Structural, electrical and magnetic properties of (Cd, Ti) modified BiFeO3

Accepted Manuscript Structural, Electrical and Magnetic Properties of (Cd, Ti) Modified BiFeO3

Nitin Kumar, Alok Shukla, R.N.P. Choudhary

PII: DOI: Reference:

S0375-9601(16)31844-8 http://dx.doi.org/10.1016/j.physleta.2017.06.012 PLA 24559

To appear in:

Physics Letters A

Received date: Revised date: Accepted date:

22 November 2016 8 May 2017 6 June 2017

Please cite this article in press as: N. Kumar et al., Structural, Electrical and Magnetic Properties of (Cd, Ti) Modified BiFeO3 , Phys. Lett. A (2017), http://dx.doi.org/10.1016/j.physleta.2017.06.012

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Highlights • As per literature survey no work has been reported for the co-substitution of Cd+2 and Ti+4 at the Fe+3 site of BiFeO3 for enhancement of its various properties with significant reduction of electrical leakage current. • A lead-free Bi(Cd1/4 Ti1/4 Fe1/2 )O3 complex perovskite compound was synthesized using a solid-state reaction technique. • The compound has stabilized in single phase of orthorhombic structure with well defined microstructure. • Complex impedance spectroscopy suggests the existence of non-Debye type of relaxation process.

Structural, Electrical and Magnetic Properties of (Cd, Ti) Modified BiFeO3 Nitin Kumar *a, Alok Shukla a, R N P Choudhary b a Department of Physics, National Institute of Technology Mizoram, Aizawl-796012, India b Department of Physics (ITER) SOA University, Bhubaneswar-751030, Odisha, India Abstract: Bismuth ferrite (BiFeO3), one of the most prominent members of multiferroics, has multiple promising characteristics useful for multifunctional applications. Multi-doped (Cd/Ti) complex bismuth ferrite [i.e., Bi(Cd1/4Ti1/4Fe1/2)O3] ceramic was synthesized through a mixed-oxide route. X-ray structural analysis of the prepared material provides its basic crystal data of a single-phase orthorhombic system. The scattered crystallite size and lattice strain of the material were estimated using Scherrer and Williamson-Hall approaches respectively using x-ray diffraction peaks. Analysis of the micrograph of field emission scanning electron microscope shows uniform and densely packed grains on the surfaces of the pellet sample suggesting the formation of good quality and high-density sample. A significant effect of substitution of multiple elements at the Fe-site on dielectric constant and tangent loss of BiFeO3 has been observed. Detailed studies of temperature (25–500 0C) and frequency (1–1000 kHz) dependence of impedance and ac-conductivity have provided the effect of grains and grain boundaries on the conduction mechanism and dielectric relaxation of the material. Based on the magnetic measurements, it is concluded that (Co, Ti) modified bismuth ferrite has provided saturation magnetisation (Ms) and coercivity (Hc) of 2.66 emu.g1 and 653.75 Oe respectively which are consistent with those of many compounds of similar type.

Keywords: Mixed oxides, Crystal structure and microstructure, X-ray structural data, Magnetic properties, Impedance Spectroscopy.

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

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1. Introduction Among a large number of ferroelectric and ferromagnetic materials available nowadays, few lead-based materials have been found useful for a wide range of devices. Because of toxic nature and creation of environmental problems, much attention is now paid on to develop lead-free materials for devices. Recently, some lead-free multiferroic materials have been found promising for an enormous range of applications including field of sensors, actuators, magnetically modulated transducers, energy harvesting devices, etc. [1–7]. Other than these applications, some materials have been found more attractive for the fabrication of devices, which are related to simultaneous occurrence of two phenomenon such as ferroelectricity and ferromagnetism in distorted systems [8-9]. In the last few years, much attention has also been paid to develop eco-friendly materials for human health protection [10-11]. Bismuth ferrite (BiFeO3) is one of such type of material that is environmental friendly lead-free multiferroic and possesses simultaneous occurrence or coupling of ferroelectric and ferromagnetic ordering at room temperature [12]. Itis one of the promising conditions for applications in non-volatile information storages, spintronic devices and magneto-electric sensors [13, 14]. It has rhombohedrally-distorted perovskite structure with high Curie (TC =850°C) and Neel (TN = 370 °C) temperatures [15-16]. Cadmium titanate (CdTiO3) is one of the another member of perovskite, has an orthorhombic structure at room temperature and undergoes phase transition from the paraelectric (space group symmetry Pnma) to ferroelectric phase (space group Pna21) at Tc ~85 K [17-19]. It is an important compound with many excellent and interesting insulating (dielectric) and optical characteristics [20-21]. It also belongs to the ilmenite structural family with distorted structure (rhombohedral) below 1000 0C, and perovskite above this temperature [22]. Detailed literature survey shows that not much attention has been paid on (Cd+2 and Ti+4) modified BiFeO3. In order to modify some important multiferroic or physical properties required for devices, it is essential to substitute some elements at different atomic sites of BFO or fabricate its solid solution or composite with other structure of similar or different compounds to solve some inherent problems of the material. Many studies of such type including Bi1-xNdxFeO3, Bi1-xSmxFeO3, Bi(Ni0.25Ti0.25Fe0.50)O3, BiMgFeCeO6 etc., have recently been carried out [23-26, 28-30] for the purpose. In the present study, polycrystalline sample of Cd/Ti modified BiFeO3 [i.e., Bi(Cd1/4Ti1/4Fe1/2)O3] (BCTF) has been prepared by a mixed oxide route and studied its various properties. In this paper, structural (crystallite size

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and lattice strain), dielectric, ac-conductivity, impedance, electrical modulus and magnetic properties of Cd/Ti modified BFO (BCTF) have been reported.

2. Materials and methods 2.1. Materials used Analytical grade (purity>99.9 %) of ingredients; bismuth oxide (Bi2O3), titanium oxide (TiO2) and cadmium carbonate (CdCO3) were procured from M/S Loba Chemie, Pvt. Ltd, India. Ferric oxide red [Fe2O3] (>99 %) was procured from M/S CDH, Pvt. Ltd, India. Methanol (M/S Merck Ltd., India) was used as solvent. Double distilled water (purified by distillation) was used for the preparation of polyvinyl alcohol (PVA) solution. The highquality conducting silver paint was obtained from the (M/S Alfa Aesar, Chemical Co, USA purity >99.7%). Cylindrical alumina crucible, disc, tray and boat (M/S ANTS Ceramics, Pvt. Ltd, India) were used for preparation of the material. 2.2. Methodology 2.2.1. Sample preparation The polycrystalline lead-free BCTF sample were prepared by a mixed oxide route at high temperature as per the following chemical reaction and steps as under a controlled heating and cooling cycles. ଵ ଶ

ଵ

ଵ

ଵ

ସ

ସ

ସ

Bi2O3 + CdCO3 + TiO2 + Fe2O3

Heat treatment

Bi(Cd1/4Ti1/4Fe1/2)O3 +

ଵ ସ

CO2↑ (g) at 750 °C

Above ingredients were mixed in a suitable stoichiometric ratios. The ingredients (oxides and carbonate) were homogeneously mixed in agate mortar and pestle in air for 8 h, and in methanol for ten hours. The mixture was heated (calcined) several time at different temperature, and finally calcined at 750 ºC for 4 h in a high-purity alumina crucible. The fine powder of calcined material, homogeneously mixed with a few drop of a binder solution (polyvinyl alcohol solution), was used to fabricate circular (disk-shape) green pellets of diameter 10 mm and thickness 1-1.5 mm at a isostatic pressure of 4×106 N /m2 with the help of a hydraulic press (M/S Technosearch Instruments Co). The binder material was removed from the green pellets during high-temperature sintering of the sample (sintering temperature =800 ºC, time =6 h and atmosphere= air). Schematic diagram for preparation of lead-free BCTF sample is shown in scheme-1. 2.2.2 Studies of material properties

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Basic crystal data of BCTF was obtained using x-ray diffraction pattern obtained at 25 oC (room temperature) by X-ray diffractometer of M/S Rigaku Instrument Co, Japan with wave length Ȝ= 0.15405 nm, 2θ range =20º ” 2θ” 70º and scan speed=2º m-1. The relative density and porosity of pellets (used for insulation and resistive measurements) were measured using Archimedes’ principle. Both the surfaces of sintered pellets were then made smooth and parallel with the help of fine emery paper. Finally, conducting silver paste (obtained from M/S Alfa Aesar Chemical Co, USA) was used to make the surfaces conducting. The painted sample was dried above 130 ºC for few hours (>2 h) before operating measurements (dielectric/electrical). Au coated pellet sample was used to record its surface micrograph using FE-SEM (field-emission scanning electron microscope, model EVO HD 15 Carl Zeiss). Analysis of the microscopic image has provided many features of sample surface. Some insulating and resistive parameters (Cp = capacitance in parallel combination, tangent loss factor (D), θ = phase angle and Z= impedance) of silver-electrode sample were obtained at different temperatures (25-500 ºC) and frequency (1000 Hz -1000 kHz) by a personal computer (PC) interfaced LCR meter (Phase Sensitive Multimeter model PSM-1735 of M/S NewtonN4 Ltd, UK). A laboratory designed-fabricated sample holder and medium temperature range heater (furnace) were interfaced with LCR meter. The temperature of the sample was sensed and recorded with a thermocouple (K2 chromel-alumel thermo-couple). For better understanding of magnetic properties of the material, B-H hysteresis loop of BCTF was recorded at 25 oC (room temperature) in magnetic field of ±15 kOe using VSM (Vibrating Sample Magnetometer of M/S Lake Shore-7410).

3. Analysis of experimental results 3.1 Basic crystal data Fig.1 (a) depicts, X-ray diffraction pattern of BCTF as-synthesized compound at room temperature. The intensity, location and good crystallinity of peaks of the pattern very much differ as compared to those of ingredients suggesting the formation of the new crystal system, Preliminary structural analysis (i.e., indexing of peaks, determination of crystal system with unit cell parameters, single-phase formation, crystallite size, etc.) was carried out using standard XRD interpretation software POWD [31-32]. Basic crystal data of the material observed (obs) and calculated (cal) inter planar spacing d of (h k l) planes and intensity are given in Table-I. The least-squares refined lattice constants of orthorhombic crystal system are; a=14.3173 Å, b=4.3105 Å, c=27.8075 Å, (standard deviation is an order of ±0·0029) and

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volume V=1716.13 Å3. The unit cell approximately corresponds to that of distorted perovskite (a~3b, b=b, c~7b) or rhombohedral cell of BiFeO3. Additionally, our previous results showing the co-substitution effects (Ni, Co, Cd with Ti) on bismuth ferrite has been summarized in Table-II. The Goldschmidt tolerance factor t [33] for XYO3-type perovskite (X = mono/divalent and Y = tri-hexavalent ions/atoms) compounds is estimated using the following expression,

– ൌ 

ሺ‫  ۄ ୶”ۃ‬൅  ”୓ ሻ ξʹ൫‫  ۄ ୷”ۃ‬൅  ”୓ ൯

 ሺ‫ܫ‬ሻ

Here ‫ۃ‬rx‫ۄ‬, ‫ۃ‬ry‫ ۄ‬and ro are ionic radii at X (bismuth) and Y (Cd, Ti, Fe) and O (oxygen) site elements. The value of tolerance factor is indicative of structural stability of perovskite compounds. If t =1, it shows an ideal cubic structure. When t < 1, Fe–O bonds are strained compressively. Based on Bi–O bonds, it is assumed that the oxygen octahedral must inter a bigger cation into a smaller space to have minimum lattice strain. Because of much smaller tolerance factor of BCTF (< 0.6), we expect greater distortion and deviation in perovskite structure of the material. 3.2 Study of crystallite size and strain Detailed analysis of peak width of diffraction pattern provides some important and useful information about crystallite/particle size and lattice strain of material. Full width at half maximum (FWHM) of peaks, radiation used, and diffraction angle are used to estimate crystallite size and lattice strain from well-known Scherrer and related formula [31]. In addition to the particle size determination, peak broadening also plays a significant role for grain refinement of large strained powder. Though particle and particle size of crystal/material is broadly taken as same, they are somehow different because of formation of polycrystalline aggregates in powder sample [34]. There are various reasons of peak broadening including instrumental, crystal imperfection, strain, and structural distortion. The broadening (ȕhkl) corresponding to a reflection plane (h k l) only for BCTF was estimated using the relation: ȕhkl = [ (ȕhkl)2 Measured - (ȕhkl)2

1/2 Instrumental]

(ii)

Using corrected peak broadening of reflection plane ȕhkl, crystallite size (Dhkl) is calculated using in the following Debye-Scherrer’s [31] equation

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Dhkl = K.λ/ ȕhkl Cosθ

(iii)

Where K= factor related to crystallite (best possible value of the factor=0.89), ȕhkl = full width at half maximum (FWHM) of intensity, detector angle 2ș, Ȝ = wavelength of radiation used (=0.15405 nm) and ș= Bragg’s diffraction angle. The calculation of Dhkl was carried out on multiple reflections of the sample widely located on 2ș scale. The average crystallite size of BCTF is found to be 30 nm. The following formula was used to calculate strain induced (s) due to structural imperfection and distortion, s = ȕhkl/ 4 tanș

(iv)

As contributions of crystallite or particle size and strain to line broadening are independent of each other, they can follow Cauchy-like relation. Therefore, observed breadth or broadening is simply a sum of the above two equations (iii) and (iv), ȕhkl = K.λ/ Dhkl Cosθ + 4 s tanș Ÿ ȕhkl Cos θ = K.λ/ Dhkl + 4 s sinș

(v)

Above expressions are known as Williamson-Hall (W-H) equations. Fig 1(b) is a plot of 4sinș (x-axis) and ȕhkl cosș (y-axis) for BCTF. The value of strain (s) and the crystalline size (Dhkl) is obtained using the slope and intercept on y-axis (i.e., y=0.001x + 0.002; R2 = 0.2960). A positive value of slope of W–H plot suggests the presence of tensile strain in BCTF [35-36]. 3.3 Micro-structural study Fig.2 displays FE-SEM (field-emission scanning electron microscope) micrograph of BCTF pellet recorded at room temperature. The micrograph image shows that grains of different shape and size are largely distributed homogeneously with agglomeration at some sites. Similar situation occurs in the micrograph of pure BFO [37] where grains of larger size (> 50 μm) have been fused together. A profession ImageJ software was used to calculate grain size of micrograph. Pure BFO has relatively large grains whereas multi-components (Cd/Ti) modified BFO (i.e., BCTF) microstructure has highly packed (high-density) smaller grains. Average grain size for BCTF is found to be 2-5 μm.

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3.4 Dielectric study Dielectric property is one of the most important properties of ceramic materials. Study of this property of insulator (dielectric) provides vital information about nature and quantity of polarizations, relaxation process, defects, origin of dielectric loss, etc. The experimental data were obtained by using an experimental set up consisting of a phase sensitive multimeter, computer, small furnace, sample holder with K-type thermocouple. The relative dielectric constant (İr) of the material was calculated using the following simple relation [38] at a particular frequency and temperature.

İr = Cp t / İo

(vi)

Where Cp = capacitance in parallel mode (obtained experimentally), t and A are thickness and area of the specimen pellet sample respectively, and İ0 = permittivity of free space. 3.4.1. Frequency dependent dielectric properties Fig. 3(a-b) illustrates the frequency and temperature dependence of relative dielectric constant (İr) and tangent loss (tan į) showing strong dielectric dispersion. The systematic and smooth variation in these dielectric (insulating) parameters is observed on change in temperature and frequencies. As both these parameters (İr and tan į) decrease on increasing frequency and temperature, studied material follows a general features of normal dielectric and ferroelectric materials [39]. Such type of variation in dielectric spectrum may be attributed to the different nature of polarization (i.e., dipolar, ionic, atomic, space charge, electronic). According to the mechanisms of different polarization, a short-range displacement can occur which leads to get total polarization of the material [40]. Based on a general theory of Koop’s [41, 42], the dielectric spectrum curve can easily be explained. It is suitable for the determination of dielectric property of inhomogeneous double structure ferrite materials. At room temperature, relative permittivity (İr) of the material was found to be 370 (1 kHz) and 293 (1000 kHz). This dielectric parameter is found to be 3182 (1 kHz) and 353 (103 kHz) at 250 oC whereas at 500 oC this parameter becomes 4311 (1kHz) and 389.8 (1000 kHz). However, the dielectric slope and peak located between 100 kHz to 1000 kHz were found independent of temperatures. Above dielectric parameters of materials are strongly affected by the presence of voids, grain boundaries and defects in the sample [43]. Room temperature value of tangent loss of the material at 1 kHz and 1 MHz is found to be very small 0.133 and 0.011 respectively. However, at higher temperatures, value of tan į increases significantly at these frequencies (1.99 and 0.143 respectively) at 250. The value further

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increases on increasing temperature [i.e., 3.21 (1 kHz) and 3.67 (1 MHz)] at 500 oC . However, it is to be mentioned that phase transition is not observed in the said (experimental) temperature range (30–500 0C) in BCTF. It is also observed that the value of tan į increases exponentially in the low-frequency range (1-100 kHz) above 350 0C, but slightly increases for higher frequencies. Due to the presence of Fe in the material, value of tan į is found to be larger in the low-frequency region which decreases rapidly with rise in frequency at higher temperatures. 3.4.2. Temperature dependent dielectric property Fig. 4 (a-b) illustrates the change of İr with rise in temperature (30-500 oC) at 10, 25, 50, 100, 250, 500 kHz and 1000 kHz) for BCTF. Analysis of this dielectric dispersion curve (İr vs. temperature at different frequency) reveals that İr increases gradually on increasing temperature and culminates at 235 and 325 0C (usually referred as transition temperature) in the low-frequency range (10 to 50 kHz). However, a small increase in İr with increase in temperature above 50 kHz was observed. Generally, the increasing tendency of the dielectric constant for BCTF on increasing temperature has been explained with electron–phonon interaction [44, 45]. From Fig. 4(a) we get the value of İr =328 (10 kHz), 294 (1000 kHz) at room temperature whereas this dielectric parameter has been enhanced to 1779 (10 kHz) and 390 (1000 kHz) at 500 oC. The value of relative dielectric constant of parent compound BFO (1200 at 1kHz and 300 oC) has been enhanced to 2900 at the same frequency and temperature [44]. Fig. 4(b) exhibits the nature of temperature dependence of tangent loss (tan į) of BCTF at different frequency. It is observed that increasing rate of tan į at low temperatures (30-365 0

C) is small, but the value of loss factor significantly increases exponentially above 365 oC.

This nature of variation in tan į may be due to the scattering of temperature induced charge carriers and defects. Because of these defects, increase in electrical conductivity is expected and large increase in tan į. However, the value of tan į, as calculated from Fig. 4(b) and 4(c)] at 104 Hz and 106 Hz at 30 oC and 500 0C, is found to be 0.07, 76, 0.01 and 3.66 respectively These results are very much consistent with the reported ones [25]. As compared to BFO, the value of tangent loss has been reduced from 500 (1 kHz and temperature 300 oC) to 10 at the same experimental conditions (frequency and temperature) [44]. It is to be mentioned that reduction of dissipation factor (tangent loss) or leakage current as much as possible of multiferroics is very much required for devices.

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Furthermore, results showing the effects of co-substitution of (Ni, Co, Cd/Ti) on dielectric parameters has been summarized in Table- III. 3.5 Electrical impedance spectroscopy Impedance spectroscopy technique offers a unique and effective method to study the various electrical parameters and effects (grain, grain boundary and electrode effect etc.) of ceramicbased materials with suitable simulated circuits. Fig.5 (a- b) depicts the effect of temperature and frequency of electric field on Z'(real) and imaginary Z'' (imaginary) components of complex impedance of Bi(Cd1/4Ti1/4Fe1/2)O3 ceramics. It is clear from Fig. 5(a) that the value of Z' is found to be in decreasing order on increasing frequency. This result is in a good agreement with our previous reported impedance spectrum [26]. In the high-frequency region (102 Hz to 10 6Hz), all the curves of impedance spectra merge. For this type of impedance spectra, release of space charges may be responsible [39, 46]. Reduction in barrier contributions could be one of the factors responsible for increasing conductivity (ac) in the high frequency and temperature range [46]. On decreasing frequency, the magnitude of Z' decreases on increasing temperature. This nature confirms semiconducting behaviour of material. In addition, frequency dependence of Z'' at different temperatures is also shown here in Fig. 5(b) with inset (room temperature effect). The decreasing trend in the value of Z'' with increase in frequency is observed here. Detailed analysis of spectrum provides information on strength and type of dielectric relaxation process at a particular frequency [47-48]. It is also assumed that low-temperature immobile species and high-temperature defects could also be responsible for relaxation process in the material. Using peak position of Z'' (on x-axis) in the relation τ = 1 / 2πfr, (where symbols denote their usual meaning), fr (relaxation frequency) and τ (relaxation time) of the material were calculated. A correlation between Z' and Z'' part of complex impedance is normally established through simulated ideal electrical circuit models to get better understanding of structure-properties relationship of ceramic materials [49]. Fig. 5(c) represents a set of Nyquist plot (Z' versus Z'') spectrum drawn at different temperature in the range of 250-500 0C. For temperature 300 to 400 0C, the plots can be ¿tted with Nyquist semicircles. Fig. 5(d) represents the simulated (equivalent) circuit diagram for BCTF. The circuit could have combination of Rb (bulk resistance), Cb (bulk capacitance) and a CPE (correspondence constant phase element) in parallel [50] connected in series and/or (ii) combination of a grain boundary resistance (Rgb) and grain

boundary

capacitance (Cgb), depending on experimental

9

temperature and

frequency range and nature of material. We have plotted spectra of the materials at different temperature. The temperature dependent resistive and capacitive parameters were calculated and presented in Table-IV. Analysis of Nyquist plots (using experimental) and theoretical (fitted) have been performed with the ZSimpWin software [51]. Nyquist spectra clearly show the separated semicircle in the temperature range of 250-400 0C. The semicircle does not tend towards the zero at higher temperatures (450 to 500 0C) in the complex plane, which can be viewed in the inset of Fig. 5(c). As centre of high-temperature semi-circles does not fall on Z' axis, and become depressed asymmetric, ideal Debye-relaxation in the material is not expected [52]. The value of real part of impedance of BFO at 300 oC reduces from 575x101 Ohm to 50x103 Ohm at the same temperature and frequency. Similarly, the imaginary component of impedance (Zʿmax) changes from -100 kOhm to -20 kOhm at the same temperature and frequency [44]. These results also suggests the improvement in the impedance parameters of BFO on addition of Cd/Ti at the Fe-site of the parent compounds. 3.6 Study of Electrical modulus Complex electric modulus (M*) first time introduced by Macedo at al. [53-54] is widely used to study dielectric relaxation and electrical conductivity of ceramics and ionic conductors [55]. A complex electric modulus (CEM) spectrum represents energy distribution of ions in structure of materials. Temperature-frequency dependent microscopic properties and electrical relaxation of materials can be obtained by this technique [56]. In order to analyse the experimental data, following mathematical relations of real M' and imaginary (M'') components of M* are used; ª (ωRC ) 2 º M'= Y« 2 » ¬1 + (ωRC ) ¼

(vii)

and M ' ' = Y ª«

ω RC º 2 » ¬1 + (ωRC ) ¼

(viii)

where M*= 1 / ε*, M*= M' + j M'' = j ωε0 Z* ; M' = ω C0 Z'' ; M'' = ω C0 Z' ; ε* =complex dielectric permittivity; ω = 2πƒ; f = frequency and the term Y= C0/C, C = measured capacitance. Fig. 6(a-b) exhibits frequency dependence of M' and M'' of BCTF at some temperatures. Fig.6 (a) shows that value of M' first become constant at higher frequencies, then saturated and finally reached

to

a

maximum value (Mmax) at a temperature whereas at lower

frequency it becomes a negligibly small contribution of suggesting no or small effect

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electrode and /or ionic polarization in the said temperature range. The mobility of the charge species is controlled by electric field in the absence of restoring force [57]. The short- range mobility of charge carriers is mainly responsible of conduction process since the frequency induced sigmoid increase of M' is observed in the material [58]. Fig.6 (b) shows that peaks of imaginary component of modulus spectrum (Mmax) is shifted slowly to higher temperature side (in 250-350 range) whereas above 400 0C, Mmax peaks shift faster. Location of peak is indication that a transition takes place between short ranges to long-range mobility on changing frequency. Imaginary modulus peaks are observed in the range of frequencies where ions move a long distance. At high frequencies, the ions are confined to their potential wells and move locally [59, 60]. Moreover, broadened peak of M'' suggests distribution of relaxation time with separate time constant. In addition, Fig.6(c) depicts Nyquist-like of plot of electrical modulus (i.e., M' versus M'') for BCTF ceramic at varying temperatures. The asymmetric M'' peaks confirm the inhomogeneous and polycrystalline behaviour of the material which can probe bulk and grain boundary effects. As observed in impedance plot, suppressed semicircle of modulus plots refers to non- Debye–type behaviour with multiple relaxation times [61]. 3.7 Electrical conductivity 3.7.1 Frequency dependence of conductivity: Fig. 7 (a) exhibits frequency dependence of ac conductivity of BCTF in 25-500 0C range. Using dielectric data, ac conductivity was calculated with an empirical equation: ıac (ω) = Ȧ İ0 İr (tan į) where symbols have their usual meaning [62-64]. The frequency dependent conductivity can also be expressed as [63]: Vୟୡ ሺZሻ ൌ  VD ൅ ZE (ix) As σα does not depend on frequency, it gives dc conductivity. β is degree of interaction between crystal lattices and mobile ions and P is polarizibility. Exponent β lies between zero to one. Conductivity spectrum (conductivity vs. frequency) depicts continuous increase in conductivity with increase in frequency. Based on nature of frequency-temperature dependent conductivity, the following conclusion can be drawn; (i) from room temperature to 400 0C, there is a continuous increase of conductivity with frequency dispersion related toVୟୡ ሺZሻDZE , (ii) above 400 0C, there is frequency independent slope. A change of slope of conductivity plot suggests existence of different conduction mechanism in materials .At the conduction anomaly (i.e., change of slope at particular frequency), polaron hopping of

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charges specie takes place [65, 66]. Using hopping model, different characteristics region is separated. Transition from long range hoping to short-range ionic motion with conductivity relaxation phenomenon takes place at high frequencies [67]. The nature of frequencytemperature dependence of conductivity supports the conclusion drawn earlier on semiconducting behaviour of the material. 3.7.2 Effect of temperature on ac-conductivity: Fig. 7(b) presents temperature dependent ac conductivity spectrum of Arrhenius-type for BCTF sample. For better explanation and understanding, spectrum is grouped into three region; region I, region II and region III. In region I (high temperature), a sharp increase in ıac on increasing temperature is observed. The perfect linear nature of plot suggests proper Arrhenius type and frequency independent region. Region-II shows partially frequency dependent region where oxygen vacancies or defects play important role in the conduction process because of their long-range motion [68]. At low temperatures (i.e., region III), the value of conductivity (ıac) is almost constant at all frequency. Strong frequency dependent dispersion curves of this region may be attributed to hopping of charge carriers to random site of variable barrier heights. The value of conductivity increases with increase in frequency. Therefore, conductivity (ıac) at low frequency and temperature is invariant. The average value of activation energy for region I, II and III, calculated from Fig. 7(b), is 0.83, 0.32 and 0.21 eV respectively. However, BCTF compound follows Arrhenius behaviour; ıac = ıo exp (íEa/kT), where the symbols have its usual meanings. 3.8 Magnetic behaviour Fig.8 shows magnetic hysteresis (M-H) loop of Bi(Cd1/4Ti1/4Fe1/2)O3 ceramic measured under applied magnetic field of about ±15kOe at room temperature. The nature of the loop clearly shows the ferromagnetic characteristics of the BCTF compound. Actually, bismuth ferrite has anti-ferromagnetic behaviour and G-type magnetic structure [69], where ferromagnetic coupling of the magnetic moment of Fe3+ cations in pseudo-cubic planes, and antiferromagnetic coupling between adjacent planes occurs. The observed value (calculated from Fig. 8) of remnant magnetisation (Mr), saturation magnetisation (Ms) and coercivity (Hc) are 0.003 emu.g-1, 2.66 emu.g-1, and 653.75 Oe respectively. The remnant magnetisation obtained in this study is much higher than that of some well-known multiferroic material reported earlier [68, 70-72]. Even with visual inspection of room temperature M-H loop of the studied compound [Fig.8] and BFO [44] shows enhanced magnetic property of Cd/Ti

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modified bismuth ferrite as compared to that of pure bismuth ferrite compound. The maximum magnetic moment of BFO at 12Oe and room temperature changes from 0.08 emu.g-1 to 3 emu.g-1 [44]. It is a significant improvement of modified BFO and may be useful for application. Thus, the co-substitution of Cd and Ti at Fe-site enhances value of magnetic parameters with respect to Ni and Ti co-substituted bismuth ferrite. The tabulated forms of results are summarized in Table-V. The enhancement of magnetic parameters on Cd and Ti co-substitution on BFO will be useful for magnetic sensors.

Conclusions Multi-elements (Cd and Ti) modified Bi(Cd1/4Ti1/4Fe1/2)O3 (BCTF) was synthesised by using a mixed oxide route at high temperatures. The orthorhombic symmetry of BCTF is determined by preliminary structural analysis of x-ray diffraction data. The scattered crystallite size and lattice strain of the sample were estimated using Scherrer equation and Williamson-Hall method. The calculated (average) crystallite size was found to be 30 nm. Study of FE-SEM micrograph shows the inhomogeneous distribution of different size of grains on the surface of sample. Porosity and density of the prepared pellet sample were found to be between 7% and 93 % respectively (of the theoretical value).Detailed analysis of iimpedance and modulus spectra reveals the non-Debye type relaxation behaviour of BCTF. Bulk resistance of the sample decreases on substitution of Cd/Ti in BiFeO3.Impedance spectroscopic studies also reveal the existence of a negative temperature coefficient of resistance (NTCR) in the materials. An important role of grain (bulk) and grain boundary effects on electrical and structural properties has also been realised to understand structureproperties relationship of the material. Modulus spectrum analysis suggests that the material has non-exponential type of conductivity relaxation. Improvement in some dielectric, impedance and magnetic of bismuth ferrite has been observed on addition of Cd/Ti at its Fe site. The value of coercivity, saturation magnetisation and remnant magnetisation of BCTF is found useful for some applications

Acknowledgement: Kind and valuable help of Magnetic Material Laboratory of Department of Physics, IIT Guwahati are gratefully acknowledged for carry out magnetic measurements of the samples.

13

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19

SCHEME CAPTION

Scheme 1-Schematic diagram for the synthesis of BCTF

20

TABLE CAPTIONS Table I: Comparison of observed and calculated d-values in (Å) of XRD peaks at room temperature with observed relative intensity (I/I0)

Bi(Cd1/4Ti1/4Fe1/2)O3 — BCTF (orthorhombic phase) Peak 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

d-spacing (obs.) 4.1229 3.9037 3.5756 3.1996 3.1099 2.9227 2.7608 2.7040 2.3828 2.2575 2.1572 2.0651 1.9844 1.9521 1.8468 1.7348 1.8680 1.6396 1.5928 1.4938 1.3806

d-spacing (cal.) 4.1172 3.9085 3.5793 3.1989 3.1174 2.9212 2.7537 2.7047 2.3837 2.2570 2.1585 2.0649 1.9834 1.9519 1.8464 1.7349 1.8674 1.6401 1.5928 1.4940 1.3807

I/Io 6 36 16 59 12 17 100 39 7 19 9 6 9 19 6 20 8 12 22 9 9

h 0 0 4 3 0 0 4 2 3 6 4 4 3 3 4 5 3 0 6 6 2

k 1 1 0 1 0 1 1 1 1 0 1 0 1 0 2 1 1 2 0 1 3

l 2 3 0 0 9 7 1 7 0 4 8 11 11 13 0 11 14 11 13 13 4

Table II: Structural symmetry for (Ni, Co, Cd with Ti) co-substituted bismuth ferrite: S. No. 1. 2. 3. 4. 5.

Compounds Name

XRD Structure

Bi(Ni1/4Ti1/4Fe1/2)O3 Bi(Ni0.45Ti0.45Fe0.10)O3 Bi(Ni0.40Ti0.40Fe0.20)O3 Bi(Co1/4Ti1/4Fe1/2)O3 Bi(Cd1/4Ti1/4Fe1/2)O3

Orthorhombic symmetry Tetragonal symmetry Orthorhombic symmetry Orthorhombic symmetry Orthorhombic symmetry

21

Average crystallite size 28 nm 30 nm 36 nm 30 nm 30 nm

Reference

[25] [27, 28] [29] [26] Present paper

Table III: Electrical characteristic summary (at 250 0C) for ilmenites co-substituted bismuth ferrite (Temperature dependent) S. No. Compounds Name

1. 2. 3. 4. 5.

Bi(Ni1/4Ti1/4Fe1/2)O3 Bi(Ni0.45Ti0.45Fe0.10)O3 Bi(Ni0.40Ti0.40Fe0.20)O3 Bi(Co1/4Ti1/4Fe1/2)O3 Bi(Cd1/4Ti1/4Fe1/2)O3

Dielectric Constant (İr) 1 kHz 1 MHz 451.23 278.87 247.96 234.76 327.39 256.95 617.47 304.22 3182.09 352.86

Dielectric Loss (tan į) 1 kHz 1 MHz 0.392 0.0744 0.0367 0.0108 0.228 0.022 0.559 0.0797 1.990 0.1421

Referen ce [25] [28] [29] [26] Present paper

Table IV. Comparison of impedance fitting parameters for Bi(Cd1/4Ti1/4Fe1/2)O3at different temperature for (RQC) (RC) circuit

Temperature Cb (0C)

Rb

Cgb

CPE

Rgb

n

250

1.124E-9

2490

1.473E-10

6.572E-7

2.394E6

0.3578

300

6.265E-9

9831

1.311E-10

5.108E-7

7.701E4

0.3735

350

8.533E-10

859.1

1.233E-10

3.171E-8

3.060E4

0.6116

400

4.937E-10

114.6

1.695E-10

1.261E-7

2.326E4

0.4787

Table V: Comparison between magnetic parameters for co-substituted bismuth ferrite (at 298 K) S. No.

Magnetic parameters Compounds Name

1. 2. 3.

Bi(Ni1/4Ti1/4Fe1/2)O3 Bi(Ni0.40Ti0.40Fe0.20)O3 Bi(Co1/4Ti1/4Fe1/2)O3

Ms (emu.g-1) 0.131 0.363 2.66

22

Hc (Oe) 298.98 389 653.75

Reference

[25] [29] [26]

FIGURE CAPTIONS

Fig. 1 (a): Room temperature x-ray diffraction pattern of Bi(Cd1/4Ti1/4Fe1/2)O3 Fig. 1 (b): Williamson-Hall plot for the sample Bi(Cd1/4Ti1/4Fe1/2)O3 Fig.2: Room temperature FE-SEM image of Bi(Cd1/4Ti1/4Fe1/2)O3 Fig.

3

(a):

Frequency–temperature

dependence

of

relative

permittivity

of

Bi(Cd1/4Ti1/4Fe1/2)O3. Inset shows room temperature behaviour Fig. 3 (b):

Frequency variation of dielectric loss of Bi(Cd1/4Ti1/4Fe1/2)O3 at different

temperatures. Inset shows room temperature behaviour Fig. 4 (a): Variation of dielectric constant of Bi(Cd1/4Ti1/4Fe1/2)O3 with temperature at different frequencies Fig. 4 (b): Temperature dependence of dielectric loss of Bi(Cd1/4Ti1/4Fe1/2)O3 at selected frequencies (c) inset shows enlarge scale for tan į at room temperature to 350 0C for selected frequencies. Fig.5: (a) Variation of frequency and temperature dependent of Z' for Bi(Cd1/4Ti1/4Fe1/2)O3 Fig.5: (b) Variation of frequency and temperature dependent of Z'' for Bi(Cd1/4Ti1/4Fe1/2)O3 Fig.5: (c) Nyquist plot for Bi(Cd1/4Ti1/4Fe1/2)O3 at selected temperatures Fig.5: (d) Equivalent simulated circuit for Nyquist plots Fig. 6: (a) Frequency variation of M' and (b) M'' at various temperatures. Inset shows room temperature plot. Fig.6 (c): Nyquist plots of complex electric modulus for Bi(Cd1/4Ti1/4Fe1/2)O3 at selected temperatures (250 to 500 °C) Fig. 7(a): Frequency-temperature dependent ac conductivity of Bi(Cd1/4Ti1/4Fe1/2)O3 Fig. 7(b): Temperature-frequency dependent (Arrhenius plot) of ac-conductivity for Bi(Cd1/4Ti1/4Fe1/2)O3 Fig. 8: Magnetic hysteresis loop of Bi(Cd1/4Ti1/4Fe1/2)O3 at room temperature

23

411

Bi(Cd1/4Ti1/4Fe1/2) O3

20

30

40

50

234

6 1, 13

5 1, 11 3 1, 14 0 2, 11 6 0, 13

420

012

400

009 017

2 0, 11 604 418 4 0, 11 3 1, 11 3 0, 13

217

013

310

Intensity (A.U.)

CuKα

60

Bragg's Angle (2θ)

70

0.6

1.0

Ι

Ι

Ι

Ι

0 2, 11

420

3 0, 13

Ι

3 1, 11

2 0, 11

217

604 418 4 0, 11

017 310

0.8

Ι

Ι

009

400

Ι

Ι Ι

ΙΙ

411

013

Ι 012

βhklCos θ

Ι

0.004

0.003

Ι Ι Ι Ι

3 1, 14

Ι

0.006

0.005

Ι

6 1, 13

Experimental points Fit to Linear

Ι

5 1, 11

0.007

6 0, 13

Fig. 1 (a): Room temperature x-ray diffraction pattern of Bi(Cd1/4Ti1/4Fe1/2)O3

1.2

1.4

1.6

1.8

2.0

2.2

4 Sinθ Fig. 1 (b): Williamson-Hall plot for the sample Bi(Cd1/4Ti1/4Fe1/2)O3

24

Fig.2: Room temperature FE-SEM image of Bi(Cd1/4Ti1/4Fe1/2)O3

5000

(a)

4000

400

0

Room Temperature

350

300

3000 10

100

1000

εr

1

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

2000

1000

0

1

10

100

Frequency (kHz)

1000

Fig. 3 (a): Frequency –temperature dependence of relative permittivity of Bi(Cd1/4Ti1/4Fe1/2)O3. Inset shows room temperature behaviour

25

350

(b)

0

0.14

300

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

Room Temperature

0.12

Dielectric loss (tan δ)

0.10 0.08

250

0.06 0.04

200

0.02 0.00

150

1

10

100

1000

100 50 0 1

10

100

1000

Frequency (kHz) 3 (b): Frequency variation of dielectric loss of Bi(Cd1/4Ti1/4Fe1/2)O3 at different temperatures. Inset shows room temperature behaviour

10 kHz 25 kHz 50 kHz 100 kHz 250 kHz 500 kHz 1 MHz

Dielectric Constant (εr)

1800 1500 1200

(a)

900 600 300 50

100 150 200 250 300 350 400 450 500 0

Temperature ( C) Fig. 4 (a): Variation of dielectric constant of Bi(Cd1/4Ti1/4Fe1/2)O3 with temperature at different frequencies

26

80

10 kHz 25 kHz 50 kHz 100 kHz 250 kHz 500 kHz 1 MHz

60 1.8 1.6

40

20

Dielectric loss (tan δ)

Dielectric loss (tan δ)

(b)

1.4 1.2 1.0 0.8

(C)

Frequency 10 kHz 25 kHz 50 kHz 100 kHz 250 kHz 500 kHz 1 MHz

0.6 0.4 0.2 0.0 50

100

150

200

250

300

350

0

Temperature ( C)

0 100

200

300

0

400

500

Temperature ( C) 4 (b):Temperature dependence of dielectric loss of Bi(Cd1/4Ti1/4Fe1/2)O3 at selected frequencies (c) inset shows enlarge scale for tan į at room temperature to 350 0C for selected frequencies.

(a)

0

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

40

Z'(kΩ)

150

30

Room Temperature

125

Z'(k Ω )

50

100 75 50 25 0 1

10

100

Frequency (kHz)

1000

20 10 0 1

10

100

1000

Frequency (kHz) Fig.5: (a) Variation of frequency and temperature dependent of Z' for Bi(Cd1/4Ti1/4Fe1/2)O3

27

-1200

(b)

Room Temperature

-1000

Z''(kΩ)

-25 -20

-800 -600 -400 -200 0 1

Z''(kΩ)

-15

10

100

Frequency (kHz)

1000

0

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

-10 -5 0 1

10

100

1000

Frequency (kHz)

Fig.5: (b) Variation of frequency and temperature dependent of Z'' for Bi(Cd1/4Ti1/4Fe1/2)O3

28

50

(c)

600 500

o

250 C o 300 C o 350 C o 400 C o 450 C o 500 C Fitting

- Z''(kΩ)

30 20

400

- Z''(Ω)

40

0

450 C 0 500 C

300 200 100 0 0

100

200

300

400

500

600

Z'(Ω)

10 0 0

10

20

30

40

50

Z'(kΩ) Fig.5: (c) Nyquist plot for Bi(Cd1/4Ti1/4Fe1/2)O3 at selected temperatures

Fig.5: (d) Equivalent simulated circuit for Nyquist plots

29

0.0030 0.0025

0.0032

M'

0.0020

(a)

Room Temperature

0.0034

0.0030 0.0028 0.0026

M'

1

0.0015

10 100 Frequency (kHz)

0

1000

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

0.0010 0.0005 0.0000 1

10

100

1000

Frequency (kHz)

0.0014

0.00040

Room Temperature

0.00035

0.0010

0.00020 0.00015 0.00010 0.00005 0.00000

0.0008

M''

(b)

0.00025 M''

0.0012

0.00030

1

0

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

0.0006 0.0004 0.0002

10 100 Frequency (kHz)

1000

0.0000 1

10

100

1000

Frequency (kHz)

Fig. 6: (a) Frequency variation of M' and (b) M'' at various temperatures. Inset shows room temperature plot.

30

0.0030

(c)

0

250 C 0 300 C 0 350 C 0 400 C 0 450 C 0 500 C

0.0025

M''

0.0020 0.0015 0.0010 0.0005 0.0000 0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

M'

Fig.6 (c): Nyquist plots of complex electric modulus for Bi(Cd1/4Ti1/4Fe1/2)O3 at selected temperatures (250 to 500 °C)

31

1E-4

−1

-1

σac (Ω m )

1E-3

1E-5

Room Temperature 0 0 250 C 300 C 0 0 350 C 400 C 0 0 450 C 500 C

1E-6

1E-7 1

10

100

1000

Frequency (kHz) Fig. 7(a): Frequency-temperature dependent ac conductivity of Bi(Cd1/4Ti1/4Fe1/2)O3

0.01

Frequency 10 kHz 25 kHz 50 kHz 100 kHz 250 kHz 500 kHz 1 MHz

I E =0.83 eV a

1E-4

III E =0.21 eV

−1

-1

σac (Ω m )

1E-3

a

1E-5

Ea=0.32 eV

II

1E-6 1.5

2.0

2.5

3.0 -1

Temperature (K )

32

3.5

Fig. 7(b): Temperature-frequency dependent (Arrhenius plot) of ac conductivity for Bi(Cd1/4Ti1/4Fe1/2)O3

Magnetic Moment (emu/g)

3

Bi(Cd1/4Ti1/4Fe1/2)O3

2 1 0 -1 -2 -3

-12

-8

-4

0

4

3

8

12

Magnetic Field H/(10 Oe) Fig. 8: Magnetic hysteresis loop of Bi(Cd1/4Ti1/4Fe1/2)O3 at room temperature

33