2 shows the temperature dependence of realpart of dielectric permittivity (e’) and loss (tand) forx=0.00, 0.01, 0.05 and 0.07 at different frequencies. Although we did measurements at twenty differentfrequencies, but data of four frequencies are presented for the sake ofclarity. An anomalous increase in the dielectric permittivity was observed athigh temperature (>400oC) and low frequency region (<10kHz), probablydue to high dc-conductivity of the sample 27, 28. Therefore, the data forlower frequency range are not presented.
PureNBT shows a well-definedshoulder in the dielectric curve followed by a diffuse peak near 360 °C (Tm),where dielectric constant attains a maximum value. In fact, e’ exhibits frequency dispersionat the shoulder which diminishes near 290 °C, considered as onset of anti-ferroelectricto paraelectric phase transition. The tand curve shows well-defined anomalies with incisive decrease of loss near 190°C.The temperature (Td) at which tand curve falls rapidly is frequency dependent as observed in relaxor ferroelectrics.These results agree well with the previous results and the temperatures areidentical to the structural phase transition temperatures of NBT 26. With increasein the BZ content the dielectric curve showsbroad maxima, indicating that the transition become diffuse. Hence, thecharacteristic temperatures (Td, TRT,and Tm) are not very clearly distinguishable.
The broadening ofphase transition is common in solid solutions and other disordered systems. Thebroadening in NBT based systems is attributed to the presence of structuraldisorder and composition fluctuations. To observe the changes in the dielectricproperties, the real part of dielectric permittivity at room temperature () and at Tm ( ) are plotted against ‘x’ inFig.3.
It is evident from the figure that the dielectricconstant decreases gradually for x ? 0.03 and then it increases abruptly forx=0.05 and 0.
07. Dielectric loss (tand) of system found toincrease slightly with increase in BZ content when compared to pure NBT(x=0.00).
Beyond 200oC a reasonable increase in the dielectric lossis observed in terms of enhanced conductivity. A rationalization of decline inthe dielectric constant value with BZ, for x ? 0.30 can be conferred byconsidering polarizability of oxygen octahedral into account. For a givenconcentration, x, in (NBT)(1-x)(BZ)x the Ti4+ions are replaced by Zr4+ ions.
Using first principle calculation,Zhou et al 29 have shown that replacing the B-site (Ti4+) ions inNBT with Zr4+ ions strongly reduces the polarizability by increasingB-O bond strength. This in turn, results in overall reduction in the dielectric constant value. It has been shown by Koo & Cheong 30 that forpolycrystalline materials the dielectric permittivity is higher for tetragonalstructure than the monoclinic. The enhancement in the dielectric constant wasattributed to 90° domain rotation in the tetragonal phase.
Recently,Singh et. al. 26 have observed tetragonaldistortion in (NBT)(1-x)(BZ)x solid solutionfor x = 0.05and 0.07.
Therefore, the enhanceddielectric constant for x=0.05 and 0.07 is due to the emergence of tetragonal distortionin these compositions