Accepted Manuscript Cation distribution, magnetic properties and cubic-perovskite phase transition in bismuth-doped nickel ferrite Shyam K. Gore, Santosh S. Jadhav, Umakant B. Tumberphale, Shoyeb M. Shaikh, Mu Naushad, Rajaram S. Mane PII: S1293-2558(17)30852-X DOI: 10.1016/j.solidstatesciences.2017.10.009 Reference: SSSCIE 5581 To appear in: Solid State Sciences Received Date: 6 September 2017 Revised Date: 22 October 2017 Accepted Date: 23 October 2017 Please cite this article as: S.K. Gore, S.S. Jadhav, U.B. Tumberphale, S.M. Shaikh, M. Naushad, R.S. Mane, Cation distribution, magnetic properties and cubic-perovskite phase transition in bismuth-doped nickel ferrite, Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.10.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Graphical Abstract Cation distribution, magnetic properties and cubic-perovskite phase transition in bismuth-doped nickel ferrite AC C EP TE D M AN U SC and Rajaram S. Mane RI PT Shyam K. Gore, Santosh S. Jadhav, Umakant. B. Tumberphale, Shoyeb M. Shaikh, Mu. Naushad ACCEPTED MANUSCRIPT Cation distribution, magnetic properties and cubic-perovskite phase transition in bismuth-doped nickel ferrite Shyam K. Gore,a Santosh S. Jadhav,a Umakant. B. Tumberphalec, Shoyeb M. Shaikh,b Mu. a RI PT Naushadd and Rajaram S. Maneb* Dnyanopasak Shikshan Mandal?s Arts, Commerce and Science College, Jintur-431509, India b Center for Nanomaterial & Energy devices, School of Physical Sciences, Swami Ramanand c d SC Teerth Marathwada University, Nanded-431606, India Microwave Research Laboratory N. E. S Science College Nanded-431606, India Department of Chemistry, College of Science, Bld#5, King Saud University, Riyadh, Saudi M AN U Arabia. Abstract The phase transition of bismuth substituted nickel ferrite from cubic to perovskite is confirmed from XRD analysis synthesized by sol-gel autocombustion routine. The changes in isomer shift, hyperfine field and cation distribution are obtained from the Mossbauer spectroscopy analysis. The cation distribution demonstrates Ni2+ cations occupy tetrahedral sites, while Fe3+ and Bi3+ TE D occupy both tetrahedral as well as octahedral sites. For higher concentrations of bismuth, saturation magnetization is increased whereas, coercivity is decreased which is related to phase change. The variations of dielectric constant, tangent loss and conductivity (ac) with frequency EP (10 Hz-5 MHz) have been explored with Bi3+-doping i.e. ?x?. According to Maxwell-Wagener model, there is an involvement of electron hopping kinetics as both dielectric constant and tangent loss are decreased with increasing frequency. The increase of conductivity with AC C frequency (measured at room temperature) is attributed to increase of number of carriers and mobility. Keywords: Bismuth-doped nickel ferrite; Structural elucidation; Morphology evolution; Magnetic studies *Authors to whom all correspondence can be addressed. Emails: [email protected] (M. Naushad, Prof.) and [email protected] (Rajaram S. Mane, Prof.) ACCEPTED MANUSCRIPT 1. Introduction Recently, modifications in structure and chemical composition of nanostructured materials have attracted considerable attention and thereby, investigators are actively engaged in altering their RI PT properties for promising applications [1, 2]. The chemical composition and structure of the nanomaterials play vital role in modifying the surface of tailored structure and properties. As various efforts, like developing new modified preparation methods, controlled preparative parameters, amount and type of dopant are being underway [3-5]. The chemical and physical characteristics SC of nanomaterials are strongly influenced by phase structure of the nanomaterials for various applications such as magnetic, electronics and electrochemical devices [6-8]. The doping is an old method to modify the properties of the host material by incorporating atoms or ions of M AN U suitable elements called impurity doping. Studies on an impact of dopant on the crystal structure, size and shape of these materials have attracted considerable attention in recent which can be the simplest approach to modify the essential properties. Ferrites were prepared by doping divalent and trivalent impurities in stoichiometric proportion. The modified stoichiometry of cation can provide an alternative route in property altering [9-11]. The modification of cation stoichiometry and its influence on the structural, magnetic and electrical properties of ferrite is the curiosity. TE D This can open a new window for functional materials for various applications. Recently, impurity doping has shown influence on the crystallographic phase, shape, size and electronic configuration of nanomaterials [12-14]. The new interest has arisen to interface the component EP for translating from old to new application and the multi-applicability of the interfaced materials. The magnetic and electrical properties of ferrite, mainly related to a number of factors, such as cation distribution, oxygen stoichiometry, and phase purity [15-17]. Mostly in ionic compounds, AC C the crystal structure is determined from the closely packed lattice of anions. The anionic lattice has power to accommodate a wide range of different cations in the largest interstices, which not only offers stoichiometry in compound but also helps in modifying the stoichiometry. Nickel aluminum ferrite is modern functional material used in electrochemical devices. In the present case inverse spinel mixed oxide i.e. NiFeAlO4 is considered as the low potential electro-catalyst for water oxidation . Zhu et al. showed electromagnetic and microwave absorbing properties of nickel ferrite, where strong absorbing properties of nickel ferrite nano-crystals for thin and light weight microwave absorber in the X- band frequencies were reported . Shan et al. ACCEPTED MANUSCRIPT prepared nickel ferrite with controlled morphology and phase formation mechanism prepared by hydrothermal synthesis was designed . The magnetic properties of bismuth ferrite were studied from the samples of bismuth ferrite (BiFeO3) showing multiferroic properties with various applications [21-23]. It is noted that magnetic properties are poorer as compared to RI PT nickel ferrite, hardly used in microwave technology. Nickel ferrite is one of the important members of ferrite family, investigated since six decades by many investigators . Many synthesis methods such as solid state reaction, chemical precipitation, chemical spray, micro emulsion, and sol-gel method have been reported to control the morphology and properties of SC nano crystalline nickel ferrite [25-28]. In this work, we have studied the sol gel synthesis of bismuth substituted nickel ferrite; the M AN U incorporation of Bi3+ causes the modification in phase and magnetic properties. The magnetic and dielectric property depends on cation distribution and hopping of electron. The structural and magnetic properties of nanocrystalline materials motivate us to study the effect of modifications of stoichiometry on magneto-structural properties of Ni1-xBixFe2O4 (NBF) ferrite structures. 2. Experimental details TE D Various magnetic parameters are also measured and reported. Nickel ferrite powders with different amount of bismuth were synthesized by sol-gel self combustion method. The starting materials, without any further purification were used as Iron EP (III) nitrate (Fe(No3)3�2O, nickel (II) nitrate (Ni(NO3)2�2O, bismuth (III) nitrate (Bi(NO3)3�2O and citric acid (C6H8O7稨2O). All hydrated A. R grade reagents were purchased AC C from S-D fine India, were used to fabricate Ni1-xBixFe2O4. The Ni1-xBixFe2O4 (NBF) powders were synthesized by novel sol-gel autocombustion route with citric acid as chelating agent. The citric acid to metal nitrate ratio was kept 3:1 during the synthesis of NBF powder. Starting materials were weighed in appropriate proportions as to form anticipated product. The Ni(NO3)2 6H2O, Fe(NO3)3 9H2O and C6H8O7 were dissolved in deionized water separately. The Bi(NO3)3 5H2O was dissolved in concentrated HCl. The solutions of all metal nitrates and citric acid were mixed together; the pH ?7 of the mixture solution was controlled by adding access ammonium hydroxide solution before heating the solution . The solution was heated while continuous stirring at ?100癈. Slowly, solution got evaporated and changed into a viscous gel and finally, ACCEPTED MANUSCRIPT formed thick gel when all water molecules were evaporated; gel automatically got ignited, yielding the black colored ashes, termed as final products. The final products were heated separately at 500癈 for 4h to get desired crystal structure with enhanced crystallinity. The RI PT sintered powders were used for further characterizations. The microstructure and phase formation of the sintered powder samples were investigated by Xray diffraction (XRD) spectra recorded in 2? range from 20� to 70� with scanning rate 10�/min, obtained from the Rigaku-denki (Japan) X-ray diffractrometer (D/MAX 2500) with Cu K? SC radiation (?= 1.5418 �). The morphology of the powder samples were investigated using scanning electron microscopy (SEM). The Mossbauer spectra were taken in transmission M AN U geometry at room temperature. A 57Co/Rh ?-ray source was used. The magnetic measurements of the samples were carried out using a vibrating sample magnetometer (VSM) at room temperature by Lake Shore: Model 7404. Dielectric properties were measured by a two probe method at room temperature. The dielectric constant (??), dielectric loss tangent (tan?) and conductivity (AC) were measured as a function of frequency by using (LCR Hi Tester 3520-50) LCR-Q meter. TE D 3. Results and discussion 3.1. Studies on structural elucidation and morphology evolution Obtained crystal phase of the samples were determined from the XRD spectra. Fig.1 shows the EP XRD patterns of NBF samples for various x value i.e. (x ?0 ?0.2). The XRD analysis of powder samples reveals that the broad peaks associated with NiFe2O4 are for samples prepared with x = AC C 0.0 and 0.05. These peaks are in accordance to standard XRD spectrum of JCPDS data file (441485) with cubic spinel phase having Fd3M space group . No impurities peaks are confirmed for these two samples. However, extra reflections peaks are found the XRD patterns of NBF samples prepared at x = 0.1, 0.15, and 0.2. The relative intensities of these extra peaks are increased with increasing x > 0.1. The comparison of these extra reflection peak patterns to standard JCPDS data card (14-0181) reveals to the presence of perovskite phase of BiFeO3 . Thus, for the x- values between 0.1 and 0.2 of Bi doping the sample shows simultaneous spinel and perovskite phases. Fig. 1 shows the (311) Brag peaks of the cubic structure and (012) peak of perovskite structure. The XRD patterns of NBF (x = 0.1, 0.15, 0.2) samples match well to the ACCEPTED MANUSCRIPT JCPDS card of the spinel and perovskite phases. Data for the NBF composition is not available in the inorganic Crystal Structure Database or any other reference data where Bi3+-substituted sample shows that the phase transformation from ?spinel? to ?spinel-perovskite? phase for the x = 0.10, 0.15 and 0.20 samples. The secondary phase of BiFeO3 is obtained when x = 0.1 to 0.2 RI PT with excessive Bi3+ -substitution. The reflections intensities of these extra peaks are increased TE D M AN U SC with x value > 0.1 supporting for the increase of perovskite phase in the ferrite crystal. Fig. 1: XRD patterns of NBF samples for x = 0.0 - 0.2 annealed at 500癈. For x ? 0.1 the appearance of (012) Bragg peak is indicating the presence of perovskite phase along with spinel phase. For x = 0.2, another peak (021) of perovskite phase corroborates EP increase of the perovskite phase in the ferrite crystal. The morphology of the NBF samples were examined by SEM images (fig. 3). Closely packed crystallites and air voids are seen in the SEM AC C image of nickel ferrite samples. The 0.05 mol of Bi3+substituted sample shows that the agglomeration decreased and crystal size increased with spherical grains are formed with average crystal size of 50 nm. Shape of the crystal have been changed from spherical to cubic with increasing size for the increasing substitution of bismuth, where majority of crystallite have grown on the surface. Smaller spherical crystallites are found on the large crystallite. The 0.1 mol of Bi3+ substituted sample confirms the difference in the shapes with increasing informality, where crystallites come closer to one another. The 0.2 mol Bi3+ substituted sample accepts an increase of crystal size where the shape of the crystal looks like a pyramid type. The elemental percentage of Bi3+, Ni2+, Fe3+ and O2- are obtained from energy dispersive X-ray spectroscopy ACCEPTED MANUSCRIPT (EDS) spectra for the as prepared samples and is shown in Table 1. The chemical compositions of above samples were analyzed by using (EDS). Analysis of samples using EDS spectra evidences good agreement of stoichiometry with the precursor ratio. Comp. O Fe Ni ?x? RI PT Table 1.The elemental percentages of O, Fe, Ni, Bi in obtained product for various x values. Bi 43.24 40.69 16.07 0 0.05 41.29 39.23 18.59 0.89 0.10 40.23 41.34 16.97 1.46 0.15 47.03 38.34 12.43 2.20 0.20 46.41 37.38 13.4 3.04 AC C EP TE D M AN U SC 0.0 Fig.2 SEM images of NBF,: (a) pristine NiFe2O4, (b) x= 0.05, (c) x= 0.10, (d) x= 0.15, (e) x= 0.20 NBF samples, and (f) EDS of NBF when x=0.2. ACCEPTED MANUSCRIPT 3.2 Mossbauer study Mossbauer spectroscopy has been used to study the structural changes and coordination difference of iron due to substitution of Bi3+ in NBF system, at the room temperature. Spectra are RI PT shown in fig. 3. The Mossbauer spectra were fitted with magnetic components associated with two sextets of iron in the tetrahedral (A-site) and octahedral (B-site) coordination. The dots in the figure present the experimental data and solid lines in the spectrums indicate computer fitted spectra. The experimental data has been fitted by using least square WinNormos fit program . EP TE D M AN U cation distributions are presented in Table 2. SC The Mossbauer parameter isomer shift (?), quadrupole splitting (?), Hyperfine field (Hf) and AC C Fig.3: Mossbauer spectrum of NBF ferrites for x=0.0, 0.1 and 0.2 at 300K. The Mossbauer spectra in Fig. 3 exhibit Zeeman sextet which indicates the occurrence of Fe3+ ions at the tetrahedral A sites and octahedral B sites . The tetrahedral A and octahedral B sites have been identified with the help of isomer shift in ferrite. Spinel ferrites generally have bond separation between Fe3+ ? O2- smaller for tetrahedral sites ions as compared to octahedral ions, larger overlapping of the orbital of Fe3+ and oxygen anions and larger covalency lead to smaller isomer shift on A site . The isomeric shift is due to the s-electron charge distribution of Fe3+ ions at tetrahedral A sites and octahedral B sites . In this study the isomer shift remains constant for three samples on tetrahedral A sites and slightly increased at octahedral B ACCEPTED MANUSCRIPT sites for x = 0.2; which indicates population of Fe3+ ions at B sites is more because of replacement of Bi3+ with Fe3+ ions from A sites. In most of the ferrites, the hyperfine magnetic field at octahedral B sites is more than that of the tetrahedral A sites. The hyperfine field for x = 0.0, 0.1, and 0.2 samples with bismuth content ?x? at A sites decreases for x ? 0.1 and increases RI PT for x = 0.2; the increase in hyperfine fine field is due to super-transferred hyperfine field components strongly influenced by neighboring ions and magnetic moments of these ions . The spinel structure is face centered cubic lattice of oxygen ions, with metal ions occupying eight tetrahedral sites and sixteen octahedral sites. The Ni2+ ions prefer octahedral B sites, Fe3+ SC and Bi3+ ions occupy both tetrahedral and octahedral sites. Since Fe3+ and Ni2+ are magnetic ions and Bi3+ is non-magnetic, for x = 0.2, transfer the Fe3+ ions from octahedral to tetrahedral sites M AN U causes increase in hyperfine field at A sites and decrease in it at B sites. Quadrupole splitting indicates the degree of deviation from cubic symmetry structure. The quadrupole splitting for system shows no variation that is constant for all samples which indicates Fe3+, Ni2+ and Bi3+ ions symmetry has not been changed between Fe3+ ions and their surrounding with addition of Bi3+ ions in the system. It indicates that the Bi3+substitution in the lattice do not disturb the cubic structure. The Mossbauer spectra have six line magnetic hyperfine splitting, the sextet TE D corresponding to Fe3+ ions at A sites and B sites. The relative intensities of A and B sites pattern can be used to determine cation distribution . The relative number of Fe3+ ions on A and B sites is determined from the ratios of the areas of the spectra of A and B sites; from this the number of Ni2+ and Bi3+ ions on A and B sites are determined. In pure nickel ferrite NiFe2O4 EP involves only one cation of (Ni2+) with two Fe3+ ions. The chemical formula for distribution of cation is shown in table 2. When bismuth ions are added into the nickel ferrite at x = 0.1 the intensity of the outer most peak for A sites is increased with respect to x = 0.0. The cation AC C distributions are given in table 2 suggest that for x = 0.0, fraction of Ni2+ ions enters the tetrahedral A sites and rest occupy octahedral B sites, Fe3+ ions occupy both tetrahedral A and octahedral B sites. For x = 0.1 and 0.2, Ni2+ ion prefers octahedral B sites only, Fe3+ ions prefer both tetrahedral A and octahedral B sites and Bi3+ ions occupy tetrahedral A and octahedral B sites. ACCEPTED MANUSCRIPT Table 2. Cation distribution, IS, QS, Hhf and A values obtained from Mossbauer spectra. Site Cation Distributions ?x? IS QS (mm/s) 0.1 0.2 (mm/s) (kOe) A (%) A (Ni0.047Fe0.953) 0.27 0.008 51.1 35.4 B [Ni0.93Fe1.067]O4 0.29 0.008 55.1 64.6 A (Bi0.046Fe0.954) 0.27 0.008 50.1 57.0 B [Ni0.9Bi0.0154Fe1.084]O4 0.29 0.008 55.1 43.0 A (Bi0.016Fe0.984) 0.27 0.008 54.1 44.8 B [Ni0.8Bi0.184Fe1.016]O4 0.33 0.008 50.1 55.2 M AN U 3.3. Magnetic study SC 0.0 Hhf RI PT Comp. Saturation magnetization (Ms), coercivity (Hc), and remanance (Mr) etc., of NBF system were measured from magnetic hysteresis loops taken on VSM at 300K (fig.4) and are listed in Table 3. The Ms value is increased with bismuth content up to x = 0.15 and then suddenly is decreased with further increase of bismuth content i.e. x = 0.2. The obtained Ms can be explained on the TE D basis of cation distribution and exchange interaction of cations between on A site and B site. The Ni2+cations prefer octahedral B sites , bismuth and iron cations occupy both tetrahedral A and octahedral B sites . The Ms is the difference between magnetization of B and A sites. When small amount of bismuth is introduced into the nickel ferrite, few Bi3+ ions may occupy EP tetrahedral A sites and remain reside on octahedral B sites. Therefore, Fe3+ ions can migrate from A sites to B sites which eventually increases the iron concentration on B sites, as a result, the AC C magnetization on B sites is increased. The migration of Fe3+ ions has continued with further substitution of Bi3+ up to x = 0.15 and strength of A-B interaction is increased for x = 0.2, Ms value was decreased due to retardation in A-B exchange interaction and spin magnetic moment on B sublattice do not held parallel to spin magnetic moment on A sublattice. The B sublattice moments were decreased due to the departure of spin from collinearity so called spin canting effect . The variation of coercivity (Hc) with bismuth content x is shown in table 3. It is observed that Hc is decreased with increasing nonmagnetic bismuth substitution. The Hc is a function of crystallite size, shape, domain structure and anisotropy of crystal. In single domain ACCEPTED MANUSCRIPT region with decrease in size the coercivity decreases because of thermal effect. The coercivity in single domain region is expressed as , H= g- h/D2 RI PT (1) where g and h are constant, D is diameter of the particle. In multi domain region, variation of corecivity with grain size can be expressed as, (2) TE D M AN U where a and b are constant and D is diameter of particle. SC Hc = a + b/D Fig. 4. Magnetic hysteresis loops of NBF ferrite at 300K with inset shows the enlarged view of EP central area of each. Hence in multidomain region the Hc is decreased as the crystallite size is increased . This is AC C due to decrease in magnetrocrystalline anisotropy, result of increased susceptibility. The saturation magnetization is related to Hc through Brown?s relation , = 2 /� (3) According to this relation coercivity Hc is inversely proportional to Ms. Here also Ms is increased with decrease of coercivity. The Mr of nickel ferrite samples are presented in table 3. The remanance is the magnetization remained even after the applied field (H) reduced to zero value. In our study the remanance is varied in range of 5.6-8.3 emu/g. ACCEPTED MANUSCRIPT Table 3. The Ms, Hc, and Mr obtained from hysteresis loops. Comp Ms ?x? (emu/g) 0.0 16.91 255 5.65 0.05 19.85 194 6.60 0.10 22.85 189 7.82 0.15 29.22 182 8.38 0.20 18.41 173 5.4 Hc Mr SC RI PT (Oe) (emu/g) M AN U 3.4. Dielectric study The fig. 5(a) shows the variation of (??) as a function of frequency at 300K for all samples of NBF ferrites. It is observed that (??) decreases very rapidly with increasing frequency and remains independent of applied field at high frequency, which is the common behavior observed in ferrite study. The dielectric behavior of the material is due to four types of polarizations such as dipolar polarization, interfacial polarization, atomic polarization and electronic polarization TE D . The dipolar and interfacial polarizations are strongly temperature and frequency dependent, electronic and ionic polarization occur at very high frequency range from THz to PHz [45, 46]. According to Maxwell-Wagner model, ferrites are the dielectric materials which are assumed to be composed of conduction grains separated by less conducting grain boundaries [47, 48]. The EP dipolar polarization in polycrystalline ferrite is reported to be hopping of electrons between Fe2+ ? Fe3+ions or Ni2+ ? Ni3+ ions of same elements in different oxidation states . The AC C interfacial polarization in the ferrite is due to gathering of charges at grain and grain boundaries. When alternating field is applied, the electrons move in the direction of field through the grains by hopping and accumulate at the grain boundaries to produce polarization . When frequency of field increases electrons cannot keep up with field and lag behind the field. As a result electrons reaching at the grain boundaries become fewer. This decreases the polarization and hence, the dielectric constant and there is decrease of dielectric loss. The inset of fig. 5 (a) shows the variation of (??) with bismuth content ?x?. Where (??) decreases with increasing bismuth content ?x?. This is attributed to the increase in grain size. Increase in the grain size decreases both number of grains and grain boundaries, results in the decreased polarization . Similarly ACCEPTED MANUSCRIPT Bi3+occupies both tetrahedral A sites and octahedral B sites by replacing Fe3+ ions in B sites. Exchange process of Ni2+ ? Ni3+ is weak compared to Fe3+ ? Fe2+, and hence Fe3+ ? Fe2+ is assumed to be dominant mechanism. In NBF system exchange mechanism can be represented as, Ni2+ + Fe2+ ? Ni3+ + Fe3+ RI PT (4) The variation of dielectric constant with bismuth content ?x? has interesting steps in changes as indicated by the x-y scattered graphs in inset of fig. 5(a). As can be seen, for x <0.05, the decreasing trend is fast indicating an involvement of dielectric polarization in spinel phase of SC nickel ferrite. For 0.05 <x <0.15, a very small decrease in values of dielectric constant is assigned to simultaneous occurrence of spinel and perovskite phases increase of Bi3+. However, M AN U the moderate decrease in values of dielectric constant for x >0.15 is due to increase of perovskite phase with Bi3+. Fig. 5b shows the variation of (tan?) with frequency for all compositions of Bi3+. The dielectric loss tangent is large at lower frequency and it decreases with increasing frequency. The tan? is the energy dissipation in dielectric system which is proportional to the imaginary part of (???). The dielectric loss tangent depends upon number of factors such as Fe2+ content, stoichiometry, structure, homogeneity and particle size etc. At low frequency, resistivity TE D of materials is high; more energy is required to exchange of electron between Fe2+ and Fe3+ ions and thus, energy loss is high. In the high frequency range, which corresponds to low resistivity, a small energy is needed for electron exchange between Fe2+ and Fe3+ions in the grains and EP accompanied by a small eddy current and hence a decrease in energy loss .The inset of fig. 5(b) shows the variation of (tan?) with bismuth content ?x?. It is evident that the (tan?) decreases with increasing bismuth content ?x?. The variation of conductivity (ac) with frequency of NBF at AC C room temperature is shown in fig. 5(c). It is observed that conductivity increases with increasing frequency. The conduction in ferrite takes place with the help of formation of Fe3+ and Fe2+ ions at octahedral B site and electron exchange between them; i.e. the hopping of electrons through the grains which result in conduction . The hopping rate is frequency dependent. As frequency of applied field increases, the hopping rate also increases which increases the conductivity. The enhancement of conductivity at high frequency is not due to increased concentration of charge carriers but it is due to rate of mobility of charge carriers. In the present study conductivity decreases with the increasing bismuth concentration ?x? which is because the ACCEPTED MANUSCRIPT doping of Bi3+ increases the size of grain (Bi3+ =1.03�) decreases grain boundaries and the number of grains. Grains of higher dimensions have less number of insulating grain boundaries which help in decreasing resistivity [53, 54]. The maximum conductivity of the doped sample is also due to availability of extra charge carriers than undoped sample are with increasing RI PT concentration of Bi3+ increase the number of Fe3+ ions on B site means increasing charges. So the maximum number of Fe2+ and Fe3+ ions and enhanced rate of electron exchange between them gives the optimum conductivity, similar behavior was reported by MohdHashim et. al for Ni-Cu- AC C EP TE D M AN U SC Zn ferrite [55, 56]. Fig. 5: (a) Variation of dielectric constant with frequency at 300K. Inset shows variation of dielectric constant with bismuth content ?x? at various frequencies. (b) Variation of loss tangent with frequency at 300K. Inset shows variation of loss tangent with bismuth content ?x? at various frequencies and (c) Variation of conductivity with frequency for NBF system measured at 300K. ACCEPTED MANUSCRIPT 4. Conclusions The bismuth doped nickel ferrites are synthesized through sol-gel autocombustion route. In NBF system, spinal phase is observed in the pristine sample for x = 0.0. For 0.1< x < 0.15, the RI PT perovskite phase is evolved in simultaneous with the spinal phase, as concentration of Bi3+ cations is increased. For x = 0.2, there is appreciable increase of perovskite phase, confirmed by the presence of (012) and (021) peaks in the XRD patterns of the ferrite samples. The saturation magnetization increases with increasing bismuth content in spinel phase and suddenly decreases when there is increase in perovskite phase at x = 0.2. Cation distribution obtained from SC Mossbauer spectroscopy suggests that, the Ni2+ ions have preferred octahedral sites, while Fe3+ and Bi3+ have preferred both tetrahedral and octahedral sites. Dielectric constant and loss tangent M AN U values diminish with increasing bismuth doping levels. The occurrence of three steps variation in the curves of variation of dielectric constant with bismuth content indicates occurrence of spinel and perovskite phase for 0.1< x < 0.15, while there is increase in perovskite phase for x = 0.2. The polarization in ferrite is due to hopping of electrons. The increase in conductivity with Acknowledgment TE D increasing frequency is due to increased mobility of carriers. Authors extend their appreciation to the International Scientific Partnership Program ISPP at AC C EP King Saud University for funding this research work through ISPP# 0032 ACCEPTED MANUSCRIPT References  B. P. Hahn, J. W. Long, A. N. Mansour, K. A. Pettigrew, M. S. Osofsky and D. R. Ralison, Energy Environ. Sci., 2011, 4, 1495 ? 1502.  A. Singh, V. Pande, R. K. Kotnala and D. Pandey, Phys. Rev. Lett., 2008, 101,  RI PT 247602. J. Xiao, N. C. Chernova, S. Upreti, X. Chen, Z. Li, Z. Deng, D. Choi, W. Xu, Z. Nie, G. L. Graff, J. Liu, M. S. Whittingham and J. G. Zhang, Phys. 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