The room temperature XRD pattern (Figure 1) of the calcined powder, which is different from those of the ingredients (JCPDS file nos. 22-1141, 02-0364 and 75- 0457), shows the formation of a single phase new compound. All the reflection peaks of the pattern are indexed in tetragonal or orthorhombic crystal system and in different cell configurations using software “POWD” [15].

Since TB compounds have tetragonal or orthorhombic crystal structure out of which an orthorhombic unit cell was selected on the basis of the best agreement between observed and calculated interplaner spacing d (i.e., ΣΔd = d_obs − d_cal = minimum). Then the selected lattice parameters are refined using the least-squares method, which are found to be a = 4.3691 (7) Å, b = 18.8181 (7) Å, c = 13.5254 (7) Å and volume V = 1112.05 ų for the compound (the number in parenthesis is estimated standard deviation of lattice parameters).

The unit cell parameters are consistent, and are in good agreement, with those reported earlier [9,16,17] for TB structural family. Unfortunately, it is not possible to determine the space group of the compound with the limited powder data. The crystallite size (P) of BDTV was roughly estimated from the broadening of a few XRD peaks (in wide 2θ range) using the Scherrer’s equation [18]: P = Kλ/(β_1/2 cos θ_hkl) (where K = constant = 0.89, λ = 1.5405 Å and β_1/2 = peak width of the reflection at half intensity). The average value of P was found to be 12 nm. The effects of strain, instrument and other defects on broadening are ignored in the calculation.

The SEM micrograph of the compounds with different magnifications at room temperature is shown in Figure 1 (inset). It is found that the grains are inhomogeneously and uniformly distributed over the entire surface of samples. The grain size evaluated from histogram is found to be in the range of 1.6 µm. A similar type of microstructure was found in many materials of this family [16,17].

Temperature dependence of dielectric constant (ε_r) for BGTV is shown in Figure 2(a). The value of relative dielectric constant (ε_r) initially increases with increase in

temperature, reaches to a maximum (peak) value at transition temperature (T_c ~386˚C), and then decreases and on farther increasing the temperature, the ε_r value increases. In our previous study using the same material, no expected dielectric anomaly was observed within the measured temperature range (30˚C - 500˚C) when Sm was doped in place of Gd, where as for Dy doped compound the transition temperature was ~430˚C [19,20]. Also it is observed that the values of tanδ (Figure 2(a) (inset)) increase with increase in temperature which may be due to space charge polarization. The low value of tanδ represents the good quality of ferroelectric material under observation. At low temperature the change in the values of tanδ of the compound is found considerably small. It then increases significantly at high temperatures. This may be due to enhanced conductivity in this compound, and also the reduction in ferroelectric domain wall’s contribution at high temperatures (paraelectric phase) [15, 16]. Ferroelectric-paraelectric phase transition was confirmed by appearance of hysteresis loops (as shown in the later section).

The broadening of dielectric peak indicates the existence of diffuse phase transition. The degree of disorder or diffusivity (γ) of the compound is estimated by an empirical relation; ln(1/ε_r – 1/ε_max) = γln (T – T_C) + lnK where T is the temperature and K is a constant. The value of γ at different frequency, calculated from the slope, is given in Figure 3. Generally, the value of γ is between 1 (for normal ferroelectrics) and 2 (for completely disordered system). The above value of γ (1 ≤ γ ≤ 2) shows a deviation from normal ferroelectric behavior, which indicates the occurrence of disordering in the system [17]. Kenzig [21,22] showed that when the paraelectric phase of BaTiO₃ transform to the ferroelectric macro domain phase, small domains of about 100 Ǻ are formed in a temperature range of about 1˚C (above T_c). They constitute the nucleation of the low temperature ferroelectric phase. Diffuse phase transition occurs due to fluctuation or substitutional disorder in the arrangement of cations at different crystallographic sites which is found in most of the niobate compounds of TB family.

Figure 3 shows the variation of polarization as a function of electric field on poled sample at room temperature.