine calcined powder and the binder (PVA) provides mechanical strength to the pellets. The pelletization of the calcined powder was done by uniaxial pressing at 3.5 × 10⁶ N/m² pressure and the obtained pellets of diameter 12 mm were sintered at an optimal elevated temperature of 1100 ⁰C for 40 minutes by means of a microwave furnace (VB Ceramics). The binder was removed during the high temperature sintering and this process yields finally strong compact pellets.

2.2 Characterization techniques

The phase formation was studied by means of a powerful X-ray diffraction (XRD) technique by an X-ray diffractometer (Rigaku, Ultima IV, Japan) using Cu-kα radiation (1.5406 Å) as source over an expanded interval of Bragg’s angles (20⁰ ≤ 2θ ≤ 80⁰) at a suitable scanning rate of 3⁰/ minute. The analysis of various molecular vibrations and functional groups was done by using Fourier transform infrared (FTIR) technique using a FTIR-ATR spectrometer. The phonon vibrations and chemical structure was studied by Raman spectroscopy technique using micro Raman system from Jobin Yvon Horibra LABRAM-HR 800 visible instrument. The microstructural analysis was done by means of a high resolution field emission gun-scanning electron microscope (FESEM) (FESEM, Zeiss Supra-40, Germany). To confirm the purity of the compound, energy dispersive X-ray spectroscopy (EDS) analysis was performed from the mentioned FESEM instrument. The detailed chemical valence of the involved elements along with their electronic structure was investigated by incorporating the X-ray photoelectron spectroscopy (XPS) technique using ESCA X-ray photoelectron spectrometer (SPECS Surface Nano Analysis GmbH, Germany). UV-Visible characterization was performed to study the optical band gap by using a double beam UV-Visible spectrometer (Shimadzu, UV-2600i). Dielectric and impedance characterizations of the silver coated pellets were performed by means of a LCR-meter (Hioki LCR, Model-3532, Japan). The room temperature (RT) ferroelectric property was investigated by using an automatic PE-loop tracer (M/s Marine India, New Delhi). The RT magnetic property was analyzed by using a vibrating sample magnetometer (VSM, Model – 7400 series, Lake Shore cryotronics, USA). To analyze the magnetic structure of the studied material, RT Mössbauer spectroscopy was performed. ⁵⁷Fe Mössbauer experiments were done in transmission mode with ⁵⁷Co (Rh) radioactive source in constant acceleration mode by using a standard PC - based Mössbauer spectrometer equipped with WissEl velocity drive. The velocity calibration of the Mössbauer spectrometer has been done by means of the natural iron absorber at RT. The experimental spectrum was analyzed by using the NORMOS-SITE program (Brand R A 1990 NORMOS Programs Universitaet Duisburg).

3.1 X-ray diffraction analysis

3.1.1 Rietveld analysis

Figure 1 (a) reveals the RT Rietveld refinement profile of the examined PFN-CST ceramic and the refinement has been carried out using computer software MAUD [12] by taking the CIF file of Pb (Nb_0.5Fe_0.5)O₃ & (Sr_0.96Ca_0.04)TiO₃ as reference. The XRD spectrum consists of well separated intense distinct diffraction peaks signifying the new single phase formation without the traces of any impure secondary pyrochlore phases. Notably, it is indeed challenging to prepare pyrochlore free PFN and the present result is much improved while compared to the earlier reports of PFN compounds [1–2, 4–6]. The optimized high temperature calcinations and addition of CST of suitable stoichiometric proportion are might be its cause. The best possible crystal structure (with minimum χ² value) is extracted to be pseudocubic with pm-3m as the space group. The obtained crystal structure is well consistent with the previous reports for similar compounds [13–15]. The reliability parameters extracted from the refinement are: R_wp(%) = 15.97, R_b(%) = 12.51, R_exp(%) = 14.21, Goodness of Fit = (chi)² = χ² = R_wp(%) / R_exp(%) = 1.12, which recommends the acceptability of the refinement. A considerably reduced χ² is achieved which indicates a better fitting. Further, to get a deep insight, we have extracted the atomic positions from Rietveld analysis illustrated in Table 1. The extracted atomic positions are used to construct the unit cell structure using computer software VESTA as shown in Fig. 1 (b). The minor distortion in the coordinates of Pb has been noticed from Table 1. The extracted atomic coordinates of Pb are (0.046, 0, 0), which shows a clear displacement in Pb from its ideal (0, 0, 0) corner position. In ideal ABO₃ cubic perovskites, the corner A atoms or cations are in 12 fold octahedral coordination with the oxygen anions and they occupy the corners of the cube with (0, 0, 0) locations. But, due to the observed displacement in Pb cations, the octahedral may get some space to fill / rotate itself which might lead to the pseudocubic structure. In addition, due to this cationic displacement, the symmetry gets reduced which can also lead to slight deviation from the ideal cubic structure [16]. The occurrence of pseudocubic phase is understandable in complex perovskites because of the presence of two or more different atoms in the respective lattice sites. The presence of the RT ferroelectricity may be the other possible reason for the atomic displacements which may lead to minor deviation from the ideal cubic phase and hence, justifies its pseudocubic nature. In the ferroelectric compounds, because of the occurrence of the RT ferroelectricity, minor spontaneous atomic displacements are usually occurred.

Table 1

Extracted atomic positions from Rietveld refinement

Atomic type	X	Y	Z
Pb1	0.046	0	0
Ca1	0	0	0
Sr1	0	0	0
Fe1	0.5	0.5	0.5
Nb1	0.5	0.5	0.5
Ti1	0.5	0.5	0.5
O1	0.5	0.5	0
O2	0	0.5	0.5

3.1.2 Crystallite size and strain analysis

Generally in perovskites, the broadening of diffraction peaks is because of the combined contributions of the crystallite size (D) and micro-strain (ε), but as the mentioned effects are independent of each other, it is expected that they can obey the Cauchy type behavior [17]. Hence, the net broadening (β_T) can be a sum of the broadening due to crystallite size (β_D) and micro-strain (β_ε) and expressed as: β_T = β_D + β_ε. The parameters β_D and β_ε can be estimated from the respective relations $D= K\lambda /{ \beta }_{D}Cos \theta$ (Scherrer’s relation with λ = 0.15406 nm for Cu-kα line) and ε = β_ε / 4 tan θ [17]. Using these equations,

β _T = Kλ/ Dcos θ + 4ε tan θ

Or, β_T cos θ = 4ε sin θ + Kλ/ D

The extracted equation is called as Williamson-Hall (W-H) equation and has been fabricated in accordance with the uniform deformation model (UDM), which assumes the uniform distribution of strain in the substance [18]. Figure 1 (c) reveals the W-H plot of the examined PFN-CST compound with the extracted equation y = 0.000968x + 0.00203. The values of ε and D are found to be around 0.000968 and 68.31 nm respectively from the linear fitting (D = kλ / 0.00203 = 68.31 nm, with shape factor k = 0.89). The type of strain can be predicted from the nature of the slope [18]. Since, the obtained value of strain is positive, it signifies to the tensile strain (otherwise compressive). Notably, the induced strain is minor. The creation of oxygen vacancies is favourable in Pb based compounds and since, oxygen vacancies do not impart much stress on large crystallites, a small value of strain is quite evident.

3.2 FTIR spectroscopy

Figure 2 portrays the RT FTIR spectra (Transmittance versus Wavenumber) of the analyzed PFN-CST material in the wavenumber interval of 4000 − 500 cm^− 1. The spectrum consists of various strong and weak transmittance bands around 504, 526, 543, 561, 582, 601, 623, 642, 687, 734, 751, 786, 841 and 1459 cm^− 1. Usually, in perovskites, the peaks formed between the ranges 500–850 cm^− 1 are attributed to the lattice / phonon vibrations which are related to the perovskite phase identifications [19]. The strong intensity bands located around 500–751 cm^− 1 are assigned to the Ti-O stretching vibrations [20], which are related to the metal oxide (M-O) B-site stretching vibrations of the ABO₃ perovskites. The weak peaks near 786 and 841 cm^− 1 are because of the Nb-O stretching vibrations [21]. The broad asymmetric band centered around 1459 cm^− 1 is assigned to the vibration of C = O groups [22]. Since, no traces of peaks around 3600 cm^− 1 are located, it confirms that the examined compound does not have any hygroscopic nature which is highly desirable. We have used the Hook’s relation to analyze the wavenumber of the most prominent Ti-O vibration [23]:

$\stackrel{-}{{\nu }}$ = $\frac{1}{2ᴨc}$ $\sqrt{\frac{k}{\mu }}$, with, $\mu = \frac{{m}_{1}{m}_{2}}{{m}_{1}+{m}_{2}}$

Where, µ be the effective mass and m_1, m₂ are the atomic weights of different bonding atoms. In addition, k = 17 / r³ [23], where, k is the average force constant and the r be the average bond length. By using the average value of wavenumber, µ, k and r for the prominent Ti-O bond are estimated to be around 1.99 x 10^− 23 gm, 2.008 N/cm and 2.044 Å respectively nearly consistent with the previous reports for Ti-O bond lengths [18, 23].

3.3 Raman spectroscopy

We have incorporated the non-destructive Raman spectroscopy technique to study the local structure of the analyzed PFN-CST ceramic. The Raman characterization has been performed using 632.8 nm He-Ne laser as the radiation excitation source at room temperature. Figure 3 reveals the RT Raman spectra of the examined ceramic in which a total of 7 Raman active modes are noticed around the wavenumbers 110, 193, 266, 455, 512, 774 and 832 cm^− 1. The sharp asymmetric peak centred at 110 cm^− 1 is ascribed to the E(TO₁) phonon mode which is seemed to be slightly shifted towards the higher wavenumber side [24]. The broad peak noticed around 193 cm^− 1 and weak intensity band around 266 cm^− 1 is possibly because of the O-Ti-O bending mode of CST [25–26]. Meanwhile, the broad asymmetric bands observed around 455 and 512 cm^− 1 are because of the TiO₆ octahedral stretching in consistent with the previous studies [26–27]. This stretching is related to the B-site octahedral vibrations of the ABO₃ perovskites. The asymmetric stretching may yield a distorted TiO₆ cluster which will lead to interesting optical properties like band gap narrowing. Due to this asymmetric stretching, the octahedral may slightly distort justifing the pseudocubic nature of the compound. The strong asymmetric band noticed around 774 cm^− 1 is ascribed to the [A₁(LO₃) + E(LO₄)] mode and this mode is found to be transferred towards the higher wavenumber side [28]. Further, the small swelling around 832 cm^− 1 is possibly ascribed to the A_1g phonon mode of PFN [24]. Notably, the A₁(LO₃) and A_1g modes are ascribed to the Nb-O-Fe stretching vibrations of lead based perovskites like PFN. The A_1g mode resembles the oxygen ion vibrations of the oxygen octahedron (typical Nb-O-Fe stretching) [24].

The structural and physical property like conduction mechanism related to the perovskites is highly relied on the effect of structural distortions of certain molecular clusters and defects like oxygen vacancies (OVs). Notably, the mentioned asymmetric stretching may yield a distorted TiO₆ cluster. Milanez et al reported two important classes of charged clusters of Titanium (Ti) in similar Ti based perovskite [29]: [TiO₆] and [TiO₅ V^Z_O]. The [TiO₅ V^Z_O] clusters are connected to the mobile OVs in which V^Z_O = V^X_O, V^’_O and V^’’_O, where the dominant carrier for conduction is the doubly ionized OVs (V^’’_O) [27]. So, the charge density in the material is heavily relied on the interactions between the network formers like [TiO₆] - [TiO₅ V^Z_O]. But, in addition to the effect of the structural dislocations in the network formers on the net charge density, as per the study of Lazaro et al, the distortions in the network modifiers like [CaO₁₂] - [CaO₁₁ V^Z_O] may also have a noticeable effect on the total charge density [30]. The observed broadening of certain Raman modes is ascribed to the effect of distribution of phonon wave vectors in accordance with the crystallographic axis as a result of the scattering of phonon momentums usually noticed in powder ceramics [28]. The slight shifting of the noticed Raman peaks towards the higher wavenumber sides is might be because of the occurrence of the tensile strain confirmed from the W-H analysis.

3.4 FESEM microscopy and EDS analysis

The RT FESEM micrograph of the examined PFN-CST material has been illustrated in Fig. 4 at 5 kX magnification. The micrograph portrays the compact arrangements of grains with specified grain boundaries. The crystalline nature of the ceramic is clearly observable. A considerably better grain growth is achieved on account of the high temperature synthesis. The packing of grains is seemed to be highly dense which hints the formation of the uniform microstructure in the compound. The grain growth is possibly resulted due to the temperature promoted matter transport process between the grains through the grain boundaries which may finally yield grains of various sizes [28]. The noticed voids are very minor and these are possibly resulted because of the volatile nature of lead during the high temperature sintering resulting in the formation of OVs. These voids hint the hopping mechanism responsible for conduction process in the substance. The average grain size is estimated using the computer software Image J and the histogram plot displayed in Fig. 5 (inset) reveals that the average grain size lies in the range from 1–12 µm. The estimated average grain size is around 4.00136 µm. To confirm the purity, we have performed the EDS analysis from the mentioned FESEM instrument. The EDS spectra portrayed in Fig. 5 propels peaks for the expected elements only i.e. for Pb, Fe, Nb, Ca, Sr and Ti. No signs of the foreign elements are traced as we do not get any additional un-indexed peaks. The FESEM and EDS studies highly recommend the suitable availability of the examined material for the multifunctional analysis.

3.5 X-ray photoelectron spectroscopy

We have incorporated the sophisticated XPS technique to analyze the chemical structure and valence states of the involved elements in the studied sample. The analyzed XPS spectra of the examined PFN-CST material in the expanded binding energy (B.E.) interval of 1–800 eV are portrayed in Fig. 6, as Figs. 6 (a) – (h). The presence of all expected entities like lead (Pb), iron (Fe), niobium (Nb), calcium (Ca), strontium (Sr), titanium (Ti) and oxygen (O) are identified through the XPS spectra displayed in Fig. 6 (a) and also well verified through the regional scans spectra portrayed in Figs. 6 (b) – (h). The peak recorded around 286 eV is because of C-1s used as the reference peak for the B.E. calibrations of all the constituent elements. The regional scan of Ca in the specified energy range revealed by Fig. 6 (b) portrays two separate peaks: Ca (2p_3/2) spaced around 349.65 eV and Ca (2p_1/2) located near 353.38 eV. The regional scan of Sr is illustrated in Fig. 6 (c) and it reveals that the spectra of Sr have been splitted into two separate components: Sr (3p_3/2) spaced near 271.71 eV and Sr (3p_1/2) traced near 275.85 eV. The presence of the two unique peaks for Ca and Sr are due to spin-orbit interactions [31]. This investigation reveals that Ca and Sr present as Ca²⁺ and Sr²⁺ in the sample [31–33]. Now, to analyze the real oxidation states of Pb, Nb, Ti, Fe and O in the examined sample, we have de-convoluted the obtained experimental XPS spectra of Pb 4f_7/2, Nb 3d_5/2, Ti 2p_3/2, Fe 2p_3/2 and O_1s as revealed in Figs. 6 (d) – (h). The de-convoluted spectra of Pb 4f_7/2 revealed one peak [Figure 6 (d)] around 141.35 eV (Pb²⁺). Nb 3d_5/2 has been de-convoluted to two peaks [Figure 6 (e)]: a suppressed peak at 207.78 eV for Nb⁴⁺ and a prominent peak at 210.03 eV indicating the presence of Nb⁵⁺ [34–35]. The Ti 2p_3/2 spectra [Figure 6 (f)] have two well de-convoluted unique peaks: one around 458.29 eV representing Ti³⁺ ion and other at 461.26 eV reflecting Ti⁴⁺ [36]. The full regional scan of Fe-2p (not shown here) predicts the occurrence of both Fe 2p_3/2 and Fe 2p_1/2 due to the well known spin-orbit interaction. The de-convolution analysis of Fe 2p_3/2 spectra were found to fit well for two separate peaks [Figure 6 (g)] with an energy gap of 3.51 eV: one peak at 707.07 eV corresponds to the Fe²⁺ and the other peak at 710.58 eV corresponds to Fe³⁺ state [31, 37]. The presence of Fe²⁺, Nb⁴⁺ and Ti³⁺ are basically because of the contributions of the OVs in the sample which is responsible for the transport mechanism. The O-1s spectra [Figure 6 (h)] has been de-convoluted to two well fitted peaks with B.E. values 530.21 eV and 532.40 eV indicating the occurrences of the lattice oxygen (O_lat) in the perovskite phase and surface adsorbed oxygen (O_ads) probably induced by the OVs in the investigated sample [31, 35]. In lead (Pb) based perovskites, OVs are created due to the volatile aspect of Pb during the high temperature sintering to maintain the charge neutralizations as per the relations: V_o ⇔ V’_o + e and V’_o ⇔ V_o” + e, where V’_o and V_o” are respectively the singly and doubly ionized OVs. This process leads to the creation of mobile electrons which are responsible for the transport properties inside the material and also, for the reduction of certain elements to their lower oxidation states (like Ti⁴⁺ → Ti³⁺, Fe³⁺ → Fe²⁺, Nb⁵⁺ → Nb⁴⁺ etc. observed from the XPS analysis as in our case).

To perform the quantitative analysis about the compositional ratio of the multivalent species like Fe, Nb and Ti, the area of the obtained de-convoluted peaks have been used. Notably, all the de-convolutions have been performed by using the Gaussian function. The compositional ratio of Nb is evaluated to be [Nb⁴⁺ : Nb⁵⁺] = [0.08 : 0.92] and its effective valence is found to be 4.92+ [4 x 0.08 + 5 x 0.92 = 4.92]. Similarly, the compositional ratio and effective valence of Ti are respectively extracted to be [Ti³⁺ : Ti⁴⁺] = [0.09 : 0.91] and [3 x 0.09 + 4 x 0.91 = 3.91]. For Fe, the compositional ratio is extracted to be [Fe²⁺ : Fe³⁺] = [0.19 : 0.81] and the effective valence of Fe is estimated to be 2.81+ [2 x 0.19 + 3 x 0.81 = 2.81]. This analysis shows that the majority oxidation states for Fe, Nb and Ti are as 3+, 5 + and 4 + respectively. The bulk oxidation state of Fe³⁺ is later confirmed by Mössbauer spectroscopy.

3.6 UV-Visible spectroscopy

The optical characteristics of the proposed PFN-CST material have been analyzed by incorporating the UV-Visible spectroscopy technique in the framework of the diffuse reflectance spectroscopy (DRS) method. The outcomes are portrayed in Figs. 7 (a) and (b). This method was proposed by Kubelka and Munk (K-M) and it has been extensively adopted for light scattering materials like powder samples [38]. Considering the strong dependence between absorbance and diffuse reflectance, the K-M function (F(R)) is expressed as [38]:

$\frac{K}{S}$ = $\frac{{(1-R)}^{2}}{2R}$ = F(R)

Where, the parameters have their standard abbreviations. For absorption process with linear absorption coefficient α, Tauc and Wood formula for parabolic band structure is expressed as [38]: αhν ∝ (hν – Eg)ⁿ, where E_g represents the optical band gap energy. Since, for a perfectly diffuse scattering, F(R) is proportional to α, the modified K-M relation is (F(R)hυ) = A (hυ-E_g) ⁿ. Here, we have used n = ½ assuming the direct transitions as per the similar reported perovskites [18, 21, 26–27, 39–40]. Usually, in Ca based perovskites, the direct E_g is linked with the electronic transition from the states of the valence band (VB) to the conduction band (CB) with the same Brillouin zone [27, 41]. The extracted E_g is found to be around 2.32 eV as displayed in Fig. 7 (a). The optical band gap values of CaTiO₃ are reported around 3.51 eV [27] and some other Ca based compounds are reported 3.2 eV [26], 2.96 eV [27] and 2.89 eV [27]. Further, the band gaps of some Pb based perovskites are reported around 3.09 eV [39], 2.92 eV [39] and 2.83 eV [39]. So, this comparison reveals that we have achieved a relatively narrow E_g in the present study.

The observed band gap narrowing is probably attributed to the structural disorder induced by the crystal defects like OVs [28, 38]. The presence OVs is confirmed from the XPS analysis. Further, OVs can induce additional localized energy states within the forbidden band basically composed of 3d states of Ti closed to the lower edge of the CB and 2p states of O lying in the vicinity of the VB [38, 42]. For SrTiO₃, Longo et al reported that as a result of the structural disorder, dislocated clusters of Ti are probably generated in the form three states of Ti: [TiO₆ → TiO₅.V^z_o] where V^z_o = V^x_o, V^’_o and V^”_o, which corresponds to the neutral (two paired ↑↓ electrons), single (one unpaired ↑ electron) and doubly ionized (no unpaired electron) states of OVs respectively [42]. These distorted clusters can also be induced as a result of the symmetry disturbances as revealed in Raman analysis because of their asymmetric stretching. These unstable networks can promote additional energy states in the band region to reduce the effective E_g. With the rise in OVs and/or the disorder, one can expect a much prominent band gap narrowing effect.

In addition to the effects of OVs and distorted clusters, we have adopted the effect of electronegativity on the lowest unoccupied molecular orbitals (LUMOs) to explain such an observed narrow E_g. To get a clear insight, we have sketched the LUMOs and the highest occupied molecular orbitals (HOMOs) in Fig. 8. Similar phenomena have been examined by Batoo et al and Zhou et al for Ti based perovskite complexes [35, 43]. As per the reports, for Ti based complex perovskites, the lower edge of the CB is set by Ti-3d states whereas the upper edge of the VB is set by O-2p states [40, 43]. When, PFN is mixed with CST, then higher electronegative elements like Fe and Nb replaces Ti in B-site of the compound. We have demonstrated this effect by taking Nb as an example, whose electronegativity is around 1.6 and that of Ti is 1.54. Because of the higher electronegativity of Nb than Ti, the Nb − 4d levels have slightly lower energy than Ti − 3d states and this will create new additional energy states below the lower edge of the CB to reduce E_g.

In order to get a clear insight about the amount of disorder present in the material, we have performed the Urbach energy (E_U) analysis which is stated as the energy of the discussed defect states. To estimate E_U, considering the dependence of α with F(R) in accordance with the DRS analysis (directly proportional), we have [44]:

α = α ₀ exp (hν / E_U)

Or, ln(α) = ln(α₀) + (hν / E_U)

Or, ln(F(R)) = ln(α₀) + (hν / E_U)

Figure 7 (b) reveals the Urbach energy plot in which ln (F(R) is taken in y-axis against energy (hν) in x-axis. The reciprocal of the slope obtained from the linear fitting of the best possible straight part of the curve gives E_U and it is extracted to be around 0.31 eV [E_U = 1/slope = 1/3.24 = 0.31 eV] as revealed in Fig. 7 (b). It should be noted that, E_U includes all sorts of disorder i.e. structural, chemical, polar etc. The obtained E_U is considerably small and this refers to the better crystallinity of the ceramic in consistent with the predictions of the XRD and FESEM analysis.

3.7 Photocatalytic studies

It has been reported that the efficient water splitting photocatalysts have band gaps just more than 2 eV to promote the visible light absorption for the optimal utilization of solar energy and this is in consistent with our result [45]. The studied compound is believed to posses improved photocatalytic activity because of its better electron excitation ability from the VB to the CB due to the effect of the band gap narrowing. In order to get a clear insight about the photocatalytic half reactions (oxidation in VB and reduction in CB), we have estimated the band positions of the valence band maximum (VBM) and conduction band minimum (CBM) using Mulliken’s electro-negativity approach as per the following relations [46–47]:

E _CBM = χ – E_c – E_g / 2

E _VBM = E_CBM + E_g

Here, χ, is the electronegativity of PFN-CST, E_c denotes the energy of the free electrons on hydrogen scale (~ 4.5 eV) and E_g be the extracted band gap (E_g = 2.32 eV). In accordance with the Mulliken’s approach, χ has been estimated as the geometric mean of the electronegativities of the involved elements where the electronegativity of a particular element is stated as the arithmetic mean of its first ionization energy and electron affinity values [46]. Using this principle and by the help of the mentioned equations, we have estimated χ = 4.52 eV, E_CBM / CBM = -1.14 eV and E_VBM / VBM = 1.18 eV.

The extracted CBM and VBM levels are labelled in Fig. 9 in accordance with the H⁺/H₂ (0 eV vs NHE, pH 0) and O₂/H₂O level (1.23 eV vs NHE, pH 0), where NHE signifies the normal hydrogen electrode potential. It is reported that if the CBM level is more negative than the H⁺/H₂ level, then hydrogen will evolve whereas, if the VBM level is more positive than the O₂/H₂O level, then O₂ emission will be favourable [46–47]. Figure 9 shows that the obtained CBM level is sufficiently negative than the reduction potential of water H⁺/H₂ (0 eV) which reveals the possible occurrence of the H₂ emission reaction [46]. By using proper sacrificial reagents (like electron or hole scavengers), one can further boost the productivity of the half reactions. Notably, the obtained VBM level is also closed to the oxygen evolution criterion. So, it may also be possible to ignite a small amount O₂ by using proper reagents.

3.8 Dielectric and impedance studies

3.8.1 Dielectric characteristics

Figure 10 portrays the thermal dependence of dielectric constant (ε_r) in a wide interval of temperature (25⁰C – 500⁰C) at four different operating frequencies 1, 10, 50 and 100 kHz. ε_r varies smoothly up to 200⁰C, thereafter, it consistently hikes up to 500⁰C. The RT values of ε_r are found to be around 342, 313, 302 and 299 for the mentioned set of frequencies respectively. The obtained variation of ε_r is nearly consistent with the previous report for CST compound [10]. The hike in ε_r is related to the transport of thermally activated space charge like OVs [10]. With the hike in temperature, the trapped carriers near the grain walls become free which will lead to hike the values of loss and dielectric constant at elevated temperatures. Further, the hopping of carriers like electrons between the Nb⁵⁺-Nb⁴⁺ sites may be the other possible factor for the rise in ε_r [48]. Further, ε_r falls with the hike in frequency which may be due to the diminished contributions of polarizations with larger relaxation times like dipolar and atomic polarizations. Figure 10 (inset) reveals the thermal dependence of tan δ in a wide temperature range of (25⁰C – 500⁰C) for the mentioned set of frequencies. Initially, tan δ remains almost constant with temperature nearly up to 300⁰C, and thereafter, it gradually hikes with the rise in temperature up to 500⁰C. We have achieved considerably small values of tan δ at such a high temperature range of 500⁰C i.e. at 500⁰C, tan δ = 1.76, 1.23, 0.77 and 0.72 for frequencies 1, 10, 50 and 100 kHz respectively. Further, the RT values of tan δ are found to be around (0.02–0.07). These observed values of ε_r and considerably better tan δ at elevated temperatures suggest the potential use of the PFN-CST material for high temperature energy storage dielectric capacitors with better stability.

3.8.2 Complex impedance spectroscopy

The correlation between the microstructural behaviour with the electrical properties of the dielectric ceramic is effectively studied by employing the complex impedance spectroscopy (CIS) technique in the framework of brick-layer model [17, 21]. The Nyquist plot technique has been adopted (shown in Fig. 11) fitted by the displayed equivalent (RQC) (RC) circuit. The fitting is performed in a broad temperature interval of 350⁰C – 500⁰C using the computer software Zsimp Win (version 3.21). Figure 11 portrays that two distorted semicircles can be traced for each temperature indicating the combine effect of grain (major) and grain boundary (minor) towards the total resistivity of the material. The arcs of the half circles found to be decreasing with the hike in temperature, indicating a fall in resistivity with temperature. This in turn reveals the hike in conductivity with the hike in temperature suggesting the negative temperature coefficient of resistivity (NTCR) response of the compound. The presence of the distorted arcs suggests the departure from the ideal classical Debye-response and it reflects the occurrence of a temperature dependent non-Debye relaxation with several relaxation times. These findings are in consistent with the previous reports [10–11, 21].

3.9 Conductivity analysis

Figure 12 reveals the frequency variation spectra of ac conductivity (σ_ac) at temperatures 425, 450, 475 and 500⁰C. σ_ac is estimated from the relation σ_ac = ωεε_o tan δ. The ac spectrum clearly exhibits two separate zones: the low frequency plateau region and the high frequency dispersion zone. With the hike in frequency, all the plotted curves posses a gradual transformation from the frequency unaffected plateau region to the heavily frequency dependent dispersion region. In the plateau zone, conductivity remains almost constant with frequency and hence, it signifies to the dc conductivity (σ_dc) of the compound. However, in this zone, conductivity rises with the hike in temperature signifying that electrical conductivity is a thermally dependent phenomenon and also revealing the NTCR response of the material. The dispersion region refers to the short ranged back and forth hopping of carriers between the lattice sites which cause the frequency dispersion.

This type of conduction profile is well fitted by the Jonscher’s power law [49]: σ(ω) = σ_dc + σ_ac = σ_o + Aωⁿ, where A and n (T) are respectively the pre-exponential parameter and frequency exponent (0 ≤ n ≤ 1). The fitting is carried out in a broad temperature range (573K – 773K). n seems to be decreasing in the whole temperature range and the variation of n gives an insight about the type of conduction mechanism occur inside the material. Based on the observed decreasing behaviour of n, the correlated barrier hopping (CBH) mechanism is best suited to explain the origin of conductivity inside the examined ceramic [50].

As per this model, conduction is believed to be implemented by the jumping of carriers over the Coulomb barrier connecting the two lattice sites [51]. The binding energy (W_m), which is stated as the energy needed for the transport of carrier from one site to the nearby site, is estimated from the formula [51]: β = 6K_BT / W_m, where β = 1-n. The minimum hopping length (R_min) is estimated using the values of W_m as per the equation [51]: ${R}_{min}$= $\frac{2{e}^{2}}{ᴨ\epsilon {\epsilon }_{0}{W}_{m}}$, where, ε is the dielectric permittivity at frequency 100 kHz. The thermal dependence of W_m is displayed in Fig. 13 and W_m falls with temperature. In accordance with Pike’s theory, the decreasing nature of W_m with temperature is probably ascribed to the overlapping of the lattice site potentials because of the effect of temperature promoted thermal energy [52]. The hike in temperature certainly induces more rapid overlapping of the site potentials which in turn causes a fall in W_m. The fall in W_m signifies that the no of species crossing the barrier will rise which in turn hikes the conductivity. Figure 13 (inset) reveals that the minimum hopping length (R_min) falls with temperature and one may expect a similar reason for this. The density of states near Fermi level (N(E_f)) has been extracted from the equation [51, 53]:

$${\sigma }_{ac}\left(\omega \right)= \frac{ᴨ}{3} {e}^{2}\omega {K}_{B}T {\left[N\right({E}_{f}\left)\right]}^{2}{\alpha }^{-5}{\left[\text{ln}\frac{{f}_{0}}{\omega }\right]}^{4}$$

Here, f₀ is the photon frequency and α be the localized wave function. We have taken f₀ = 10¹³ Hz and α = 10¹⁰ m^− 1 [51]. Figure 14 portrays the frequency variation of N(E_f) at various temperatures (673K − 748K). For a given temperature, N(E_f) falls with frequency and generally, N(E_f) hikes with the rise in temperature. A merging type behaviour is noticed at the extreme frequency zone. A close observation portrays that N(E_f) hikes rapidly with temperature in the low frequency zone but, it rises very slowly with temperature (almost temperature independent) at high frequency zone. The large values of N(E_f) reveals that the conduction in the investigated ceramic is basically relied on the hopping of carriers and so also, the hooping is short ranged [53]. Figure 14 (inset) portrays the variation of σ_dc with the inverse of the absolute temperature [σ_dc versus 10³/T (K^− 1)]. The values of σ_dc have been extracted from the Jonscher’s law fittings and σ_dc hikes with the hike in temperature indicating the NTCR response of the compound. The above plot follows the Arrhenius relation [11]: σ_dc = σ₀ exp (-E_a / KT) and the activation energy (E_a) for conduction is extracted to be 1.03 eV from the linear fitting nearly consistent with the previous calculations [11].

3.10 Ferroelectric study

The RT P-E hysteresis loop of the studied ceramic is displayed in Fig. 15. The extracted remnant polarization (2P_r) and coercive field (2E_c) are around 0.033 µC/cm² and 1.46 kV/cm respectively which reveal the presence of weak ferroelectricity in the investigated compound. The presence of OVs is confirmed from the XPS analysis and this will lead to mobile electrons formed due to maintain the charge neutralizations. These electrons may result in generation of minor leakage currents which may yield the observed round shape corner of the P-E loop. Similar types of PE loops have been reported for CST based compounds [7]. The obtained 2P_r is comparable to some lead based ferroelectric compounds [17, 21, 54]. The ferroelectric ordering is probably be because of the 6s² electrons of Pb²⁺ that boosts the off centre shift of B’’ in A(B’B”)O₃ perovskites exhibiting d⁰ configurations [40].

3.11 M-H analysis

The RT M-H hysteresis loop of the examined PFN-CST ceramic is shown by Fig. 16. The magnetic investigation has been performed in a wide interval of field strength (up to ± 16 kOe). The extracted remnant magnetization (2M_r) and coercive filed (2H_c) are respectively around 0.057 emu/g and 0.41 kOe indicating the presence of weak ferromagnetism (FM) in the investigated compound. The extracted 2M_r is consistent with the previous studies for Pb based compounds [55–56] and some other Fe based compounds [57–58]. Even if for such a high applied field, the loop is not seemed to be completely saturated. Towards the high field regions, the magnetization seems to be rising with the advancement in the applied field which indicates the occurrence of the paramagnetic component, but towards the centre (near the low field region), one can clearly notice a ferromagnetic behaviour although it is weak. The magnetic behaviour of such compounds can be analyzed in accordance with the report of Bhoi et al for PFN based compound as per the following equation [59]:

$$M \left(H\right)=\left[2\frac{{M}_{s}}{\pi }{tan}^{-1}\left\{\left(\frac{H \pm {H}_{ci}}{{H}_{ci}}\right)\text{tan}\frac{\pi {M}_{r}\left(FM\right)}{2{M}_{S}\left(FM\right)}\right\}\right]+\chi H$$

Here, M_s, M_r, H_ci and χ denote the saturation magnetization, remnant magnetization, intrinsic coercivity and magnetic susceptibility. The equation can be analyzed in two parts: first part in the square bracket denotes the ferromagnetic part and the second part which is the linear component signifies to the paramagnetic (PM) and/or anti-ferromagnetic (AFM) part [59]. Similar PM component analysis has also been performed by Xiaoyan et al [58] and Arshad et al [60]. But we will later discuss this PM behaviour through the Mössbauer studies.

As per the origin of FM is concerned, the F-centre exchange mechanism (FCE) is best suited for the cause of the RT FM. The occurrence of OVs is confirmed from the XPS analysis and the occurrence of Fe³⁺ species (as bulk) is evident from both XPS analysis and Mossbauer spectroscopy (discussed later). As per the reports of Coey et al and Ren et al, we may expect the presence of the OVs modulated Fe³⁺ ̶ V_O ̶ Fe³⁺ networks inside the material which will lead to the desired RT FM [56, 61].Obviously, more OVs will construct more such clusters but it also depends on the density of Fe-ions. In accordance with this model, an electron blocked inside OV creates an F-centre and the trapped electronic orbital may gain a position which will overlap with the d-orbitals of the two Fe-neighbours. The ferromagnetic coupling is possibly created due to the exchange interactions between the two coupled Fe-atoms through the generated F-centre [56, 61] which is similar to the bound magnetic polaron. Since, Fe³⁺ ions are paramagnetic, this coupling may yield anti-ferromagnetism and one may expect a mutual competition which may yield the observed weak ferromagnetism [56].

3.12 Mössbauer spectroscopy

To analyze the magnetic structure of the Fe-nuclei, we have performed the sophisticated RT ⁵⁷Fe Mössbauer spectroscopy. Figure 17 reveals the RT Mossbauer spectra of the analyzed PFN-CST compound along with the obtained listed hyperfine parameters in which gamma-ray intensity has been plotted as a function of the velocity. A sharp dip is located around the origin indicating the resonant absorption by the Fe nuclei of PFN. The Mössbauer spectrum has been fitted well by a computer software NORMOS-SITE program (Brand R A 1990 NORMOS Programs Universitaet Duisburg) and it reveals a paramagnetic doublet around the centre. The hyperfine parameters extracted from the fitting of experimental data for Fe nuclei are: Width [Γ (mm/s)] = 0.37 ± 0.05, Isomer shift [(δE) (mm/s)] = 0.40 ± 0.02 and Quadrupole splitting [(ΔE_Q) (mm/s)] = 0.34 ± 0.03. The width of the peak is sufficiently broad in comparison to the natural peak width of iron (Fe) [62]. These doublet parameters signify the presence of the high spin Fe³⁺ species in the octahedral environment [62–63]. The occurrence of quadrupole splitting is obvious for PFN compounds and this might be because of the oxygen polyhedron distortion [63].The observed broadening of the Mössbauer line is due to the random orientations of the Fe³⁺ and Nb⁵⁺ species [62–63]. The absence of the traces of the sextet / hextet in the Mossbauer spectrum is might be due to the absorption effect of gamma ray intensity by lead (Pb). Usually, from the literature of PFN compounds, due to the absorption effect of Pb, similar spectra have been recorded [62–63].

Now, it is essential to discuss the slightly contradictory predictions of the obtained results of the M-H magnetization analysis and the Mössbauer spectroscopy. The M-H analysis reveals a weak ferromagnetic behaviour with a linear superimposed paramagnetic component. However, the Mössbauer analysis predicts a high spin paramagnetic doublet with all the Fe ions in the Fe³⁺ states occupying the octahedral positions and no such residual impurities like Fe²⁺. Notably, the VSM instrument is relatively more sensitive in the detection of magnetism in comparison to the Mössbauer equipment [58]. Therefore, although there are no traces of the residual iron oxides in the ⁵⁷Fe Mössbauer spectrum, still there might be a probability of their residual occurrences [58]. This assumption is quite obvious considering the observed outcome of the XPS analysis which reveals the compositional ratio of Fe as [Fe²⁺ : Fe³⁺] = [0.19 : 0.81]. So, the bulk oxidation state is only evident from the Mössbauer studies. It is reported that no sextet in the Mössbauer spectrum does not always guarantee the absence of the FM [64]. Similar observations have reported for iron (Fe) based compounds by the previous researchers: Ahn et al and Lin et al for Zn_1 − xFe_xO [57, 65], Xiaoyan et al for Ti_1 − xFe_xO₂ [58] and Bennett et al for Fe_xCo_1−xTi [66]. The observed discrepancies between the M-H and Mössbauer analysis is possibly ascribed to the spin fluctuations created by the short spin lattice relaxation times [57–58, 64–65]. This phenomenon will possibly arise a result of the effect of the interaction between the lattices of PFN with the Fe-spins from the relaxation and the observed contradictory behavior is attributed to the paramagnetic-spin lattice relaxation [57]. As the other possibility, the linear part arises in the M-H loop may origin from the paramagnetic zone (s) contained in our material due to the occurrence of the nearly separate Fe-ions, for example, the places where the local concentrations of Fe-ions are lowest in consistent with the previous report for Fe-based compounds [64]. This expectation cannot be ruled out as because of the CST addition in PFN, the concentration of Fe-ions is definitely reduced in the examined material.

Single phase PFN-CST compound has been manufactured using high temperature solid state technique with optimal calcinations and sintering temperatures. Rietveld analysis revealed the single phase formation best fitted to pseudocubic structure with a minimized χ² of 1.12. The W-H studies predicted the possible presence of the tensile strain (0.000968) and the crystallite size is near 68.31 nm. The wavenumber of the most prominent Ti-O vibration was analyzed by the FTIR analysis with force constant (2.008 N/cm) and average bond length (2.044 Å). Various expected phonon vibrations and the distortion of TiO₆ networks were analyzed by Raman spectroscopy. The estimated average grain size is around 4.00136 µm from FESEM studies and EDS analysis predicts the purity. The detailed chemical valence analysis has been performed by XPS analysis. The dual oxidation states of Fe, Ti, Nb and Pb were evident from the XPS analysis. In addition, the presence of OVs is also evident. The estimated band gap from the UV-Visible analysis is 2.32 eV and Urbach energy is around 0.31 eV. The photocatalytic studies revealed that the compound can be a potential candidate for H₂ production. Dielectric and ferroelectric studies revealed the ferroelectric nature of the compound with considerably small amount of loss tangent at elevated temperatures making this studied compound a promising candidate for the energy and information storage device applications. The thermal variation of the extracted frequency exponent (n) revealed the presence of CBH conduction mechanism. The RT M-H analysis revealed weak FM with a superimposed linear component. The RT Mössbauer spectroscopy predicted a paramagnetic doublet corresponding to high spin Fe³⁺ octahedral species. The contradictor behaviours are assigned to the paramagnetic spin-lattice relaxation.

Conflict of Interest

We wish to confirm that there are no conflicts of interest associated with this publication and there has been no financial support for this work that could have influenced its outcome. We also confirm that this article has not been submitted to any other journal.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Acknowledgements

We acknowledge the timely support of the Central Research Facility (CRF), IIT Kharagpur for room temperature magnetic (VSM) measurements. We also acknowledge the support of UGC-DAE CSR for providing us the facility to carry out the room temperature Mossbauer, Raman and XPS characterizations.

Credit Author Statements

Mr. Dinesh Kumar Pati: This manuscript is related to Mr Pati's PhD thesis work. Almost all experimental part such as synthesis, Raman, electrical characterizations, magnetic characterizations, P-E loop, Mössbauer spectroscopy, XPS, etc. has been contributed by him. In addition, he has prepared the manuscript draft, studied the literature survey, result and discussion. Dr. Rajib Padhee: Dr. Padhee is the PhD supervisor of Mr. Pati. His contribution is vital because he has designed the research problem and provided guidance for analysis of the results. Dr. Piyush R. Das: Dr. Das is another supervisor of Mr. Pati who is an expert in the field of multiferroics. These expertises of Dr. Das have been utilized by Mr Pati for magnetic analysis of the results. Dr. B. N. Parida: Dr. Parida helped Mr. Pati in experimental works relating to XRD and FESEM along with its analysis. S. Bhattacharjee: Mr. Bhattacharjee helped Mr. Pati in Reitveld analysis. Miss. M. Mahapatra: Miss. Mahapatra helped Mr. Pati in UV-Visible and FTIR experiments and analysis.

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Structural investigations, band gap narrowing and magnetic characteristics of multiferroic Pb(Fe0.5Nb0.5)O30.4 - (Ca0.2Sr0.8)TiO30.6

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Abstract

Figures

1. Introduction

2. Experimental Details

2.1 Sample preparation

2.2 Characterization techniques

3. Results And Discussion

3.1 X-ray diffraction analysis

3.1.1 Rietveld analysis

3.1.2 Crystallite size and strain analysis

3.2 FTIR spectroscopy

3.3 Raman spectroscopy

3.4 FESEM microscopy and EDS analysis

3.5 X-ray photoelectron spectroscopy

3.6 UV-Visible spectroscopy

3.7 Photocatalytic studies

3.8 Dielectric and impedance studies

3.8.1 Dielectric characteristics

3.8.2 Complex impedance spectroscopy

3.9 Conductivity analysis

3.10 Ferroelectric study

4. Conclusions

Declarations

References

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