Comparative study of structure, dielectric and electrical behavior of Ba(Fe0.5Nb0.5)O3 ceramics and their solid solutions with BaTiO3

Dielectric properties of (1-x)Ba(Fe0.5Nb0.5)O3-xBaTiO3 (where x = 0.00, 0.05 and 0.10) solid solution ceramics at high temperature range of RT  270 o C have been characterized in this paper. The above said polycrystalline ceramics with (x = 0.0, 0.05 and 0.10) have been produced via a mixed oxide route. The effects of BaTiO3 substitution on the structure and on the electrical and ferroelectric properties of Ba(Fe0.5Nb0.5)O3 samples have been studied by performing x-ray diffraction and dielectric measurements. The dielectric properties ( and tan) were investigated in the temperature range of 30-270 °C and in the frequency range of 100 Hz-5 MHz. The variation of relative dielectric permittivity () and tangent loss (tan) has suggested a significant role of hopping of trapped charge carriers, which is resulted in an extra dielectric response in addition to the dipole response. It is observed that: (i) the relative dielectric permittivity() and tangent loss (tan) are dependent on frequency, (ii) the temperature of dielectric permittivity maximum shifts toward lower temperature side and (iii) dielectric permittivity and tangent loss rapidly increase by making solid solution of BFN with BaTiO3. X-ray diffraction analysis of the compound suggests the formation of single-phase compound with monoclinic structure. SEM photographs exhibit the uniform distribution of grains. The maximum ferroelectric transition temperature (Tc) of this system was 250-270 °C with the dielectric constant peak of 72500 at 1.09 kHz for x = 0.05. Copyright © 2011 VBRI press.


Introduction
Recently, perovskite like ceramics have been at the center of research to study the ferroelectric and piezoelectric materials because of the diversity of their physical properties. These compounds have found applications in several industrial fields, such as high density capacitors, memory devices, actuators and so on [1]. Many ferroelectrics of the perovskite family of a general formula ABO 3 (A = mono or divalent, B = triad to pentavalent ions) are very important because of their potential applications such as dynamical and volatile memory components, transducers, sensors and many other active and passive devices [2][3][4]. During the intervening years, a large amount a good understanding of the many inter-related properties and phenomena associated with these materials has been achieved. Most of dielectric materials with perovskite structures contain more or less lead, which pollutes the environment. So the, leadfree materials with high dielectric constants are becoming more attractive for research. Recently, high dielectric constant and dielectric relaxation in A(Fe 0.5 B 0.5 O 3 ), (A = Ba, Sr and B = Nb, Ta) [5][6][7][8][9][10][11], CaCuTi 4 O 12 [12], CdCr 2 S 4 [13] etc have been pursued to understand the relaxation mechanism, which describes the dielectric relaxation (i.e., charge redistribution, structural frustration or polaron redistribution, ferroelectric relaxor, and Maxwell-Wangner space charge (pseudo relaxor)). The high value of dielectric constant over a very wide range of temperature interval is due to disorder in the distribution of B-site ions in the perovskite unit cell. Various relaxation processes seem to coexist in real perovskite crystals or ceramics, which contain number of different energy barriers due to point defects appearing during technological process. Therefore, the departure of the response from the ideal Debye model in the solid-state samples, resulting from the interaction between the dipoles, cannot be disregarded [14]. BaTiO 3 -based dielectrics are more focused in research field, due to its not detrimental to the environment both in synthesis and its application. The compositions of the majority dielectric materials used for ceramic capacitors are based on barium titanate [15][16][17].
In view of the above, we have made an attempt to study Ba(Fe 0.5 Nb 0.5 )O 3 (BFN) and its solid solution with BaTiO 3 . In this paper we will discuss the structural, dielectric and electrical behavior of the synthesized ceramics.

Experimental
The compounds used in this investigation were prepared by a routine solid-state reaction technique. The starting materials were BaCO 3 (M/S Burgoyne Buirbridger, India), Fe 2 O 3 (M/S Indian rare earth Ltd.), Nb 2 O 5 (M/s. Loba Chemie Pvt. Ltd., India) and TiO 2 (M/S John Baker Inc., USA). The raw powders were based on the formula (1x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 .The doping concentration of was varied from 0.05 to 0.10 mole%. These materials were taken in a suitable stoichiometry ratio and mixed thoroughly with acetone in an agate-mortar for 4 h. The mixtures of ingredients of compounds were calcined in an alumina crucible at 1200°C for ~8 hr in air atmosphere. The calcined powders were mixed with the binder ( polyvinyl alcohol (PVA)) and compacted into cylindrical discs (pellets) of diameter ~10 mm and thickness 1-2 mm at the pressure of 5×10 7 N/m 2 using a hydraulic press. The pellets were sintered in air at 1250°C for 5 hr in high purity alumina crucibles. The formation and quality of the compounds were checked with X-ray diffraction (XRD) technique. The XRD patterns of the compounds were recorded at room temperature using x-ray powder diffractometer (Rigaku Minifiex, Japan) with CuKα radiation (λ = 1.5405 Å) in a wide range of Bragg angles 2θ (20° ≤ 2θ ≤ 60°) at a scanning rate of 2° min −1 . SEM micrographs were obtained with a JEOL-JSM-5800 scanning electron microscope operated at 20 KeV. SEM images were taken from freshly broken surface of the specimens.The flat polished surfaces of the sintered samples were electroded with air-drying silver paste. Before taking electrical measurements, the pellets were kept at 150°C for two hours. The dielectric constants () and loss tangents (tan) of samples were measured using a PSM (Comm.1735) while heating at the rate of 0.5ºC min -1 with a laboratory-made three-terminal sample holder.

Structural analysis
Room temperature XRD patterns of the (1x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 (where x = 0.00, 0.05 and 0.10) perovskite ceramics are shown in Fig. 1. All the reflection peaks were indexed using observed inter-planar spacing d, and lattice parameters of all the samples were determined by using least-squares refinement method. The calculated and observed d values of all diffraction lines (reflections) of above compounds suggest that there is a no change in the basic crystal structure from monoclinic as x changes from 0.00 to 0.10. A good agreement between the observed (obs) and calculated (cal) d-values (Table 1), suggests the correctness of the selection of crystal system and cell parameters. Our results are in good agreement with those by Saha and Sinha [7] and Shanming Ke et al. [8] deduced BFN having a monoclinic structure by using a standard computer program (POWD) analysis. The lattice parameters of different (1-x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 compounds are given in Table 1.

Microstructural analysis
Microstructure development during sintering was investigated by scanning electron microscopy (SEM). The surface micrographs of all compounds at magnification of 2µm at room temperature are shown in Fig. 2. The SEM micrographs of as-sintered ceramics show a few pores at grain boundaries, but cracks or micro cracks were not detected. The average grain size of compounds was around 2 -5 µm. Grains are distributed randomly in the samples surface. As in the Fig. 2 for the solid solution (1x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 , (x = 0.00, 0.05, 0.10), the grain sizes are reduced to favor the matter transport mechanism during the sintering process. When the powders are submitted to the sintering process performed at 1250°C for 5 h, the movement of atoms or molecules is driven by differences in curvature between the particles in contact. In order to reduce surface free energy, atoms supposedly move from particles with smaller radius to those with larger radius. Particularly, for the (1-x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 ceramics with concentrations of x = 0.00,0.05 and 0.10, it is possible that the matter transport between several aggregated particles and the high anisotropy in the grain boundary energies induced the formation of compact and irregular particles.

Dielectric analysis
The logarithmic angular frequency dependence of the dielectric constant () and loss tangent (tanof all compounds at 30°C and 130°C are plotted in Fig. 3(a-b). The dielectric constant () is decreases with increase of frequency at 30°C. But at 130°C, the  value in low frequency region (below 1 kHz) increases significantly. This behavior is consistent with that of common ferroelectrics. 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 abovementioned polarizations may have less contribution in . 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. The grains of the sample are separated by more insulating intergrain barriers, as shown in a boundary layer capacitor [23]. It is observed that tan maximum increases as temperature increases from 30°C to 130°C for x = 0.00, 0.05 and 0.10 , indicating that the concentration of conduction electrons increases as temperature due to thermal activation [24]. Table 1. Comparison of observed (obs) and calculated (cal) d-values (in nm) of reflections of (1-x)Ba(Fe0.5Nb0.5)O3-xBaTiO3 samples with different concentration (x = 0, 0.05, 0.10) compounds at room temperature. The estimated error in d is ± 0.00010 nm.
The variation of dielectric constant () as a function of temperature (30°C-270°C) at 1.09 kHz and 2.26 kHz frequencies of (1-x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 ceramics is shown in Fig. 4. For x = 0.00 the dielectric constant (') is found to increase with rise in temperature for a given frequency. There are two possible explanations for this behavior. One possibility is that it is due to the increased conductivity in the samples caused by the presence of Fe 2+ in sintered BFN 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. Due to the finite hopping (or jump) probability of electrons, this conduction mechanism tends to come into effect only at lower frequencies. An alternative explanation is related to disorder in the B-site cations of complex perovskites, as suggested by Majumdar et al. [27]. The dielectric constant (') of x = 0.05 and 0.10 ceramic showed a ferroelectric to paraelectric transition around 260°C. The peak position at the transition shows its dependence on the measured frequency. From Fig. 5, it is observed that tan remains almost constant up to 215°C, afterwards, it increases rapidly [28]. Fig. 6 shows a complex-plane impedance plot (Z * -plot) for x = 0.00, 0.05 and 0.10 ceramics, between the imaginary part Z against the real part Z of complex impedance Z * = Z-jZ at temperature 240°C. Generally, this type of plots can be characterized by three zones: (i) low-frequency zone where electrode polarization dominates, (ii) intermediatefrequency zone where polarization by migration of charges plays an important role, and (iii) high-frequency zone where dipole relaxation takes place. In all ceramics, a single semicircular arc is observed in the complex plane, and this can be explained by a RC equivalent circuit (Fig. 6  (a), inset)). Dipole relaxation process occurs in all the materials since semicircular arc observed in the highfrequency zone. The resistance was obtained from the intersection of the semicircle and the Z-axis. It is clear from the figure that the resistance decreases on increasing temperature. The semicircle has their center located away from the real axis, indicating the presence of relaxation species, and non-Deby type of relaxation process occurs in the materials.
The logarithmic angular frequency dependence of Z' and Z" of (1-x)Ba(Fe 0.5 Nb 0.5 )O 3 -xBaTiO 3 samples, at several temperatures between 120°C and 270°C is plotted in Fig. 7. At lower temperature, Z' decreases monotonically with increasing frequency up to certain frequency and then becomes frequency independent in all samples. At higher temperatures, Z' is almost constant and for even higher frequencies decreases sharply. 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" 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" for x = 0.05 and 0.10 starts appearing. The peak shifts towards higher frequency with Copyright © 2011 VBRI press. increasing temperature showing that the resistance of the bulk material is decreasing. Also, the magnitude of Z" decreases with increasing frequencies. Corresponding figure indicates the spreading of the relaxation time. This would imply that relaxation is temperature dependent, and there is apparently not a single relaxation time. There by relaxation processes involved with their own discrete relaxation time depending on the temperature. It is evident that with increasing temperature, there is a broadening of the peaks and at higher temperatures, the curves appear almost flat.   The origin of frequency dependence of conductivity in the relaxation phenomenon arises due to mobile charge carriers. The ac conductivity provides the insight of the mobility of charge carriers. Fig. 8 shows the ac conductivity of the all the compounds as a function of frequency at different temperature. The ac conductivity of the system depends on the dielectric properties and sample capacitance of the material. This behavior may be attributed to the presence of space charge in the material. The two plateaus separated by frequency region are observed in BFN(x= 0.00) ceramic. The low-frequency plateau represents the total conductivity whereas the highfrequency 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. The investigated material followed universal Jonscher's power law [29] (i.e., where  () is the total conductivity,  (0) is the frequency independent conductivity(, i.e.,  dc and A depends on temperature). The exponent n in the lower frequency region increases with the increase of temperature and is found to be in the range of 1.5-2. 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 x = 0.05 and 0.10 with the frequency independent conductivity below 10 kHz which is a typical feature of perovskite at elevated temperature (›180°C).

Conclusion
BFN-BT solid solution ceramics were prepared using solid state reaction method. BFN-BT ceramics have monoclinic crystal structure at room temperature exhibiting better homogeneity and formation of compounds. BaTiO 3 doping in BFN exhibits many interesting feature. Studies of the dielectric constant (') and tangent loss (tan ) of compounds suggest that the higher value of ' at the low frequency region has been explained using Maxwell-Wagner polarization effect. This is associated with heterogeneous conduction in the grains and grain boundaries of the compounds. The impedance plots showed only bulk contributions, and non-Debye type of relaxation process occurs in the material. The abnormal behavior of dielectric constant (') of x = 0.05 and 0.10 shows ferroelectric to paraelectric transition around 260°C.