Fig 2 shows the temperature dependence of real part of dielectric permittivity and loss tan for x 0 00 0 01 0 05 and 0 07 at different frequencies Although we did measurements at twenty different frequencies but data of four frequencies are presented for the sake of clarity An anomalous increase in the dielectric permittivity was observed at high temperature 400oC and low frequency region 10kHz probably due to high dc conductivity of the sample 27 28 Therefore the data for lower frequency range are not presented Pure NBT shows a well defined shoulder in the dielectric curve followed by a diffuse peak near 360 C Tm where dielectric constant attains a maximum value In fact exhibits frequency dispersion at the shoulder which diminishes near 290 C considered as onset of anti ferroelectric to paraelectric phase transition The tan curve shows well defined anomalies with incisive decrease of loss near 190 C The temperature Td at which tan curve falls rapidly is frequency dependent as observed in relaxor ferroelectrics These results agree well with the previous results and the temperatures are identical to the structural phase transition temperatures of NBT 26 With increase in the BZ content the dielectric curve shows broad maxima indicating that the transition become diffuse Hence the characteristic temperatures Td TRT and Tm are not very clearly distinguishable
