Figure 1(a) shows the XRD patterns of Sb (0.0, 0.01, 0.02 and 0.03) doped Bi_0.5Na_0.5TiO₃ ceramics sintered at 950˚C for 4 h. All the compositions exhibit a pure perovskite structure and no second phases are observed, which implies that Sb ceramic has diffused into the Bi_0.5Na_0.5TiO₃ lattices completely. All the reflection peaks of the X-ray profile were indexed and lattice parameters were determined using a least-squares method with the help of a standard computer programme (POWD) [27] . The values of observed (d_obs) and calculated (d_cal) interplanar spacing, (hkl) indexing and intensity are shown in Table 1. Good agreement between the observed and

calculated inter-planar spacing (d-values), suggests that the compositions are stabilized in rhombohedral phase. The unit cell is selected for which ΣΔd = Δ(d_obs − d_cal) is minimum. It can be seen from Figure 1 that the prepared compositions have pure phase without any detectable impurity phases. The lattice constants obtained for different compositions are shown in table 1 for rhombohedral and hexagonal unit cells. The estimated lattice parameters are very close to those obtained in earlier reports [28] [29] (JCPDF No-36-0340). Also Figure 1(b) shows the XRD patterns of the ceramics in the 2θ range of 44 - 50 degree. A distinct 002/200 peak splitting appears when doping percentage changes from 0.02 to 0.03, referring to a hexagonal symmetry. To characterize the phase compositions in a more quantitative way, the XRD patterns of the MPB compositions in the 2θ ranges of 46˚ - 48˚ were fitted as shown in Figure 2. The data shows the Lorentzian deconvolution of the 002 and 200 peaks of the tetragonal phase and the 202 peak of the rhombohedral phase. These results suggest that the rhom-

bohedral-hexagonal morphotrophic phase boundary (MPB) appears in ceramic near to doping level at 0.03% Sb content.

Figure 3 represents the Raman spectra of pure BNT and Sb doped BNT with different percentage. There are only five Raman-active modes observed in the range from 100 to 1000 cm⁻¹ in agreement with earlier report [30] [31] . BNT ceramics with rhombohedral structure presents 13 Raman-active modes (Γ_Raman = 4A₁ + 9E) due to the disorder in A-site related to distorted octahedral [BiO₆] and [NaO₆] clusters [32] . The first Raman active A₁ (TO₁) mode at around (144 cm⁻¹) is related to distorted octahedral [BiO₆] and [NaO₆] clusters. The second Raman active E (TO₂) mode was observed in the regions of 276 cm⁻¹ which is assigned to stretching arising from the bonds due to presence of octahedral [TiO₆] clusters at short-range.

The third Raman-active (LO₂) mode is related to short-range electrostatic forces associated with the lattice iconicity [33] . The (TO₃) mode situated at around 541 cm⁻¹ may be due to the (O-Ti-O) stretching symmetric vibrations of the octahedral [TiO₆] clusters [34] . The (LO₃) mode found at 813 cm⁻¹ is due to the presence of the sites within the rhombohedral lattice pre containing octahedral distorted [TiO₆] clusters [35] . There is no significant change in the spectra for the prepared compositions. For better observation of the Raman spectra of BNT pure and Sb doped BNT (Sb = 0.03%) along with the curves fitted to individual peaks are shown in Figure 4. The spectra of BNT (Sb = 0.03%) shows additional peaks around 151, 281, 585 and 853 cm⁻¹ compared to the peaks observed in BNT. The occurrence of these bands splitting may be due to structural change at doping level 0.03% of Sb, which are well in line with the studies of XRD phase analysis. However, from Figure 3 it is possible to detect that all the Raman peaks are very broad in BNT and Sb doped BNT ceramics. It is believed that this behaviour is due to the presence of the disorder structural or distorted octahedral [TiO₆] clusters at short-range and the overlapping of Raman modes due to the lattice anharmonicity.

For closer investigation, the variation of full width of half maximum (FWHM) and intensity of individual peaks are plotted in Figure 5(a) and Figure 5(b). The mode intensity and FWHM undergo slope change at Sb = 0.03. The variation of intensity and FWHM of all peaks shows a similar type of anomaly at Sb = 0.03. On the basis of these considerations, it is possible to conclude that the rhombohedral-tetragonal phase co-exists at x = 0.03 which is also observed in the XRD results.