Synthesis, Structural and Dielectric Properties of SrBi_1.8Ce_0.2Ta₂O₉

This Ce-doped strontium bismuth tantalate SrBi _1.8 Ce _0.2 Ta ₂ O ₉ was prepared by solid-state reaction. X-ray diffraction was used to determine the crystal structure of the powders. The Raman spectrum of SrBi _1.8 Ce _0.2 Ta ₂ O ₉ sample was measured to confirm X-ray diffraction result. The microstructure of ceramic was observed by Scanning Electron Microscope (SEM). The Temperature dependence of the dielectric properties of ceramic was investigated from the room temperature to 400°C.

• Aurivillius
• Solid-State
• Electron Microscopy
• Raman
• Dielectric Properties
• Electrical Conductivity

1. Introduction

These template, Aurivillius phase compounds, as bismuth layer-structured ferroelectrics are generally formulated (Bi₂O₂)²⁺(A_m−1B_mO_3m+1)²⁻, where A is a mono, bi or trivalent ion, B a tetra, penta or hexavalent ion, and m the number of BO₆ octahedral in each pseudo-perovskite block (m = 1 to 5) [1] . Bismuth layer-structured ferroelectric materials have attracted an increasing attention for non-volatile Ferroelectric Random Access Memory (FeRAM) applications [2] [3] .

SrBi₂Ta₂O₉ has attracted much attention of researchers due to its fatigue-free properties in nonvolatile ferroelectric thin film random access memory applications [4] . The crystal structure has orthorhombic symmetry with a = 0.5306 nm, b = 0.55344 nm and c = 2.49839 nm; the theoretical density is 8.789 g/cm [5] .

To our knowledge, there few studies talk about the substitution of bismuth by cerium in bismuth layered structured ferroelectrics systems. Cerium (Ce⁴⁺) modified bismuth layered ferroelectric SrBi₂Ta₂O₉ with general formula SrBi_2−xCe_3x/4Ta₂O₉ (x = 0, 0.025, 0.05, 0.075 and 0.1) was prepared by solid state reaction route. The morphological study was done by Scanning electron microscopy, which shows plate like structure. The temperature dependent dielectric study shows a diffuse phase transition with a linear decrease in transition temperature and dielectric constant with an increase in Ce⁴⁺ content [6] .

Effect of Ce and La substitution on the microstructure and dielectric proprieties of Bi₄Ti₃O₁₂ ceramics was investigated by Nikolina pavlovic et al. [7] . Bi_4−xA_xTi₃O₁₂ ceramics were prepared by modified sol-gel method. Briefly, the addition of Ce improves diffuse phase transition and frequency dispersion of dielectric constant. It could be due to the characteristic nature of Ce. Cerium can change its oxidation sates easily between Ce³⁺ and Ce⁴⁺.

In order to investigate, we report solid solution of the Aurivillius type SrBi_1.8Ce_0.2Ta₂O₉, on microstructure and dielectric proprieties. However, our results and discussion were supported by the literature researches.

2. Prepare Experimental

First, SrBi_1.8Ce_0.2Ta₂O₉ (SBCT) powder was prepared by conventional solid-state reaction method using Bi₂O₃, SrCO₃, Ta₂O₅ and Ce₂O₃ as starting materials. All raw materials were weighed at stoichiometric proportion and then mixed manually by a gate mortar. The mixed powder was calcined at 1200˚C for 12 h. After calcination, the mixture was milled again and pressed into pallet with a diameter of 6 mm and a thickness of 1 mm under the pressure of about 1 MPa. The ceramic was sintered at 1250˚C for 8 h.

The crystal structure of the powder was determined by X-ray diffraction (XRD) using a Cu K_α radiation (λ = 1.54178 Å). The morphology, structure and size of the ceramic were characterized with scanning electron microscopy (SEM). The temperature dependence of the dielectric properties of the ceramic was performed using a HP 4284A LCR meter.

3. Results and Discussion

Figure 1 shows XRD pattern, which was identified as orthorhombic (JCPDS 49-0609) with space group A2₁ am. The lattice parameters were calculated using program Unit Cell: a = 5.53290 Å, b = 5.52195 Å and c = 25.02979 Å. The strongest diffraction peak at 30˚ is correlated to the (1 1 5) orientation, which is consistent with the (1 1 2m+1) highest diffraction peak in bismuth layer-structured ferroelectrics [8] . The crystallite size was calculated from the (1 1 5) XRD peak using the Debye-Sherrer’s equation [9] , this was calculated to be 1221 μm.

Figure 2 shows room temperature Raman spectra of SBCT powder. The bands (89 and 131 cm⁻¹) assigned to the Bi-O bonds [10] , which reflect the vibration of Bi³⁺ ions in (Bi₂O₂)²⁺ layer and Sr-Site Bi³⁺ ions. According to references [11] [12] , the band around 130 cm⁻¹, it may be attributed to the intercorporation of Ce/Bi.

J. S. Zhu et al. [13] , reported that the Raman mode at 160 cm⁻¹ corresponds to the Ta z-axis vibration, the 206 cm⁻¹ band represent the SrO vibration with a rock salt structure, the band at about 602 cm⁻¹ is associated with

Figure 1

X-ray diffraction pattern of SBCT

the internal vibration of the TaO₆ octahedron and the one at about 812 cm⁻¹ is also related to the stretching mode of TaO₆ octahedron.

Figure 3 shows SEM images of the SBCT ceramic. It can be seen that the ceramic has a plate-like morphology. This plate-like morphology of the grain is a characteristic feature of bismuth layer compounds [14] . The grain size is found to be slightly coarsened to 2 μm - 900 nm, small amounts of pores still exist.

Figure 4 shows the frequency dependence of the dielectric constant and loss tangent measured at room temperature. The dielectric constant tends to be constant and the overall tangent loss was found to be below the 5 × 10⁻² through the frequency range studied.

Figure 5 shows the temperature dependence of the (b) dielectric constant (ε') and (a) dielectric loss (tanδ) of SBCT ceramic at 100 Hz and 1 kHz. The dielectric dispersion with frequency is significant at higher temperature. Well, it can due to the phenomena of space charge effects. This phenomenon was reported in detail by D. Dhak et al. [15] . The Curie temperature was around 330˚C and dielectric peak was found to be 115 at 1 kHz.

The dielectric loss (tanδ) values as a function of temperature tend to be constant below 300˚C. But, above the latter temperature, tanδ increases with the increase of temperature, which might be due to the oxygen vacancies.

Figure 6(a) shows the variation of reciprocal dielectric constant with temperature at 100 Hz. It was found that the dielectric of SBCT deviates slightly the Curie-Weiss low.

where C is the Curie constant and T_CW is the Curie-Weiss temperature. The Curie-Wiess constant was found to be 0.8 × 10⁵ K and the Curie-Wiess temperature is 320˚C. These results suggest using Curie-Weiss modified [16] .

where C is the modified Curie-Weiss constant and ε_m' is the maximum dielectric constant. However, the relaxation factor γ was found to be approximately 0.9, according to the fitting result shown in Figure 6(b). This is why the para-ferroelectric phase transition of the SBCT was regarded as non-relaxation.

Figure 7 shows the temperature dependence of ac conductivity of SBCT sample. The curve shows two regions: 1) at lower temperatures, the conductivity tends to be constant. It may be attributed to extrinsic conduction and a lattice defect; 2) in the high-temperature region, the conductivity increases with increasing temperature. Also, the activation energy calculated using the Arrhenius equation [17] was found to be 0.4 eV. According to Yun Wu et al. [18] , the activation energy for SrBi₂Ta₂O₉ is close to 1 eV. This difference, it may be due to the bond dissociation energy (enthalpy change) for a bond Ce-O (795 kJ/mole) is higher than Bi-O (343 kJ/mole) [19] .

Figure 2

Raman spectrum of SBCT sample

Figure 3

SEM images of sintered pellet of SBCT ceramic

Figure 4

Frequency dependence of the dielectric of constant and loss tangent measured at room temperature of SBCT ceramic

Figure 5

Variation of (b) dielectric constant (ε') and (a) dielectric loss (tanδ) respectively with temperature of SBCT ceramic

Figure 6

(a) Inverse dielectric constant (ε') as function of temperature at 100 Hz; (b) ln(1/ε' − ) vs. ln(1/T − 1/T C ) at 100 Hz of SBCT ceramic

Figure 7

The variation of conductivity in SBCT ceramic as a function of temperature

4. Conclusion

SrBi_1.8Ce_0.2Ta₂O₉ was prepared by solid state reaction route. XRD analysis in SrBi_1.8Ce_0.2Ta₂O₉ showed the orthorhombic crystal structure. The Raman study confirms the XRD result. Plate-like structure and poor microstructure were observed from the SEM figures. The point of view dielectric measurements, a normal ferroelectric is observed and the activation energy calculated is assumed to the chemical bond.

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