Effect of Multiple Substitutions ( Pb , Ti , Zr ) on Structural , Permittivity and Electrical Properties of BiFeO

Three complex polycrystalline samples of Bi1-xPbxFe1-x Zrx-yTiyO3,with the combination of (i) x, y = 0.0 (BFO), (ii) x = 0.5 and y = 0.5 (BFPTO)and (iii) x = 0.5 and y = 0.25 (BFPZTO) were prepared by a standard mixed oxide (solid state reaction route) method at optimized calcinations (900 o C) and sintering (930-950 o C)temperatures. The effect of multiple (Pb, Ti and/or Zr) substitutions of different amount onthe permittivity, impedance and modulus characteristics of the materials has been investigated at different frequencies and temperature.Limitation associated with BiFeO3 (BFO) due to its semiconducting behavior acts as constraint in polling the material at room temperature leading to large dielectric loss. Attempts were made to minimizethe loss by substitution of Pb on the Bi site and Ti and/or Zr at the Fe site of BFO. The study indicates that the phase transition temperature of BFO is lowered,the degree of diffuse phase transition is enhanced and electrical parameters (dielectric constant, electrical resistivity, remnant polarization and maximum polarization) areincreased with a significant reduction in dielectric loss as a consequence of the substitution. Detailed studies of Nyquist plots with impedance and electric modulus data suggest that the existence of non-Debye type of relaxation phenomena in the materials. The ac conductivity study reveals that the conduction mechanism in the material obeys universal Jonscher’s power law. [


INTRODUCTION
Though a large number of ferroelectric oxides are available, some perovskite oxides of a general formula,ABO3(a=mono-divalent and B=tri-hexavalent ions) such as BaTiO3, PbTiO3, Pb(ZrTi)O3 (PZT)etc. exhibit high dielectric constant(permittivity), spontaneous polarization, piezoelectric coefficient, pyroelectric current, transition temperature above room temperature, etcwhich enhance their potentiality for technological applications. These unique properties are resulted from their crystal structures that affectthe domain wall formation and orientation under external stimulation.Attempts have been made to develop new materials with unique and improved properties by substitutingsuitable element(s) at the A or/ B site (s) or combining different perovskites in suitable proportions [1][2][3][4].Among the perovskites, BiFeO3 (BFO) is considered as a unique material which goes to various structural phase transitions [5]. Literature survey indicates that bismuth ferrite has ferroelectric ordering due to its distorted perovskite structure below the critical temperature (Tc ≈ 850 o C) [6] and anti-ferromagnetic ordering with a cycloidal canted spin arrangement up to Neel temperature (Tc ≈ 370 o C -380 o C) [7]. Due to its semiconducting behavior,and high leakage current generated due to defects, it is difficult to pole the material by applying electric field at room temperature to get the proper ferroelectric hysteresis loop [8]. Other than the above processes,leakage problem can be reduced by working at low temperature. It has been reported that the conductivity of the material is less and the hysteresis loop of the material shows a saturation polarization of 3.5μC/cm 2 [9]. Both PbTiO3 and PZT are associated with lead I S S N 2 3 4 7 -3487 V o l u m e 1 3 N u m b e r 4 J o u r n a l o f A d v a n c e s i n P h y s i c s 4811 | P a g e A p r i l 2017 w w w . c i r w o r l d . c o m vacancies due to evaporation of Pb during the processing, which leads to the increase in acceptor levels in the crystal, therefore, the conductivity problem and dielectric loss of BFO can be reduced significantly by substituting different elements (Pb, Ti, Zr) of PZT into A/B-sites of BFO in suitable proportions [10][11][12][13].
In the present work, attempts have been made to solve the conductivity or leakage problem of bismuth ferrite by Pb substitution at its Bi site and Ti and/or Zr substitution at the Fe site. The effect of multiple substitution on the structural, dielectric and electrical properties of the material (BFO) at high temperatures in a wide frequency range (1kHz -1MHz) is reported here.

Synthesis of materials
These ingredients weremixed in stoichiometric proportion, and grinded to fine powder with the help of agate mortar and pestle for 2h in dry (air)and as well as wet atmosphere (methanol) for 2h to obtain a homogeneous mixture. The homogeneously mixed powders of each sample are calcinated separately at an optimized temperature of 900 o C for 5 h using pure alumina crucible.

Characterization
The formation of pure phase compounds and the basic crystal structure of the samples were analyzed by powder Xray diffraction (XRD) method using a power diffractometer (Rigaku Mini-flex, Japan). The CuKα radiation of wavelength (λ) = 1.5405Å was used to record the XRD patterns and data in a wide range of Bragg angle 2θ such that    Figure 1.All the peaks of the XRD patterns were indexed in tetragonal system using standard computer software POWDMULT [14] and the lattice parameters of the samples were obtained.

Structural Analyses
The difference between the observed (obs) and calculated value (cal) of inter planner distance d [∑(dobs -dcal) = minimum] was found to be minimum in tetragonal crystal system (with space group P4mm) for the Pb and Ti and/or Zr modified BFO whereas it was rhombohedral (R3c space group) for pure BFO [15][16][17][18]. The strongest diffraction peak was found at (110) plane for all the compounds. It is also found that in the Pb and Ti doped BiFeO3 compound, the diffraction peaks shifts towards the higher angle while Zr added compound the diffraction peaks shifts towards the lower angle. Due to the larger ionic radius of Pb 2+ (1.33Å) than that of Bi    It is clearly observed that the tolerance factor of BFPTO and BFPZTO compounds is larger than that of pure BFO and is more for Zr doped compound. This indicates that the structural stability of compounds increases due to substitution (i.e., distortion in the perovskite structure of pure BFO increases due to substitution [19]). As in all samples, t<1, the A-O bond length is smaller than the B-O bond length, hence end members of Bi1-xPbxFe1-x (Zrx-yTiy) O3 influence more the dielectric and electric properties of the compounds [20]. The scanning electron micrograph of Bi1-xPbxFe1-x(Zrx-yTiy)O3pellets are compared in Figure-2(a, b, c). It is observed that the grain size decreases with substitution. The grains are uniformly distributed, andare very closely packed, hence porosity of BFO is reduced (i.e., density increases due to doping). In all the compounds, the value of εr decreases with increase in frequency at the selected temperature, and almost become frequency independent at higher frequencies (which is clearly observed in Zr doped compound) in the observed frequency range, which reflects the dielectric nature of the materials [14]. However, at all the frequencies the dielectric constant of BFPZTO and BFPTO compounds is larger than that of pure BFO.This indicates that the polarization stability increases with Pb, Ti and/or Zr substitution. This may be due to decrease in the grain size when BFO is modified. Figure 4shows the variation of dielectric loss ofBi1-xPbxFe1-x(Zrx-yTiy)O3 with frequency. It is seen that loss decreases with increase in frequency. The dispersion is large at low frequency and gradually decreases with increase in frequency, thus the tanδ value become almost frequency independent at higher frequency.It is observed that the Zr and/or Ti substitution reduces the dielectric loss or leakage current of BFO effectively. This fact may be due to the decrease in the Fe ion concentration, which is expected to cause large leakage current in BFO. Hence, the conductivity of BFPZTO and BFPTO is less than that of BFO [15]. Figures5 (a-c) and 6 (a-c) exhibit the variation of dielectric constant (εr) and dielectric loss (tanδ) of Bi1-xPbxFe1-

Dielectric and Ferroelectric Properties:
x(Zrx-yTiy)O3 with temperature at selected frequencies.Usually, in ferroelectric materials, dielectric constant increases with increase in temperature up to a critical temperature, called transition temperature [16]. The anomaly in the temperature profile is associated with the ferroelectric-paraelectric transition. No dielectric anomaly is observed in BFO sample in the experimental temperature range but for BFPTO and BFPZTO samples dielectric anomaly was observed in temperature range of 400 o C to 450 o C. It indicates that the transition temperature decreases due to Pb, Zr/Ti substitution. The temperature corresponding to the dielectric anomaly shifts towards the lower side as Zr is added to BFPTO sample. This may be due to the lower ferroelectric transition temperature of PZT than that of lead titanate [17].The temperature rate of increase of dielectric constant is more at low frequency. The tanδ temperature graph is the complementary of εr temperature graph. For all the observed frequency range the loss factor shows a peak at the dielectric anomaly temperature. The peak value of loss factor decreases with increase in frequency. The loss factor in BFPTO compound is very small as compared to that of BFPZTO. But in both the modified BFO compounds, the dielectric loss is observed to be very small as compared to that of pure BFO. At higher temperature, the loss due to leakage current is a major constraint to the material for its device applications. In pellet samples, the high temperature loss is primarily due to domain wall motion, inter-grain spacing and other defects. The decrease in dielectric loss due to substitution of Pb, Zr and/or Ti may be attributed to the increase in density of crystal, which produces larger grain boundary resistance.

Impedance and modulus analysis:
Complex impedance spectroscopy is a flexible and non-destructive technology to analyze dielectric and electric characteristics of ceramic materials simultaneously over a wide frequency and temperature range [18].  behavior [20]. The transformation of arcs with temperature indicates single relaxation process in the material characterized by a distribution of relaxation times with a mean relaxation time [21]. The depressed semicircular impedance spectra at high temperature suggest that the relaxation in the material is non-Debye type [22]. The existence of a full, partial or no semicircular (observed at low temperature and not presented here) indicates the presence of multiple relaxation process coexists in the studied material, which consists of a number of energy barriers due to point defects appearing during the synthesis. Hence the non-Debye type relaxation in the studied materials may have resulted from dipole interaction, temperature and frequency dependent space charge and ionic polarization and grain boundary sliding.   [23][24][25]. The real and imaginary parts of electric modulus in terms of R and C (for a parallel combination) can be expressed as: Where M = (Co/C), τ = RC (relaxation time)

Figure 9(a-c): Complex modulus plots of Bi 1-x Pb x Fe 1-x (Zr x-y Ti y )O 3 compounds at selected high temperatures
The complex modulus plots of BFO and BFPZTO show two resolved semicircular arc at different temperatures. The first one represents the capacitive grain boundary effects in low temperature region and the second one corresponds to the capacitive effect of grains at elevated temperatures [26]. This suggests single relaxation phenomenon in these materials. But two semicircles are clearly seems to be merged in the complex modulus plot of BFPTO compound indicating existence of multiple relaxation in the material [27].

Ac conductivity analysis
The ac conductivity of the polycrystalline ceramic materials intrinsically depends on its composition, structure, bulk size, distribution of grains, capacitive and resistive effect of grains and/or grain-boundary [28]. The conduction may be due to long range hopping of the charge carriers and/or due to localized transportation accompanied with oxygen vacancies.The ac conductivity of the ceramic compounds under study was calculated by using following equation; = Where, permittivity of the material, dielectric loss of conductivity with increase in temperature for all frequencies shows that the temperature coefficient of resistivity of the materials is negative i.e., the material exhibit NTCR behavior [34]. At high temperature, the dispersion in conductivity decreases and all the curves tends to merge. It is also noticed from the plots that curves are merging with increase in the concentration of PZT in the compounds of BPFZTO, which indicates that the conductivity is dominated by charge carriers such as oxygen vacancies. The values of activation energy are found to increase with increase in temperature in all compounds which justify the fact that more energy is required to overcome the fluctuations occurring due to the thermal excitation of charge carriers.This is due to the fact that at low frequencies the overall conductivity is due to the hopping/mobility and transportation of charge carriers over a long distance while at higher frequencies the hopping is localized near the neighboring defect site because the charge carriers have smaller response time to external excitations [35]. Comparison of activation energy (eV) of Bi1-xPbxFe1-x (Zrx-yTiy)O3 compounds at selected frequencies in high temperature range are presented in Table-2.
This indicates that the ac activation energy decreases with increase in frequency in all the studied compounds.

CONCLUSIONS:
Three polycrystalline samples of Pb, Ti and/or Zr modified bismuth ferrite compounds [i.eBi1-xPbxFe1-x(Zrx-yTiy)O3, with for BFO compound but tetragonal crystal system (with space group P4mm) for the Pb and Ti and/or Zr modifiedBFO compounds. The dielectric constant (εr) is observed to be larger in the modified compounds than that in BFO, and also the dielectric loss decreases significantly in the doped materials. Hence the leakage current in the modified compounds is less than that in BFO. Thus the modified compounds have larger potentiality for device applications.
The SEM picture of the studied compounds indicates that the grain size decreases due to doping and the grains are distributed uniformly with low porosity. The P E hysteresis loop suggests that the ferroelectric behavior of BFO is enhanced due to the substitution of Pb, Ti and/or Zr on Bi-site and Fe-site of bismuth ferrite (BFO). The frequency dependence of ac conductivity obeys the Jonscher's universal power law for all compositions. Detailed study of electrical conductivity of the materials exhibits its NTCR behavior.