Structural, electrical and magnetic properties of Bi(Ni0.45Ti0.45Fe0.10)O3

Accepted Manuscript Structural, electrical and magnetic properties of Bi(Ni0.45Ti0.45Fe0.10)O3 Nitin Kumar, Alok Shukla, C. Behera, R.N.P. Choudhary PII:

S0925-8388(16)32041-2

DOI:

10.1016/j.jallcom.2016.07.009

Reference:

JALCOM 38177

To appear in:

Journal of Alloys and Compounds

Received Date: 15 April 2016 Revised Date:

30 June 2016

Accepted Date: 1 July 2016

Please cite this article as: N. Kumar, A. Shukla, C. Behera, R.N.P. Choudhary, Structural, electrical and magnetic properties of Bi(Ni0.45Ti0.45Fe0.10)O3, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.009. 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.

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Structural, Electrical and Magnetic Properties of Bi(Ni0.45Ti0.45Fe0.10)O3 Nitin Kumar *a, Alok Shukla a, C Behera b, R N P Choudhary b Department of Physics, National Institute of Technology Mizoram, Aizawl-796012, India b Multifunctional Materials Research Laboratory, Department of Physics, SOA University, Bhubaneswar-751030, Odisha, India

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a

ABSTRACT:

Nowadays, much attention is paid on the development of pure as well as mixed metal oxides to be used for multifunctional devices. In this paper, synthesis and extensive studies of

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electric and magnetic characteristics of bismuth ferrite (BiFeO3 (BFO) modified bismuth nickel titanate Bi(Ni0.5Ti0.5)O3 (BNT) have been reported. A complex polycrystalline

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multiferroic of these lead-free compounds (BNT and BFO) in the ratio of 9:1 (i.e., Bi(Ni0.45Ti0.45Fe0.10)O3) was synthesized using a standard high-temperature solid-state reaction technique at 1073 K in air atmosphere. The formation of a single-phase compound with tetragonal symmetry was determined by preliminary X-ray structural analysis. The average crystallite size, determined using X-ray peak broadening, was estimated to be 30 nm. The field emission scanning electron microscope (FE-SEM) was employed to investigate the surface morphology of the compound. The grains were found to be uniformly distribution on

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the surfaces of the sample. Detailed studies of the effect of grains and grain boundary in the resistive and capacitive characteristics of the material at different frequencies and temperatures using the impedance spectroscopy have provided many interesting and useful

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results for applications.

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KEYWORDS: Ceramics, Solid state reaction, X-ray diffraction, Dielectric response.

*Author for Correspondence:- Phone:- +91-76690-76853; Fax: +91-38923-91774 E-mail:- [email protected] (Nitin Kumar)

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1 INTRODUCTION Though a large number of lead based- and lead free- multiferroics is known today, bismuth ferrite (BiFeO3) is considered to be the most promising material of the family for multifunctional devices because of the coexistence of both ferroelectric and anti-

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ferromagnetic phase transition in it at room temperature (TC= 1103 K and (TN=643 K) [1]. BiFeO3 (BFO) is also known as one of the most interesting multiferroic materials because of its excellent properties and several technological applications for sensors, magnetically modulated transducers, ultrafast optoelectronic devices, multistate memory devices and so on [2–4]. At room temperature, bismuth ferrite has distorted rhombohedral perovskite (ABO3)

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structure with R3c space group. It has also the coupling interaction between multiferroic orders, which in turn, produces some novel physical phenomenon such as magneto-electric

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effect and their possible use in spintronics [5–6]. As a result, multiferroic materials, in general, considered as prominent candidates for magneto-electric coupling [7-8]. In BFO, the ferroelectric and anti-ferromagnetic properties are originated due to the presence of 6s2 lone pair electrons of Bi+3 and Fe+3 ions [9]. In spite of its multiferroic characteristics, due to the defects and the high-leakage current, this compound gives rise to low resistivity. In principle, BFO with a large band gap of 2.8 eV, should show negligible small intrinsic leakage current [10]. The high-leakage current, attributed to the oxidation–reduction of Fe ions, and creation

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of oxygen vacancies for charge compensation [11-12] have been minimised or eliminated by various methods including substitution of suitable elements at different atomic sites or fabrication of solid solution and composites.

Titanium based ilmenite-type perovskite compounds, generally formulated as MTiO3 (M=

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Ni, Co, Fe, Cd, Sr, Ba, etc.), have been regarded as intelligent materials because of their multifunctional applications. Nickel titanate (NiTiO3) belongs to the ilmenite family with

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centro-symmetry rhombohedral structure at room temperature. In this structure, both Ni and Ti atoms prefer octahedral coordination with alternating cation layers occupied by Ni and Ti alone [13]. It is one of the prominent key materials having a great interest in a wide range of applications in photo catalysis, sensor, fuel cell, etc. [14–17]. Over the past few years, in order to solve the leakage current problem, the substitution at the A, B or AB–sites of perovskite has been attempted and found satisfactory. Substitution effect in BFO with other perovskite materials of the A-site (bismuth) and B-site (iron) atoms such as La, Gd and Nd [18–20], Zr, Ti and Mn atoms [21–23] for suppressing generation of oxygen vacancies have also been attempted and provided many new results. Our recent work suggests that the partial co-substitution at the B-site (such as Ni/ Ti and Co/Ti of various percentages) cation of the

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Fe+3 site in BFO, by isovalent elements leads to improvement of dielectric and magnetic properties [24-25]. In addition to the above, different other approaches have been proposed to improve the performance of multiferroic properties of BFO. Also, some new recent studies have been published on the preparation of nanocrystalline materials by using solid-state

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reaction techniques [26-29]. In view of the above, it has been proposed to synthesize lead-free bismuth nickel titanate Bi(Ni0.5Ti0.5)O3 [BNTO] modified with a small amount of BFO. To the best of our knowledge and literature survey, the proposed material has not been developed so far for the

impedance and magnetic properties of BNTFO. 2 EXPERIMENTAL PROCEDURES

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2.1 CHEMICALS

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above purpose. The present paper reports detailed study and analysis of structural, dielectric,

The polycrystalline complex lead-free Bi(Ni0.45Ti0.45Fe0.10)O3 (BNTFO) compound was synthesised by a multi-step mixed oxides route. All the chemicals used in our experiments were of analytical grade, purchased from Merck. The high-purity (>99.5%) carbonate and oxides such as; bismuth carbonate ((BiO)2CO3.H2O), nickel oxide (NiO), titanium dioxide (TiO2) and iron oxide (Fe2O3) were used as starting ingredients in a desired stoichiometric.

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The high-purity (>99.7 %) cylindrical alumina crucible, tray and boat were used for synthesising BNTFO compound. Double distilled water (purified by distillation) was used for the preparation of polyvinyl alcohol (PVA) solution. BNTFO was prepared according to the following chemical reaction:

heat treatment

2Bi(Ni0.45Ti0.45Fe0.10)O3

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(BiO)2CO3.H2O + 0.9 (NiO) + 0.9 (TiO2) +0.10 (Fe2O3)

+H2O + CO2↑ (g) at 1023 K for 6 h under a controlled heating and cooling cycles. The

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schematic representation of synthesis procedure for BNTFO is summarized in Fig. 1. 2.2 SYNTHESIS AND CHARACTERIZATION PROCEDURES The starting ingredients were first thoroughly mixed by using agate mortar and pestle in dry (air) medium for 10 h, and then in wet (methanol) medium for 12 h. The homogeneous powder of the ingredients was calcined at an optimised temperature (1023 K) for 8 h. At room temperature, the phase formation (single phase) was checked using X-ray diffraction data collected by Rigaku Powder X-ray Diffractometer (PXRD) with CuKα radiation (λ= 0.15405 nm) in a wide range of Bragg angles (20º ≤ 2θ≤ 70º) at a scan speed of 2º m-1. The micro-structural characteristics of the sample were investigated by using field emission

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scanning electron microscope (FE-SEM, EVO HD 15 Carl Zeiss). The elemental analysis of the prepared compound was carried out by using energy dispersive X-ray (EDX) microanalysis which confirms the formation of desired phase and composition of the synthesized compound. The micro-structural analysis of the prepared compound was carried

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out by using transmission electron microscope (TEM, JEOL-2100F) with an accelerating voltage of 200 kV equipped with a high-resolution CCD Camera. The calcined powder was used to prepare circular disc-shaped pellets (10 mm diameter and thickness 1-1.5 mm) at the applied pressure of 5×106 N /m2 by using a KBr hydraulic press. The pellets were sintered at an optimized temperature of 1073 K for 6 h in air atmosphere. The density of the pellets was

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measured using Archimedes’ principle. The sintered pellets were then polished with fine emery paper to make their surfaces smooth and parallel. Finally, opposite faces of the pellet

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is coated with conducting silver paint and dried at 473 K for 3 h to remove the moisture, if any, from the sample prior to electrical measurement. Dielectric (constant and loss), impedance and electric modulus, phase angle (θ) of a pellet sample (with silver-electrode) have been carried out by using phase sensitive multimeter (PSM-1735, Newton 4th Ltd, UK) with a computer interfaced furnace and laboratory fabricated sample holder from 1 kHz to 1 MHz in the temperature range 298 to 673 K. The dielectric and impedance data were

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recorded at an interval of 5 K and 25 K respectively. A digital voltmeter (Rishabh) and chromel-alumel thermo-couple were used to measure the temperature of furnace. At room temperature, the magnetic behaviour of an un-silvered pellet was studied with a maximum applied field up to ± 15 kOe by using vibrating sample magnetometer (VSM, Lake Shore-

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7410).

3 CHARACTERIZATIONS

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3.1 STRUCTURAL AND MICRO-STRUCTURAL ANALYSES Fig. 2 shows the X-ray diffraction (XRD) pattern of the complex Bi(Ni0.45Ti0.45Fe0.10)O3 ceramics at the room temperature. The preliminary structural analysis and crystalline nature of the as-prepared BNTFO sample were carried out by using ‘POWDMULT’ software [30] using XRD data. The figure shows that the intensity of most of the peaks is sharp and narrow suggesting good crystalline nature of the sample. However, few peaks of small intensity also appear which correspond to the Bi25FeO40 and Bi2Fe4O9 crystal system (shown as by a symbol, * in the figure). The occurrence of impurity phase of bismuth based multiferroics is a common problem in synthesizing such type of materials [31]. Based on the best agreement

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between observed and calculated inter planar spacing d [(i.e., ∑∆d = d(obs.) – d(cal.) = minimum)], all the X-ray reflection peaks (except impurities ones) were indexed in the tetragonal

symmetry. The least-squares refined lattice parameters were found to be

a=8.872(7) Å, b=8.872(7) Å and c=13.083(8) Å (with a minimum standard deviation is

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parenthesis). These parameters a(r)=a/√3 and c=c(r) are very much consistent with rhombohedral (r) unit cell of BFO (with minor distortion). By using broadening of some reflection peaks of XRD data, the average crystallite size (S) of the sample was estimated to be more than 100 nm using the following Scherrer’s equation [32] Shkl = Kλ / β1/2 Cos θ

(i)

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where K is a constant (best possible value= 0.89), λ is the wavelength of CuKα radiation ( = 0.15405 nm), and β1/2 is the value of full peak width at half-maximum (FWHM) of (hkl)

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reflection. The average crystallite size of the prepared compound is consistent with the reported ones [24-25, 29]. The distortion of the perovskite structure is generally estimated by tolerance factor. The calculation of Goldschmidt tolerance factor ‘GTF’ [33] for ABO3-type perovskite compound is based on the following simple formula; GTF = (〈RA〉 + RO) / √2 . (〈RB〉 + RO)

(ii)

Herein, 〈RA〉, 〈RB〉 and RO are the average ionic radii of A(Bi) and B(Ni, Ti and Fe) and O

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(oxygen) sites respectively. The tolerance factor is used to quantify the structural stability of the perovskite family compounds. The calculated value of tolerance factor is 0.83 for BNTFO compound, which suggests a lot of distortion in an ideal perovskite structure (GTF =1). Moreover, the surface morphology of as-prepared material was studied by using field-

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emission scanning electron (FE-SEM) because it is one of the most versatile instruments for the examination and analysis of the micro-structural morphology of solid specimens. Fig. 3(a) depicts the room temperature FE-SEM micro-graph of the BNTFO compound. It is

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clear from the micro-graph that the most of the grains are densely packed, but small different in shape and size suggesting the existence of polycrystalline nature. A very few voids were found to be uniformly distributed throughout the surface of the sample. Based on literature survey, the standard value of grain size of bismuth ferrite [34] ceramic is found to be in the range of greater than 50 µm. In order to confirm the composition and purity of as-prepared compound, the energy dispersive X-ray microanalysis (EDX) spectrum was utilized. In Fig. 3(b), the EDX spectrum of BNTFO sample, all elements (Bi, Ni, Ti, Fe and O) are observed. Fig. 4 shows the transmission electron micrograph (TEM) of BNTFO fine powder. The micrographs show the complete view of crystallite size, morphology and its micro structure.

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The micrograph also suggests the particles are of cubic shape and formed loose aggregates, which was a good agreement to XRD result. Some separated particles are also seen in those samples. Furthermore, some moderately agglomerated particles as well as separated particles also present in the image. However, the particle size of as-obtained BNTFO compound has

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diameter in the range of 2–5 µm which is within the range of earlier reported ceramics [3536]. The ImageJ software was used as a tool for the calculation of the grain size. BFO has relatively larger grains whereas nickel and titanium co-substituted BFO exhibit highly dense smaller grains. Furthermore, the image illustrates that as-synthesized sample has average grain size of 4 µm. However, XRD is useful for the bulk study whereas FE-SEM is

grain size, which consists of particles.

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3.2 DIELECTRIC MEASUREMENTS

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applicable for surface morphology. Also, XRD gives particles size and FE-SEM provides the

The dielectric spectrum is one of the most important properties of ceramic materials. Thus, dielectric study provides the information about the relative permittivity of material under test for a specified orientation of electric field and frequency depending on the types of polarization responsible for relaxation, defects in the specimen sample, etc. Relative permittivity (εr) is the constitutive parameter of dielectrics. It is a complex-valued parameter

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that generally depends on frequency and operating temperature. The dielectric and related parameters were reordered by using phase sensitive multimeter set up and with computer and temperature-controlled furnace. The dielectric constant (ε) of the ceramic pellet was calculated by using the Eq. (iii) from the capacitance measurements [37] at different

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frequencies as well as temperature.

 

ε = ε 

(iii)

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Here, ε= dielectric constant, C = parallel capacitance obtained experimentally, d = thickness of the specimen pellet, A = area of the electrode specimen pellet and ε0 = permittivity in free space. 3.2.1 TEMPERATURE DEPENDENCE OF DIELECTRIC PARAMETERS Fig. 5 (a-b) shows the variation of relative dielectric permittivity (εr) and tangent loss (tan δ) of the prepared sample. It has been recently shown by us [24] that value of both the dielectric parameters (εr and tan δ) increases gradually with rise in temperature al all the selected frequencies. But the value of parameters decreases with increasing frequency which is a general characteristic of the dielectrics [38]. At low frequency, all types of polarization (i.e., dipole, ionic, atomic, interfacial, etc) exit in dielectrics. These polarizations slowly vanish

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with increasing frequency, as a result, the dielectric constant decreases. The dielectric spectrum curves show that as-synthesized material possesses temperature dependence of dielectric constant. The rate of increasing trend for εr and tan δ of the material in the lowtemperature region (up to 298-500 K) slightly rises, but beyond the operating temperatures

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(500-673 K), the both values of the dielectric parameters sharply increase for all the selected frequencies. The relative dielectric constant can be utilized to detect structural phase transitions [39]. As a result, the increasing trend in the value of dielectric constant for BNTFO with rise in temperature may be due to the electron–phonon interaction [40-41]. In addition, the sharp increase in the value of tan δ at higher temperatures may be due to the

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scattering of thermally activated charge carriers and presence of oxygen vacancies in the materials. From Fig. 5(a) the calculated value of relative dielectric constant at 1 kHz and 1

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MHz at room temperature and 673 K are 233, 405, 229 and 249 respectively. However, the value of tan δ at 1 kHz & 1 MHz at room temperature and 673 K are 0.023, 1.293, 0.003 and 0.030 respectively, which is calculated from Fig. 5(b). A similar trend in the variation of both the dielectric parameters with temperature was observed in our earliest reported results [24].

3.2.2 FREQUENCY DEPENDENCE OF DIELECTRIC PARAMETERS The variation of relative permittivity (constant) and tangent loss of Bi(Ni0.45Ti0.45Fe0.10)O3

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with respect to frequency (1 kHz to 1 MHz) at selected temperatures (298-673 K) is shown in Fig. 6 (a-d). Fig.6 (a-b) shows the dielectric dispersion curves possess strongly frequency and temperature dependent. The value of relative dielectric constant of the BNTFO material decreases linearly with raising frequencies. It is due to the fact that only few dipoles follow

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the applied electric field at higher frequencies. The polarization mechanism (short-range displacement) helps to lead the total polarization of the ceramic materials [42]. The general

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theory of Koop’s [43, 44] is a suitable approach to provide suitable explanation of dielectric property of inhomogeneous double structure ferrite materials. On the other side, Fig.6 (c, d) shows that the value of tangent loss (tan δ) increases sharply in the low-frequency range while almost similar in the high-frequency range. However, the rate of variation of tan δ has higher value in low-frequency region due to the iron containing compounds. At higher temperatures, the value of (tan δ) decreases rapidly with rise in frequency. The value of relative dielectric constant and tangent loss increases with increase in space charge polarization. It may be due to some defects [45] in the material. The electronic and ionic polarizations always exist at higher frequencies. The change in the value of relative permittivity of the materials depends

on grain boundaries and different types defects

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presents in the materials. Generally, these important factors lower the value of relative permittivity [46]. However, no phase transition is observed in the operating temperature range (273–673 K) for the BNTFO compound. The calculated value of relative permittivity at 1 kHz and 1 MHz at room temperature and higher temperature are 240, 225, 375 and 231

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respectively (Fig. 6a & 6b). The value of loss tangent factor at 1 kHz and 1 MHz at room temperature and higher temperature are 0.35, 0.02, 0.84 and 0.059 respectively (Fig. 6c & 6d). 3.3 ELECTRICAL IMPEDANCE SPECTROSCOPY

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Impedance spectroscopy is a powerful technique for characterizing the various electrical parameters such as grain, grain boundary and electrode effect, etc. of ceramic based materials and their interfaces with suitable simulated circuits [47-49]. It is also possible to investigate

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the dynamics of mobile charge in the bulk or interfacial regions in liquid or solid materials. However, in this section we will discuss the effect of the above (grains, grain boundary, and interface) in the operating temperature (298-673 K) and frequency (1-kHz-MHz). Fig .7 (a-d) depicts

the

variation

of

real

and

imaginary

part

of

impedance

factor

for

Bi(Ni0.45Ti0.45Fe0.10)O3. In this figure, the value of real component of impedance (Z') is found decreasing with increasing frequency. This result is in best agreement with our earlier

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reported impedance spectrum results [24].

At higher frequency (50 kHz-1 MHz), impedance spectrum merges irrespective of temperature, which is probably due to the release of space charges [50], between the assynthesized BNTFO sample and electrode. Although, act of reduction in barrier attributes of

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the specimen with rise in temperature may be a responsible factor for the enhancement of acconductivity at high frequency [51]. Furthermore, in the low-frequency region, there is a decrease in the magnitude of Z' with rise in temperature, which indicates the presence of

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negative temperature coefficient of resistance (NTCR) behaviour of the material. In addition, the variation of the imaginary part of impedance (Z'') with frequency from 1 kHz to 1 MHz at selected temperatures (298 to 673 K) for BNTFO sample is shown in Fig. 7(c-d). The (Z'') versus frequency plots show various features such as, the presence of peak point at a particular frequency, marked asymmetry in the peak pattern, increase in peak broadening with rise in temperature and merger of the spectrum at higher frequency. Figure also depicts the decrease in the magnitude of Z'' with increase in frequency. On increasing temperature, a

significant rising in

the broadening of

the peaks suggests the temperature

dependence of the electrical relaxation phenomenon. The peaks shift to higher frequency

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side indicates the increase of tangent loss behavior occurred in the material. The peakpattern trend shows the occurrence of the strength and type of electrical relaxation process present at a particular frequency [52]. Immobile species at low temperatures and defects at higher temperatures are responsible for relaxation process.

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It may be possible that the relaxation species (defects) is responsible for electrical conduction in the material. The conduction may take place by hopping of oxygen, electrons, defects and ion vacancies among the available localized sites [53]. The peak position on frequency plan (x-axis) indicates the existence of relaxation frequency (fr), and then relaxation time (τ) can be calculated by using the relation τ = 1 / 2πfr, where symbols denotes their usual meaning.

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The complex impedance spectroscopy (CIS) [54] plots give the correlation between the response of a real and idealized simulated circuit composed of various electrical components.

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Fig. 8 represents the Nyquist plot (Z' versus Z'') for Bi(Ni0.45Ti0.45Fe0.10)O3 in a wide frequency range of 1 kHz-1 MHz at selected temperatures from 523-673 K. At lower temperature (298-498 K), the Nyquist features are not observed, while the presence of single semicircle arcs appears at higher temperatures (523-673 K) which reveals the existence of single conduction process in the sample. Therefore, the grain boundary effect does not contribute for the impedance and electrical parameters. But the appearance of semicircle

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exhibits some degree of depression on the real axis indicating the existence of non-Debye type of relaxation processes in the material [55]. The Nyquist plot also shows the radius of the semicircle arc which decreases with rising the temperature. The experimental impedance data has been analyzed with suitable equivalent circuit using the ZSimpWin software [56]. An equivalent circuit consists of parallel combination of CR and CQR, where Q is known as

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constant phase element (CPE). Using the fitted curves, the values of bulk resistance (Rb) and bulk capacitance (Cb) at different temperature were calculated. As results, on increasing

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temperature the value of bulk resistance decreases, which is summarized in Table I.

3.4 ELECTRIC MODULUS ANALYSIS The electrical modulus of the materials was analysed to study the relaxation mechanism under the influence of temperature and frequency. The utility of the electrical modulus representation in the analysis of the relaxation properties has been demonstrated for polycrystalline ceramic [57]. The main advantage of complex modulus formalism is to detect or discriminate electrode polarization and grain boundary conduction effect /mechanism. Electric modulus is represented as a reciprocal of the permittivity as given below,

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M*= 1 / ε* = M' + jM'' [58]. The mathematical expression for real and imaginary parts of the electrical modulus (i.e, M' and M'') can be written as; .(iv)

 ω RC  M ''= γ  2  1 + (ω RC ) 

(v)

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and

 (ω RC ) 2  M '= γ  2  1 + (ωRC ) 

where M*= 1 / ε* = M*= M' + j M'' = j ω ε0 Z* ; M' = ω C0 Z'' ; M'' = ω C0 Z' ; ε* is the dielectric permittivity; ω= angular frequency =2πƒ; and the term γ= C0/C.

Fig. 9(a-b) shows the variation of real and imaginary parts of electric modulus with frequency

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(1kHz-1MHz) for Bi(Ni0.45Ti0.45Fe0.10)O3 at different temperatures. Fig.9 (a) suggests some of the most significant features of M'-frequency spectrum which are: (i) in the low-frequency

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region, the magnitude of M' decreases on increasing the temperature, (ii) the magnitude of M' nearly approaches to zero at higher temperatures in the low-frequency region, which confirms the existence of electrode and /or ionic polarization [59], and (iii) the value M' increases with rise in frequency and takes almost a constant value at higher frequencies. Thus, as a result, the lack of restoring forces governs the mobility of the charge species under the influence of electric field [60]. Such observation for sigmoidal increasing

behaviour

of M' with

of charge carriers [61].

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frequency proposes the conduction mechanism which is due to the short- range mobility

Fig.9 (b) shows the variation of imaginary part of electric modulus versus frequency plots. The M''max peak shifts systematically toward higher frequencies side at high temperatures i.e.,

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the relaxation rate for the process increases with rising in temperature. At low frequencies, the peaks position suggests the transition from short range to long range mobility in which the ions are capable for moving long distances whereas, in the high-frequency region, the

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ions are confined and can execute only localized motion [57, 62]. Moreover, the asymmetric broadening of the peak suggests the spread of relaxation with different time constants, which suggests the presence of non-Debye–type relaxation behaviour in the material [63].

3.5 STUDIES OF ELECTRICAL CONDUCTIVITY 3.5.1 FREQUENCY DEPENDENCE OF AC-CONDUCTIVITY The ac-conductivity was calculated by using the dielectric data of empirical relation σac (ω) = ω ε0 εr (tan δ) (where symbols denotes their usual meaning) [64, 65]. Fig. 10 (a) indicates the

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variation of ac conductivity with broad range of frequencies (1 kHz-1 MHz) at few selected temperature (298-673 K) of Bi(Ni0.45Ti0.45Fe0.10)O3. It is clear from conductivity plot that the material exhibits dispersion in the low-frequency region. Spectrum plot also depicts that the value of conductivity rises continuously with rises in frequency. The conductivity plots could

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be fitted through the empirical relation σ ω = σα + Pωβ , known as Jonscher’s universal power law [66], where σα represents the frequency independent term which gives dc conductivity value, P is a thermally activated quantity and β is the frequency dependent exponent that values lies between (0< β < 1). However, the origin of frequency dependence of conductivity lies in the relaxation process arising due to mobile charge carriers. Thus, the

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increase in conductivity with rise in frequency and temperature suggests the existence of negative temperature coefficient of resistance (NTCR) phenomenon occurred in the sample.

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3.7.2 TEMPERATURE DEPENDENCE OF AC-CONDUCTIVITY

Fig. 10(b) shows the variation of ac conductivity with inverse of absolute temperature at some selected frequencies of Bi(Ni0.45Ti0.45Fe0.10)O3. The magnitude of ac conductivity is almost constant at the selected frequencies at low temperatures. This strongly frequency dependent dispersion region may be attributed to hopping of charge carriers to random sites having variable barrier heights. The separation and the value of conductivity increase with

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increase in frequency. At higher temperatures, the value of σac shows a faster rise with increasing temperature and the nature of the curve is perfectly linear which obeys the Arrhenius behaviour. This Arrhenious region reveals proper Arrhenius region or frequency independent region. Therefore, as a result, the change in conductivity (σac) for selected

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frequencies is very small at low temperature and is larger at higher temperature. 3.6 MAGNETIC STUDIES

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The magnetic hysteresis (external magnetic field versus magnetization) loop recorded at 298 K for Bi(Ni0.45Ti0.45Fe0.10)O3 was measured with a maximum applied magnetic field of about ±15kOe (Fig. 11). However, as a result, the M-H spectrum of the as-prepared polycrystalline BNTFO specimen clearly exhibits saturation at higher magnetic field with enhanced magnetic properties. This happens due to the enhanced surface spin contribution in comparison to the bulk perovskite ferrite. Magnetic hysteresis plot show enhanced magnetic property of multidoped bismuth ferrite Bi(Ni0.45Ti0.45Fe0.10)O3. However, the observed value of remnant magnetisation (Mr), saturation magnetisation (Ms) and coercivity (Hc) are 0.004 emu.g-1, 4.98 emu.g-1, and 529.84 Oe respectively. Thus, the magnitude of remnant magnetisation obtained in this study is much higher than that of earlier reported some well known multiferroic

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materials [67-70]. The enhanced value of magnetization may be due to the simultaneous doping of nickel at Fe-site of bismuth ferrite.

Moreover, this compound having anti-

ferromagnetic of G-type [71], because of the ferromagnetic coupling of the magnetic moment of Fe3+ cations in pseudocubic planes and anti-ferromagnetic coupling between adjacent

4. SUMMARY AND CONCLUSION

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planes.

The polycrystalline sample of lead-free Bi(Ni0.45Ti0.45Fe0.10)O3 was prepared using a hightemperature solid-state reaction technique. Preliminary structural analysis and indexing of

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most of the reflections confirmed the formation of the material in a single phase The crystallite size was found to be in the range of 24-30 nm (as calculated from the broadening

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of reflection peaks by using Scherrer’s technique). The FE-SEM morphology shows the uniform grain-distribution of different dimension. The grain size of the sample was found to be of a few µm (by FE-SEM micrograph). The impedance spectroscopic studies reveal the existence of the non-Debye type of relaxation and a negative temperature coefficient of resistance (NTCR) phenomenon of the BNTFO. The coincidence of the real part of impedance (Z') at higher frequencies at all the temperatures indicates a possible release of space charge. Thermal nature of ac conductivity reveals the change in slop and found to

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merge at higher temperatures due to intrinsic conductivity. The observed values of remnant magnetisation, coercivity and saturation magnetisation of prepared compound enhance as compared to BiFeO3. The enhancement of above magnetic parameters will be useful for

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sensor applications.

ACKNOWLEDGEMENT

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The authors are thankful to Magnetic Materials Laboratory, Department of Physics, Indian Institute of Technology Guwahati and University sophisticated instruments facility (USIF), Aligarh Muslim University, for providing various characterization of the samples.

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FIGURE CAPTIONS Fig. 1:- Schematic synthesis procedure of lead-free Bi(Ni0.45Ti0.45Fe0.10)O3 by solid state reaction techniques

temperature.

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Fig. 2:- Powder X-ray diffraction pattern of Bi(Ni0.45Ti0.45Fe0.10)O3 ceramics at the room Fig. 3(a):- The FE-SEM micrograph of Bi(Ni0.45Ti0.45Fe0.10)O3 sintered compound at room temperature Fig. 3(b):- EDX spectra of Bi(Ni0.45Ti0.45Fe0.10)O3 compound

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Fig. 4:- TEM micrograph of Bi(Ni0.45Ti0.45Fe0.10)O3 compound

Fig. 5:- (a) Variation of relative dielectric permittivity (b) tangent loss of

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Bi(Ni0.45Ti0.45Fe0.10)O3 compound with temperature at selected frequencies Fig. 6 (a): Variation of relative dielectric permittivity for Bi(Ni0.45Ti0.45Fe0.10)O3 compound as a function of frequency at selected temperature (b) inset shows enlarge scale of relative dielectric permittivity at 298-498 K

Fig. 6 (c): Variation of dielectric loss for Bi(Ni0.45Ti0.45Fe0.10)O3 compound as a function of frequency at selected temperature (d) inset shows enlarge scale of dielectric loss at 298-498 K b)

Variation

Bi(Ni0.45Ti0.45Fe0.10)O3 Fig.7:(c,

d)

Variation

Bi(Ni0.45Ti0.45Fe0.10)O3

of

frequency

and

temperature

dependent

of

Z'

for

of

frequency

and

temperature

dependent

of

Z''

for

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Fig.7:(a,

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Fig.8: Nyquist plot (Z' versus Z'') for Bi(Ni0.45Ti0.45Fe0.10)O3 sample at selected temperatures with equivalent simulated circuit

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Fig. 9: (a) Frequency dependent of M' at selected higher temperature (inset shows lower temperature nature of M') for Bi(Ni0.45Ti0.45Fe0.10)O3 (b) Frequency dependent of imaginary part of modulus (M'') at selected higher temperature (inset shows lower temperature nature of M'') for Bi(Ni0.45Ti0.45Fe0.10)O Fig. 10(a): Variation of ac conductivity as a function of frequency at different temperature for Bi(Ni0.45Ti0.45Fe0.10)O3 ceramic Fig. 10(b): Arrhenius plot of ac conductivity for Bi(Ni0.45Ti0.45Fe0.10)O3 ceramic Fig.11: Magnetic hysteresis curve of Bi(Ni0.45Ti0.45Fe0.10)O3 specimen recorded at 298 K

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TABLE

8.311E-11 8.476E-11 8.572E-11 8.542E-11 8.661E-11 8.698E-11 8.805E-11

5.818E7 5.879E7 5.593E7 1.862E7 2.448E7 6.102E7 2.107E7

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CPE

2.819E-11 1.304E-10 5.657E-10 7.407E-10 1.025E-9 9.907E-10 2.812E-9

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523 548 573 598 623 648 673

Rb

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Cb

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Temperature (K)

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Table I: Comparison of impedance fitting parameters [bulk capacitance (Cb), resistance (Rb)] for Bi(Ni0.45Ti0.45Fe0.10)O3 at different temperature for (CQR) simulated circuit

n

0.8665 0.7076 0.5958 0.6386 0.5755 0.6080 0.5466

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FIGURES

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Fig. 1:- Schematic syntheses procedure of lead-free Bi(Ni0.45Ti0.45Fe0.10)O3 by solid state reaction techniques

20

30

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*

* **

40

*

50

(5 1 6) (6 0 3)

(4 3 2) (3 3 6) (4 0 7) (5 1 4)

(3 2 3) (3 1 5) (3 3 0) (4 2 0) (2 0 8)

(0 0 7)

*

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Pyrochlore

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(2 2 0) (2 2 2)

Bi(Ni0.45Ti0.45Fe0.10)O3

(0 1 0)

(1 1 3) (1 0 4) (1 1 4)

Intensity (arb. unit)

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* 60

* * * 70

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Bragg's Angle (2θ)

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Fig. 2: Powder X-ray diffraction pattern of Bi(Ni0.45Ti0.45Fe0.10)O3 ceramic at room temperature

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Fig. 3(a):- The FE-SEM micrograph of Bi(Ni0.45Ti0.45Fe0.10)O3 sintered compound at room temperature

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Fig. 3(b):- EDX spectra of Bi(Ni0.45Ti0.45Fe0.10)O3 compound

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Fig. 4:- TEM micrograph of Bi(Ni0.45Ti0.45Fe0.10)O3 compound

0.8 0.6

1 kHz 25 kHz 100 kHz 500 kHz 1 MHz

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300

1 kHz 25 kHz 100 kHz 500 kHz 1 MHz

1.0

0.4 0.2 0.0 300

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Temperature (K)

250

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εr

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Dielectric Loss (tan δ)

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450

500

550

600

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700

Temperature (K)

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Fig. 5:- (a) Variation of relative dielectric permittivity and (b) dielectric loss of

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Bi(Ni0.45Ti0.45Fe0.10)O3 with temperature at selected frequencies

340 320 300

240

235

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225 1

10

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Frequency (kHz)

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280 260 240 220

298 K 348 K 398 K 448 K 498 K

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Relative Permittivity (ε r)

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Frequency (kHz)

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Fig. 6 (a): Variation of (a) relative permittivity(dielectric constant) for Bi(Ni0.45Ti0.45Fe0.10)O3 as a function of frequency at selected temperature (b) enlarge

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view of relative dielectric permittivity between 298-498 K

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tan δ 523 K 548 K 573 K 598 K 623 K 648 K 673 K

0.8 0.7 0.6 0.5

0.035

tan δ

(d)

298 K 348 K 398 K 448 K 498 K

0.030 0.025 0.020 0.015 0.010 0.005 0.000

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Dielectric Loss (tan δ)

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tan δ

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Frequency (kHz)

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Fig. 6 (c): Variation of dielectric loss for Bi(Ni0.45Ti0.45Fe0.10)O3 compound as a function

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of frequency at selected temperature (d) enlarge view of dielectric loss of 298-498 K

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Z'(kΩ )

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298 K 348 K 398 K 448 K 498 K

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0 1

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Fig.7: (a, b) Frequency-temperature dependent Z' for Bi(Ni0.45Ti0.45Fe0.10)O3

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-1000

298 K 348 K 398 K 448 K 498 K

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-1000

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Z''(kΩ )

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Fig.7: (c, d) Frequency-temperature dependent Z'' of B i(Ni0.45Ti0.45Fe0.10)O3

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1750

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Z''(kΩ )

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250 0

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750 1000 Z'(kΩ)

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523 K 548 k 573 K 598 k 623 k 648 K 673 K Fitting

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Fig.8: Nyquist plot (Z' versus Z'') for Bi(Ni0.45Ti0.45Fe0.10)O3 at selected temperatures with equivalent simulated circuit

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0.0045

(a)

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0.0040 0.0035 0.00440 0.00435

0.0030

0.00430

0.0015

0.00415

298 K 348 K 398 K 448 K 498 K

0.00410 0.00405 0.00400

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M'

M'

523 K 548 K 573 K 598 K 623 K 648 K 673 K

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Frequency (kHz)

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Fig. 9: (a) Frequency dependence of M' at selected higher temperature (inset shows lower temperature nature of M') for Bi(Ni0.45Ti0.45Fe0.10)O3

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0.0012

M''

0.0010 0.0008

0.00016 M''

0.00014

298 K 348 K 398 K 448 K 498 K

0.00012 0.00010 0.00008 0.00006 0.00004 0.00002 0.00000

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523 K 548 K 573 K 598 K 623 K 648 K 673 K

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Fig. 9: (b) Frequency dependence of imaginary part of modulus (M'') at selected higher temperature (inset shows lower temperature nature of M'') for Bi(Ni0.45Ti0.45Fe0.10)O3

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1E-6 1E-7 1E-8 1E-9

1

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σac (Ω m )

1E-5

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298 K 323 K 373 K 473 K 573 K 673 K

1E-4

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Fig. 10(a): Variation of ac conductivity as a function of frequency at different

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temperature for Bi(Ni0.45Ti0.45Fe0.10)O3 ceramic

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1E-6

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σac (Ω m )

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1E-8

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Fig. 10(b): Arrhenius plot of ac conductivity for Bi(Ni0.45Ti0.45Fe0.10)O3

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Bi(Ni0.45Ti0.45Fe0.1)O3

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-15

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Magnetic Moment (emu/g)

6

-5 0 5 10 3 Magnetic Field H/(10 Oe)

15

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Fig. 11: Magnetic hysteresis loop of Bi(Ni0.45Ti0.45Fe0.10)O3 recorded at 298K

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HIGHLIGHTS Bi(Ni0.45Ti0.45Fe0.10)O3 was synthesized by the mixed-oxide route.




The compound has single-phase tetragonal symmetry and well defined microstructure.




(Ni+2/Ti+4) substitutions made notable changes in the physical properties of BiFeO3.




Dielectric spectrum reveals the high permittivity and low tangent loss.




Impedance spectroscopy exhibits the non-Debye type of relaxation in the material.

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