**6.4 Comparison of lattice vibration BTO and Sm3+ doping**

In order to enhance the understanding of the doping effect from the structural point of view, Raman scattering study is a very useful tool for investigating the lattice vibrational modes, which can provide details of lattice vibrations changes. **Figure 5** shows the Raman spectra of BTO and BSmT powders at room temperature from 100 to 2000 cm-1. Theoretically, the Raman selection rules allow 24 Raman active modes for orthorhombic BTO. (Kojima, 2000, Kojima ,Shimada, 1996). However, as shown in **Figure 5a**, the Raman spectrum of BTO less than 9 active modes were observed which is partially due to the possible overlap of the same symmetry vibrations or the weak features of some Raman bands (Liang et al., 2009). As can be seen in **Figure 5a**, the Raman modes at 193, 228, 267, 330, 353, 537, 563, 614 and 850 cm-1 were observed in BTO. All the Raman modes are also characterized as the vibrational modes of BTO which can be classified as internal modes of TiO6 octahedra. According to Kojima et al. (Kojima ,Shimada, 1996), the internal modes of TiO6 octahedra appear above 200 cm-1. The mode at 850 cm-1 is attributed to the symmetric Ti – O stretching vibration of atom inside the TiO6 octahedron whereas the mode at 614 cm-1 corresponds to the symmetry one. The two modes at 537 and 563 cm-1 correspond to the opposing excursions of the external apical oxygen (O) atoms of the TiO� octahedron. The 228 and 267 cm-1 modes are ascribed to the O – Ti – O

Sintering and Characterization of Rare Earth Doped

(b) 1000oC and (c) 1100oC.

factor (Yang et al., 2008):

orientation.

**7.2 Effect of Sm3+ and Pr3+ doping on grain orientation** 

Bismuth Titanate Ceramics Prepared by Soft Combustion Synthesis 369

Fig. 6. XRD patterns of BTO ceramics sintered at different temperatures for 3 hour: (a) 900oC,

The preferred grain orientation was obviously affected by rare-earth doping in BTO particularly after sintering. Kannan et al. (Kannan et al., 2006) reported the reflections corresponding to (00*l*) plane along the *c*-axis was observed with increasing Nd3+ content from 0 to 0.25. It can give us another hint that preferred grain orientation would be different when rare-earth was introduced into BTO lattice. As discussed in **Figure 6**, the pure BTO was formed with no preferred grain orientation. In **Figure 7**, the XRD patterns of BST and BPT ceramics sintered at 1100oC for 3 hour were presented. As seen from this figure, the Sm3+ and Pr3+ doping show a highly *c*-axis oriented growth with increasing Sm3+ and Pr3+ contents. The XRD peak corresponding to (00*l*) plane was clearly observed with higher intensity as compared to (117) plane. To simplify the discussion, the peak at (0014) and (117) are taken into consideration to determine the degree of *c*-axis orientation, α*c* by Lotgering

α*c* = *I*(0014)/[*I*(0014) + *I*(117)] The values of α*c* calculated for the Sm3+ and Pr3+ doping with various contents are listed in **Table 5**. It was found that the degree of *c*-axis orientation increased with increasing Sm3+ and Pr3+ content. This indicates that the doping content has a significance result on grain

Sm3+ content α*c* (%) Pr3+ content α*c* (%) 0.25 41.40 0.25 48.34 0.5 58.12 0.5 86.61 0.75 58.89 0.75 96.78 1.0 86.17 1.0 98.79

Table 5. Lotgering factor of the degree of *c*-axis orientation.

bending vibration. Although the mode at 228 cm-1 is Raman inactive according to the *O*h symmetry of TiO6, it is often observed because of the distortion of octahedron. The mode at 330 cm-1 was from a combination of the stretching and bending vibrations of the TiO6 octahedron. In addition, the formation of BTO with orthorhombic structure is identified by the splitting mode at 193 and 228 cm-1, and 537 and 563cm-1. Nevertheless, the Raman modes of the lower wavenumber at 116 cm-1 was not found in this spectrum, to show the vibrations between Bi and O atoms. However, the reason of missing mode is still not clear. The effect of Sm3+ doping on the structure change of BTO on the basis of the Raman modes is presented in **Figure 5b**. It was clearly observed that the peak intensity decreased with increasing Sm3+ contents from 0.25 to 0.75. It is believed to be associated with strong interactions between the ionic bonds; corresponding to the smaller ionic radius of Sm3+ (0.108 nm) compared with Bi3+ (0.117 nm). With the increase of Sm content, the distortion structure would be more and the grain size would be smaller. This finding is in line with the XRD pattern and FESEM micrograph. It was reported that the duplet peaks observed in Raman spectra tend to merge into one mode when the Bi3+ is substituted by rare-earth elements (Wu et al., 2005). Similar observation was discovered in BST. In the present work, the duplet peaks at 228-267 cm-1, 330- 353 cm-1 and 537-563 cm-1 were found to merged into a single peak at 264 cm-1, 322 cm-1 and 542 cm-1 for x= 0.25 and 264 cm-1, 330 cm-1 and 553 cm-1 for x= 0.75.

Fig. 5. Raman spectra of (a) BTO and (b) BSmT at different Sm3+ content.

#### **7. Bulk ceramic characterization**

#### **7.1 Effect of sintering temperature of BTO**

Figure 6 shows the XRD patterns of BTO ceramics sintered at different temperature for 3 hour. As can be seen from this figure, the BTO was formed with random oriented grains in which the strongest peak was found at (117) instead of (00l). Besides that, the increase in calcination temperature also implies the improvement of crystallinity and the enhancement of crystallite size. This can be explained by the width of the diffraction lines, which decreased, whilst the intensity increased. The crystallite sizes for ceramics sintered at 900, 1000 and 1100oC were calculated to be approximately 104.66, 126.52 and 130.22 nm, respectively.

bending vibration. Although the mode at 228 cm-1 is Raman inactive according to the *O*h symmetry of TiO6, it is often observed because of the distortion of octahedron. The mode at 330 cm-1 was from a combination of the stretching and bending vibrations of the TiO6 octahedron. In addition, the formation of BTO with orthorhombic structure is identified by the splitting mode at 193 and 228 cm-1, and 537 and 563cm-1. Nevertheless, the Raman modes of the lower wavenumber at 116 cm-1 was not found in this spectrum, to show the vibrations between Bi and O atoms. However, the reason of missing mode is still not clear. The effect of Sm3+ doping on the structure change of BTO on the basis of the Raman modes is presented in **Figure 5b**. It was clearly observed that the peak intensity decreased with increasing Sm3+ contents from 0.25 to 0.75. It is believed to be associated with strong interactions between the ionic bonds; corresponding to the smaller ionic radius of Sm3+ (0.108 nm) compared with Bi3+ (0.117 nm). With the increase of Sm content, the distortion structure would be more and the grain size would be smaller. This finding is in line with the XRD pattern and FESEM micrograph. It was reported that the duplet peaks observed in Raman spectra tend to merge into one mode when the Bi3+ is substituted by rare-earth elements (Wu et al., 2005). Similar observation was discovered in BST. In the present work, the duplet peaks at 228-267 cm-1, 330- 353 cm-1 and 537-563 cm-1 were found to merged into a single peak at 264 cm-1, 322 cm-1 and

Figure 6 shows the XRD patterns of BTO ceramics sintered at different temperature for 3 hour. As can be seen from this figure, the BTO was formed with random oriented grains in which the strongest peak was found at (117) instead of (00l). Besides that, the increase in calcination temperature also implies the improvement of crystallinity and the enhancement of crystallite size. This can be explained by the width of the diffraction lines, which decreased, whilst the intensity increased. The crystallite sizes for ceramics sintered at 900, 1000 and 1100oC were calculated to be approximately 104.66, 126.52 and 130.22 nm,

Fig. 5. Raman spectra of (a) BTO and (b) BSmT at different Sm3+ content.

**7. Bulk ceramic characterization** 

respectively.

**7.1 Effect of sintering temperature of BTO** 

542 cm-1 for x= 0.25 and 264 cm-1, 330 cm-1 and 553 cm-1 for x= 0.75.

Fig. 6. XRD patterns of BTO ceramics sintered at different temperatures for 3 hour: (a) 900oC, (b) 1000oC and (c) 1100oC.
