**6.1 Effect of calcination temperature on BTO**

X-ray diffraction (XRD) was conducted on the as-combusted powders which calcined at different temperatures and the result is shown in **Figure 2**. The study on different calcination temperature is essential in this work. The main reason is to determine the optimum calcination temperature, which will be used for the following characterization in particular dielectric study. As seen in **Figure 2**, the BTO phase was observed in as-combusted powder. The main peak corresponding to BTO was found at ~29o. However, the presence other intermediate phases such as BTO7.7 and BTO2 were also identified and marked in XRD pattern. At calcination temperature of 600oC, there was a tremendous increase at the main peak (~29o). This inedicates that the presence more BTO phase was observed after calcination process. In addition, the peaks corresponding to intermediate phases were decreased. Further increase calcination temperature, the intermediate phases were gradually eliminated. The intermediate phases completely disappeared at 750 and 800oC. In other words, the BTO phase was successfully formed as single phase at temperature as low as 750oC. It also suggest that the optimum calcination temperature for BTO is 750oC. This temperature is probably lower than other processing route such as conventional solid state reaction and some other wet chemical synthesis (Kan et al., 2003, Pookmanee, 2008). Moreover, as the calcination temperature was increased, the XRD peaks were sharper and the stable phase BTO powders with higher crystallinity could be obtained. **Table 2** presents the variation of lattice parameters and

Sintering and Characterization of Rare Earth Doped

that Sm3+ and Pr3+ had substituted in (Bi2

whereas the BSmT and BPrT were around 650oC.

different Sm3+ and Pr3+ contents. ●:Bi4Ti3O12 or BTO.

Bismuth Titanate Ceramics Prepared by Soft Combustion Synthesis 365

(Hu et al., 2005). In addition, the increase in dopant contents would also result in difference in structure distortion (Kan et al., 2004, Kan et al., 2008). In order to determine the effect of Sm3+ and Pr3+ doping on crystal structure and lattice parameter, the calcined powder for respective contents were analyzed by XRD. Interestingly, the calcination temperature was successfully reduced from 750oC (for BTO) to 650oC (for BSmT and BPrT). According to **Figure 3**, the XRD patterns of BSmT and BPrT powders calcined at 650oC for 3 hour with different Sm3+ and Pr3+ contents were presented. Regardless of Sm3+ and Pr3+ contents, the formation of a single phase BTO was observed in **Figure 3(a)** and **Figure 3(b)** for both doping powders. This indicates that the perovskite phase was fully formed in calcined powders and all of them have a bismuthlayered structure. In comparison to the BTO calcined powder at 650oC for 3 hour (see **Figure 2**), the intermediate phases such as BTO2 and BTO7.7 were eliminated as a result of Sm3+ and Pr3+ doping. This result indicates that the Sm3+ and Pr3+ ions in the BSmT and BPrT, respectively, are incorporated into the pseudo-perovskite structure, substituting for the Bi3+ ions. Besides that, the lattice parameters and crystallite size of BSmT and BPrT were greatly influenced by Sm3+ and Pr3+ doping, as shown in **Table 3** and **Table 4**, respectively. Based on both tables, the *a*-parameter rapidly approached the *b*-parameter with increasing Sm3+ and Pr3+ content. The closed value of the *a-* and *b*-parameters was obtained at 1.0 mole of Sm3+ and Pr3+, corresponding to the increment in the symmetry of the crystal structure. This also suggested that the orthorhombic structure was formed when doping content was equivalent to 0.25, 0.5 and 0.75 whereas the tetragonal structure was formed doping content was equivalent to 1.0. Further observation shows that the *c*-parameter slightly changed with increasing Sm3+ and Pr3+ content. It was reported that the variation of *c*-parameter is attributed to the rotation of the TiO6 octahedron and the reduction in the oxygen deficient (Yoneda et al., 2006). It is evident

Ti3

Fig. 3. XRD patterns of (a) BSmT and (b) BPrT powders calcined at 650oC for 3 hour with

the TiO6 octahedron; these would eventually result in the shrinkage of unit cell. The crystallite size of BTO was found to be 37.19 nm and decreased continuously to 24.69 nm and 24.56 nm with 1.0 of Sm3+ and Pr3+, respectively. It is attributed to the reduction of space charge density and oxygen vacancies with increasing Pr3+ content, which act as grain growth inhibiter. This is supported by the finding of Xiang et al. (Xiang et al., 2006). Based on XRD studies, it can conclude that the optimum calcination temperature for BTO was determined at 750oC,

O10)2- perovskite-type layers and O vacancies in

crystallite sizes of BTO calcined at different temperatures. As can be seen in **Table 2**, the corresponding lattice *a*-, *b*-, and *c*-parameters as well as volume cell units, V were observed to exhibit in various values as a result of calcination temperature. In addition, the crystal structure of calcined powder was confirmed to belong orthrhombic, in which it is a typical structure for BTO (Hervoches ,Lightfoot, 1999, Kim ,Jeon, 2004). Besides that, the crystallite size of calcined powder was increased with increasing calcination temperature. The increase in a such way was observed in many studies (Hou et al., 2010, Pookmanee ,Phanichphant, 2009, Umar Al-Amani et al., 2010).

Fig. 2. XRD patterns of BTO powders calcined at different temperatures for 3 hour. ●:Bi4Ti3O12 or BTO; α:Bi7.7Ti0.3O12.16 or BTO7.7; φ:Bi2Ti2O7 or BTO2.


Table 2. Lattice parameters and crystallite sizes of BTO.

#### **6.2 Effect of Sm3+ and Pr3+ doping on crystal structure and lattice parameter**

The introduction of Sm3+ and Pr3+ to form Bi4-xSmxTi3O12 (BSmT) and Bi4-xPrxTi3O12 (BPrT) was expected to have a major changes on calcination temperature, crystal structure, lattice parameter and crystallite size. Based on previous studies, the ionic radii of Sm3+ and Pr3+ were reported around 0.108 nm and 0.113 nm, respectively, whereas the ionic radii of Bi3+ was about 0.117 nm (Garg et al., 2005, Hu et al., 2005). In general, the substitution of Sm3+ or Pr3+ for Bi3+ with larger difference in ionic radii size would lead to larger structure distortion of BTO lattice

crystallite sizes of BTO calcined at different temperatures. As can be seen in **Table 2**, the corresponding lattice *a*-, *b*-, and *c*-parameters as well as volume cell units, V were observed to exhibit in various values as a result of calcination temperature. In addition, the crystal structure of calcined powder was confirmed to belong orthrhombic, in which it is a typical structure for BTO (Hervoches ,Lightfoot, 1999, Kim ,Jeon, 2004). Besides that, the crystallite size of calcined powder was increased with increasing calcination temperature. The increase in a such way was observed in many studies (Hou et al., 2010, Pookmanee ,Phanichphant, 2009,

Fig. 2. XRD patterns of BTO powders calcined at different temperatures for 3 hour.

Calcination, oC 600 650 700 750 800

**6.2 Effect of Sm3+ and Pr3+ doping on crystal structure and lattice parameter** 

The introduction of Sm3+ and Pr3+ to form Bi4-xSmxTi3O12 (BSmT) and Bi4-xPrxTi3O12 (BPrT) was expected to have a major changes on calcination temperature, crystal structure, lattice parameter and crystallite size. Based on previous studies, the ionic radii of Sm3+ and Pr3+ were reported around 0.108 nm and 0.113 nm, respectively, whereas the ionic radii of Bi3+ was about 0.117 nm (Garg et al., 2005, Hu et al., 2005). In general, the substitution of Sm3+ or Pr3+ for Bi3+ with larger difference in ionic radii size would lead to larger structure distortion of BTO lattice

a/ Å 5.418(2) 5.4138(6) 5.4110(6) 5.4093(2) 5.4066(2) b/ Å 5.433(2) 5.4385(6) 5.4438(6) 5.4418(2) 5.4429(2) c/ Å 32.765(6) 32.810(3) 32.853(4) 32.830(1) 32.817(1) V/Å3 964.4709 966.0297 967.7028 966.4148 965.7186 Crystallite Size/ nm 24.87 37.19 50.54 71.34 98.07

●:Bi4Ti3O12 or BTO; α:Bi7.7Ti0.3O12.16 or BTO7.7; φ:Bi2Ti2O7 or BTO2.

Table 2. Lattice parameters and crystallite sizes of BTO.

Umar Al-Amani et al., 2010).

(Hu et al., 2005). In addition, the increase in dopant contents would also result in difference in structure distortion (Kan et al., 2004, Kan et al., 2008). In order to determine the effect of Sm3+ and Pr3+ doping on crystal structure and lattice parameter, the calcined powder for respective contents were analyzed by XRD. Interestingly, the calcination temperature was successfully reduced from 750oC (for BTO) to 650oC (for BSmT and BPrT). According to **Figure 3**, the XRD patterns of BSmT and BPrT powders calcined at 650oC for 3 hour with different Sm3+ and Pr3+ contents were presented. Regardless of Sm3+ and Pr3+ contents, the formation of a single phase BTO was observed in **Figure 3(a)** and **Figure 3(b)** for both doping powders. This indicates that the perovskite phase was fully formed in calcined powders and all of them have a bismuthlayered structure. In comparison to the BTO calcined powder at 650oC for 3 hour (see **Figure 2**), the intermediate phases such as BTO2 and BTO7.7 were eliminated as a result of Sm3+ and Pr3+ doping. This result indicates that the Sm3+ and Pr3+ ions in the BSmT and BPrT, respectively, are incorporated into the pseudo-perovskite structure, substituting for the Bi3+ ions. Besides that, the lattice parameters and crystallite size of BSmT and BPrT were greatly influenced by Sm3+ and Pr3+ doping, as shown in **Table 3** and **Table 4**, respectively. Based on both tables, the *a*-parameter rapidly approached the *b*-parameter with increasing Sm3+ and Pr3+ content. The closed value of the *a-* and *b*-parameters was obtained at 1.0 mole of Sm3+ and Pr3+, corresponding to the increment in the symmetry of the crystal structure. This also suggested that the orthorhombic structure was formed when doping content was equivalent to 0.25, 0.5 and 0.75 whereas the tetragonal structure was formed doping content was equivalent to 1.0. Further observation shows that the *c*-parameter slightly changed with increasing Sm3+ and Pr3+ content. It was reported that the variation of *c*-parameter is attributed to the rotation of the TiO6 octahedron and the reduction in the oxygen deficient (Yoneda et al., 2006). It is evident

that Sm3+ and Pr3+ had substituted in (Bi2 Ti3 O10)2- perovskite-type layers and O vacancies in the TiO6 octahedron; these would eventually result in the shrinkage of unit cell. The crystallite size of BTO was found to be 37.19 nm and decreased continuously to 24.69 nm and 24.56 nm with 1.0 of Sm3+ and Pr3+, respectively. It is attributed to the reduction of space charge density and oxygen vacancies with increasing Pr3+ content, which act as grain growth inhibiter. This is supported by the finding of Xiang et al. (Xiang et al., 2006). Based on XRD studies, it can conclude that the optimum calcination temperature for BTO was determined at 750oC, whereas the BSmT and BPrT were around 650oC.

Fig. 3. XRD patterns of (a) BSmT and (b) BPrT powders calcined at 650oC for 3 hour with different Sm3+ and Pr3+ contents. ●:Bi4Ti3O12 or BTO.

Sintering and Characterization of Rare Earth Doped

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

(e) BPrT:0.25 and (f) BPrT:1.0.

Fig. 4. Morphologies of (a) BTO:650oC, (b) BTO:750oC, (c) BSmT:0.25, (d) BSmT:1.0,

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

Bismuth Titanate Ceramics Prepared by Soft Combustion Synthesis 367

size of plate-like particle decreased relatively with increasing Sm3+ and Pr3+, corresponding to the greater relaxation in the perovskite-layer. In order to see the difference of the particle size between Sm3+ and Pr3+, the doping content was fixed at 0.25. As can be seen in **Figure 4c** and **Figure 4e** for Sm3+ and Pr3+, respectively, the particle size of Sm3+ doping was found to substantially larger than Pr3+ doping. This might be attributed to the difference in ionic radii which also resulted in different diffusivity.


Table 3. Lattice parameters and crystallite sizes of BSmT.


Table 4. Lattice parameters and crystallite sizes of BPrT.

#### **6.3 Grain morphology of BSmT and BPrT powders**

To gain an insight into the formation of BTO prepared using different doping content, the calcined powders were monitored by taking field emission scanning electron microscopy (FESEM) micrographs. **Figure 4** shows the morphology of BTO, BSmT and BPrT powders. In order to observe the increase in particle size of BTO, the morphology at 650oC and 750oC were displayed in **Figure 4a** and **Figure 4b**, respectively. It was found that the particle size is relatively expanded with increasing temperature. It was also determined that the particle size in range of 0.1 – 0.2 μm and 0.3 – 0.5 μm were found at 650oC and 750oC, respectively. It clearly observed that plate-like morphology was formed at 750oC instead of 650oC. The formation of such morphology was observed in many studies in which the plate-like structure with highly anisotropic properties is one of typical shape for pure BTO (Chen et al., 2006). In addition, the variation in particle size is also attributed to a greater distortion of perovskite-layer along *ab*-plane (particle length) as compared with *c*-axis (particle thickness). The morphology of BSmT and BPrT powders with different doping contents were observed and depicted in **Figure 4c – 4f**. In this section, the selected micrographs for each dopant with doping content of 0.25 and 1.0 were presented. The selection of the minimum and maximum doping contents is necessary to determine the variation size and shape of resultant particles. It was found that the particle size decreased with increasing doping content, corresponding to the lower diffusivity of both doping content compared to Bi3+, resulting to the suppression of the grain growth (Goh et al., 2009). It was determined that the particle size in range of 0.2 – 0.4 μm and 0.1 – 0.2 μm were observed when Sm3+ contents were equivalent to 0.25 and 1.0, respectively. Meanwhile, the average particle size in range of 0.1 – 0.2 μm and 0.05 – 0.1 μm were found when Pr3+ contents were equivalent to 0.25 and 1.0, respectively. It is also noticed that the

Sm3+ content 0.25 0.5 0.75 1.0

Pr3+ content 0.25 0.5 0.75 1.0

a/ Å 5.4210(9) 5.4157(8) 5.4099(1) 5.4067(8) b/ Å 5.4306(9) 5.4213(8) 5.4161(1) 5.4054(8) c/ Å 32.783(3) 32.810(2) 32.816(3) 32.817(3) V/Å3 965.0938 963.3097 961.527 959.0734 Crystallite size/ nm 36.44 32.86 28.55 24.56

To gain an insight into the formation of BTO prepared using different doping content, the calcined powders were monitored by taking field emission scanning electron microscopy (FESEM) micrographs. **Figure 4** shows the morphology of BTO, BSmT and BPrT powders. In order to observe the increase in particle size of BTO, the morphology at 650oC and 750oC were displayed in **Figure 4a** and **Figure 4b**, respectively. It was found that the particle size is relatively expanded with increasing temperature. It was also determined that the particle size in range of 0.1 – 0.2 μm and 0.3 – 0.5 μm were found at 650oC and 750oC, respectively. It clearly observed that plate-like morphology was formed at 750oC instead of 650oC. The formation of such morphology was observed in many studies in which the plate-like structure with highly anisotropic properties is one of typical shape for pure BTO (Chen et al., 2006). In addition, the variation in particle size is also attributed to a greater distortion of perovskite-layer along *ab*-plane (particle length) as compared with *c*-axis (particle thickness). The morphology of BSmT and BPrT powders with different doping contents were observed and depicted in **Figure 4c – 4f**. In this section, the selected micrographs for each dopant with doping content of 0.25 and 1.0 were presented. The selection of the minimum and maximum doping contents is necessary to determine the variation size and shape of resultant particles. It was found that the particle size decreased with increasing doping content, corresponding to the lower diffusivity of both doping content compared to Bi3+, resulting to the suppression of the grain growth (Goh et al., 2009). It was determined that the particle size in range of 0.2 – 0.4 μm and 0.1 – 0.2 μm were observed when Sm3+ contents were equivalent to 0.25 and 1.0, respectively. Meanwhile, the average particle size in range of 0.1 – 0.2 μm and 0.05 – 0.1 μm were found when Pr3+ contents were equivalent to 0.25 and 1.0, respectively. It is also noticed that the

Table 3. Lattice parameters and crystallite sizes of BSmT.

Table 4. Lattice parameters and crystallite sizes of BPrT.

**6.3 Grain morphology of BSmT and BPrT powders** 

a/ Å 5.4076(3) 5.4018(4) 5.3952(5) 5.3910(1) b/ Å 5.4283(3) 5.4155(4) 5.4059(5) 5.3940(1) c/ Å 32.798(2) 32.803(2) 32.800(2) 32.777(2) V/Å3 962.74 959.6153 956.6382 953.0192 Crystallite size/nm 54.89 48.47 45.27 24.69

size of plate-like particle decreased relatively with increasing Sm3+ and Pr3+, corresponding to the greater relaxation in the perovskite-layer. In order to see the difference of the particle size between Sm3+ and Pr3+, the doping content was fixed at 0.25. As can be seen in **Figure 4c** and **Figure 4e** for Sm3+ and Pr3+, respectively, the particle size of Sm3+ doping was found to substantially larger than Pr3+ doping. This might be attributed to the difference in ionic radii which also resulted in different diffusivity.

Fig. 4. Morphologies of (a) BTO:650oC, (b) BTO:750oC, (c) BSmT:0.25, (d) BSmT:1.0, (e) BPrT:0.25 and (f) BPrT:1.0.
