Structural and Electrical Characteristics of Ba ( Fe 0 . 5 Nb 0 . 5 ) O 3-SrTiO 3 Ceramic System

A complex structure of barium iron niobate, Ba(Fe0.5Nb0.5)O3 (BFN) and strontium titanate SrTiO3 (ST) was fabricated by a solid-state reaction method. The phase formation of Ba(Fe0.5Nb0.5)O3-SrTiO3 was checked using X-ray diffraction (XRD) technique. The X-ray structural analysis of BFN and BFN-ST ceramics, showed the formation of single-phase compound in the monoclinic system, which is a distorted structure of an ideal cubic perovskite. Careful examination of microstructures of the individual compounds of the system was done by the scanning electron micrograph (SEM), and confirms the polycrystalline nature of the systems. Detailed studies of dielectric and electrical impedance properties of the systems in a wide range of frequency (100 Hz 5 MHz) and different temperatures (30 ̊C 285 ̊C) showed that these properties are strongly dependent on temperature and frequency.


Introduction
Complex perovskite with general formula ABO 3 (A = mono-divalent, B = trid to hexavalent) are widely used for detectors, sensors, actuators, multi-layer ceramic capacitor (MLCC), computer memories, pyroelectric detectors, wireless communication systems, microelectronics, global positioning systems and other electronic devices.Ferroelectric perovskites have been the subject of extensive studies due to their promising electrical characteristic which has a potential usefulness in fundamental research and technological applications.Investigation of the electrical properties of these materials is desirable to predict their suitability for electronic applications.Various relaxation processes seem to coexist in complex perovskite ceramics, which contain a number of different energy barriers due to point defects appearing during their fabrication.Therefore, the departure of the response from an ideal Debye model in ceramic samples, resulting from the interaction between dipoles, cannot be disregarded.A method of predicting the relaxation behavior of a perovskite is through electric modulus theory [1].
The dielectric constant of a material (dielectric) changes due to the change of polarization by an applied electric field.This change in polarization is time dependent.Because of the resistance (resistivity) to the motion of the atoms in the dielectric, there is always a delay between changes in the field and changes in the polarization, which gives the dissipation factor tanδ and is proportional to the energy absorbed per cycle by the dielectric from the field [2].
Alternating current (AC) impedance spectroscopy is an appropriate method to study 1) the properties of the intragranular and interfacial regions and their interrelations, 2) their temperature and frequency dependent phenomena in order to separate the individual contributions from the total impedance and 3) their interfaces with electronically conducting electrodes [3][4][5][6].AC impedance spectroscopy allows measurement of the capacitance (C) and tangent loss (tanδ) over a frequency range at various temperatures.From the measured capacitance and tangent loss, four complex dielectric functions can be computed: impedance (Z * ), permittivity ( * ), electric modulus (M * ) and admittance (Y * ).Studies of electrical data in the different functions and forms allow different features of the materials to be recognized.
Materials having a diffuse phase transition have attracted the most attention due to their broad maximum in the temperature dependence of their dielectric constant.The high value of the dielectric constant over a very wide temperature interval is due to disorder in the distribution 0.5 0.5 3 3 of B-site ions in the perovskite unit cell.This may lead to composition fluctuations and, as a consequence, to different local Curie temperatures in the different regions of the ceramic [7].
Recently, high dielectric permittivity (') has been reported for ternary perovskite BaFe 0.5 Nb 0.5 O 3 (BFN) of certain compositions.Many researchers have studied BFN, including Saha and Sinha [8], Intatha et al. [9], Fang et al. [10], Raevski et al. [11] Nedelcu et al. [12], Tezuka et al. [13], Yokosuka et al. [14] and Rama et al. [15].They have reported that the BFN-based electroceramics exhibit a relaxor behavior by showing very attractive dielectric and electric properties over a wide range of temperatures.However, there still exist considerable debates concerning the physical mechanisms governing their electrical behavior [9].In the present work, Ba(Fe 0.5 Nb 0.5 )O 3 (BFN) and strontium titanate SrTiO 3 (ST) was fabricated by a solid-state reaction method.The dielectric characteristics of BFN(a), BFN-ST5(b) and BFN-ST10(c) ceramics are evaluated in broad temperature and frequency ranges.1200˚C for 8 h.The calcined powder of above mentioned ceramics were regrinded and used to make pellet of diameter ~10 mm and thickness 1 -2 mm using polyvinyl alcohol as binder.The pellets were sintered at 1250˚C for 5 h and then brought to room temperature under controlled cooling.The formation and quality of the compounds were checked with X-ray diffraction (XRD) technique.The frequency dependence of the capacitance and conductance is measured using an LCR meter in the temperature range from 30˚C to 285˚C and in the frequency range from 100 Hz to 5 MHz.The ac electrical conductivity 0 ac      were obtained from the temperature dependence of the real (ε′) and imaginary (ε″) components of the complex dielectric constant ε* = (ε′-jε″).The X-ray powder diffraction pattern of the sample is taken at room temperature using a X-ray powder diffractometer (Rigaku Miniflex, Japan) using Cuk α radiation (λ = 1.5418Å) in a wide range of Bragg angles 2θ (20˚ ≤ 2θ ≤ 80˚) with scanning rate 2˚/min.The mi-crographs are recorded using scanning electron microscopy JEOL-JSM-5800 to study the surface morphology/microstructure of the sintered pellets.

Results and Discussion
The room temperature X-ray diffraction (XRD) patterns of BFN(a), BFN-ST5(b) and BFN-ST10(c) ceramics is compared in Figure 1.The nature of XRD patterns appears to confirm the formation of single-phase with monoclinic crystal structure of the compounds.All the reflection peaks of the XRD pattern of the samples were indexed, and the lattice parameters were determined in the monoclinic crystal systems using a computer program "POWDMULT" [16].
On the basis of best agreement between the observed (obs.This behavior is very much consistent with that of common ferroelectrics.The higher values of ' at lower frequencies are due to the presence of all different types of polarizations (i.e., dipolar, atomic, ionic, electronic contribution) in the material.At high frequencies, however, some of the above-mentioned polarizations may have less contribution in '.This behavior is also found in other compounds studied by us [17][18][19][20][21][22].
The high value of ' in the low frequency region has been explained using Maxwell-Wagner (MW) polarization effect.Thus high values of permittivity are not usually intrinsic, but rather associated with heterogeneous conduction in the grain and grain boundary of the compounds.That is due to the grains of the sample are separated by more insulating intergrain barriers, as shown in a boundary layer capacitor [23].
The logarithmic angular frequency dependence of the loss tangent (tanδ) of above mentioned ceramics at 30˚C and 130˚C are plotted in inset of Figure 4.It is observed that (tanδ) max maximum increases as temperature increases from 30 to 130˚C, indicating that the concentration of conduction electrons increases as temperature due to thermal activation [24].
Temperature dependence of dielectric constant (′) and tangent loss (tanδ) of BFN(a), BFN-ST5(b) and BFN-ST10(c), ceramics is shown in Figure 5 and Figure 6 respectively at selected frequencies (1 and 20 kHz), respectively.The dielectric constant (′) is found to increase with rise in temperature at selected frequency for BFN(a) ceramics.One possibility for this behavior is that   it is due to the increased conductivity in the samples caused by the presence of Fe 2+ in sintered BFN (a) ceramics, as suggested by Ananta and Thomas [25].The concentration of Fe 2+ ions is known to be very sensitive to temperature, and it increases as temperature increases [26].It is known that the co-existence of Fe 2+ and Fe 3+ ions on equivalent crystallographic sites can give rise to an electron-hopping conduction mechanism.But for BFN-ST5(b) and BFN-ST10(c) ceramics, the dielectric constant increased with increasing temperature and become broad curve from 180˚C onward.In addition, the dielectric constant was found to decrease with increasing frequency.This behavior may be due to fact that the dipoles are able to follow the applied voltage at low frequency, but they not follow to the field at higher frequency.
From Figure 6, it is clear that in the low temperature region, tanδ is almost constant up to a certain temperature and then increases faster up to a highest temperature (~282.5˚C) in BFN(a), BFN-ST5(b) and BFN-ST10(c), ceramics.
The higher values of Z′ (BFN (a)), Z′ (BFN-ST5 (b)) and Z' (BFN-ST10 (c)) at low frequencies and low temperatures means the polarization is larger.The temperatures where this change occurs vary in the material with frequencies.This also means that the resistive grain boundaries become conductive at these temperatures.This also shows that the grain boundaries are not relaxing even at very high frequencies even at higher temperatures.
At lower temperatures Z″ (BFN (a)), Z″ (BFN-ST5 (b)) and Z″ (BFN-ST10 (c)) decreases monotonically suggesting that the relaxation is absent.This means that relaxation species are immobile defects and the orientation effects may be associated.As the temperature increases, the peak of Z″ (BFN-ST5 (b)) and Z″ (BFN-ST10 (c)) starts appearing.The peak shifts towards higher frequency with increasing temperature showing that the resistance of the bulk material is decreasing.Also the magnitude of Z″ (BFN-ST5 (b)) and Z″ (BFN-ST10 (c)) decreases with increasing frequencies.This would imply that relaxation is temperature dependent, and there is apparently not a single relaxation time.There by relaxa-Copyright © 2011 SciRes.MSA Figure 9 shows the ac conductivity of the BFN(a), BFN-ST5(b) and BFN-ST10(c) ceramics, as a function of frequency at different temperatures.The ac conductivity of the system depends on the dielectric properties and sample capacitance of the material.The two plateaus separated by frequency region are observed in BFN (a) ceramics.The low-frequency plateau represents the total conductivity whereas the high-frequency plateau represents the contribution of grains to the total conductivity.The presence of both the high and low frequency plateaus in conductivity spectra suggests that the two processes are contributing to the bulk conduction behavior.One of these processes relaxes in the higher frequency region and contribution of the other process appears as a plateau in the higher frequency region [27].The conductivity shows dispersion which shifts to higher frequency side with the increase of temperature.The strong frequency dependent conductivity has been found above (10 kHz) for BFN-ST5(b) and BFN-ST10(c) with the frequency independent conductivity below 10 kHz which is a typical feature of perovskite at elevated temperature (>180˚C).

Conclusions
In this work, we reported structural, dielectric and impedance properties of barium iron niobate, Ba (Fe 0.5 Nb 0.5 )O 3 (BFN) and strontium titanate SrTiO 3 (ST) prepared by a high-temperature solid-state reaction method.Preliminary structural analysis suggests that formation of single-phase compound in the monoclinic system.Detailed studies of electrical (dielectric, impedance and conductivity) properties exhibited a strong frequency dependent dielectric dispersion of the compound.The microstructure of the ceramics was examined by the scanning electron microscopy (SEM), and shows the polycrystalline nature of the samples with different grain sizes.

Figure 3 .
Figure 3. Frequency dependence of dielectric constant () of BFN(a), BFN-ST5(b) and BFN-ST10(c) ceramics, at 30˚C and 130˚C.BFN-ST10(c) at 100 Hz, and gradually decreases as frequency increases, But at 130˚C, the value of ' in the low frequency region (below 1 kHz), increases significantly for BFN(a) and BFN-ST5(b) ceramics.This behavior is very much consistent with that of