We report on dielectric properties of polycrystalline (BBTF) ceramic system ( , 0.06, 0.08, 0.10, 0.12, and 0.16). The materials were synthesized by solid state ceramic route. Solid solution formation has been confirmed by powder X-ray diffraction for compositions with . Crystal structure is tetragonal for and cubic for . Microstructures show that the average grain size is less than one micrometer (1？μ). Dielectric behavior has been studied as a function of temperature (100？K–400？K) and frequency. Composition with exhibits diffuse phase transition. Compositions with show ferroelectric relaxor behavior. This shows that diffuse ferroelectric transition behavior changes to relaxor type ferroelectric transition with increasing . Plots of dielectric loss (D) versus temperature shows broad maxima which shift to high temperature with increasing frequency, dispersion in dielectric loss decreases with below peak maxima and increases above. It may be attributed to Maxwell Wagner type relaxation process for low (~0.02) and relaxation of nanopolar regions for . 1. Introduction Compositions based on BaTiO3 (BTO), a well-known ferroelectric material, show significant change in the dielectric behavior near Curie temperature. Its properties are modified by a wide variety of substitutions, possible either at Ba or Ti sites independently or simultaneously. Extensive research work on the effect of isovalent as well as offvalent substitutions on the transition temperature and the dielectric properties of BaTiO3 has been done during the last few decades [1–3]. The substitution of trivalent ions either at A or at B sites causes charge imbalance and requires creation of vacancies in A or B or oxygen sublattice or generation of electrons or holes to maintain the electrical charge neutrality. It has been reported that for small concentration of La3+ substitution, the charge neutrality is maintained by electronic compensation in accordance with the formula [1], and for large concentration, excess donor charge on La3+ ions is compensated by Ti vacancies as represented by the formula [4]. Substitution of Bi3+ at Ba2+ site in small concentration is reported to exhibit positive temperature coefficient of resistance (PTCR) [5]. Diffuse phase transition has been observed for large concentration Bi3+ [6]. Possibility of creation of defects is very much reduced by valence compensated substitutions either on A site or on B site or on both sites simultaneously. For example, solid solutions of BaTiO3-NaNbO3 and Zr-doped BaTiO3 have charge compensation internally. These systems have been
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