Structural and morphology studies of praseodymium-doped bismuth titanate prepared using a wet chemical route

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Abstract

Praseodymium-doped Bi4Ti3O12 (BIT) with various compositions of dopant, Pr (x = 0.5, 0.6, 0.7, 0.8) in Bi4−xPrxTi3O12 (BPT) were synthesized using a low temperature wet chemical technique. Powders calcined at 800 °C exhibit a single phase polycrystalline perovskite bismuth-layered structure. Randomly oriented plate-like structures were observed under Scanning Electron Microscope (SEM). A small amount of Pr doping (x = 0.5) resulted in dramatically reduced of grain size from 2 μm to less than 50 nm in which Pr plays the role as a grain growth inhibitor. However, by increasing the composition of Pr, bigger grain size of up to 1 μm was observed for x = 0.8 that was caused by diffusion of Pr in the perovskite structure. Dielectric properties showed that dielectric permittivity decreased with the addition of x = 0.50, and increasing with further addition of Pr. Dissipation factor (tan δ) followed the same trend as dielectric constant that was attributed to the grain size of the samples.

Introduction

Bismuth titanate, Bi4Ti3O12 (BIT) is a ferroelectric material that has potential applications as capacitors, pyroelectric sensors, high temperature piezoelectric or electrooptic devices [1], [2], [3]. BIT has a layered perovskite structure in which (Bi2Ti3O10)2− and (Bi2O2)2+ layers alternate, named Aurivillius structure [4]. BIT is a typical layer-structured perovskite-type ferroelectric material and promising for electronic applications due to its high dielectric permittivity, high Curie temperature, and high breakdown strength [5]. However, BIT shows fatigue and unexpectedly higher leakage electric current, which have appeared as obstacles for further technological adoption [6]. Recent studies revealed that Bi3+ ions in BIT structure could be substituted by trivalent lanthanide ions, and their ferroelectric properties were in reasonable ranges for memory applications. Lanthanide-substituted BIT is an attractive lead-free material for memory applications because of their relatively large remanent polarizations and fatigue-free characteristics [2], [5], [6]. It is worthwhile to study ferroelectric properties of praseodymium-substituted bismuth titanate, Bi4−xPrxTi3O12 (BPT), since Pr is a member of lanthanide that is expected to reduce the Bi and O vacancies in BIT. Furthermore, the six-coordinated ion radius of Pr3+ (0.113 nm) is similar to that of Bi3+ (0.117 nm), which suggest that an easy replacement of Pr to Bi without a large lattice distortion [2].

Since the last decades, BIT ceramics have been synthesized by various methods, including chemical coprecipitation, sol–gel process, hydrothermal synthesis, and conventional mixed oxide technique [7]. In most works, Bi-based powders are prepared using a conventional mixed oxide technique [8], [9], [10]. Therefore, high temperature sintering up to 1400 °C is essential to obtain a dense sample. However, Bi ions are volatile at high temperature above 1130 °C. Thus, sintering temperature need to be reduced to avoid Bi volatilization. It is widely recognized that chemical synthesis route provides a better homogeneity, fine powders and a lower formation temperature, due to molecular scale mixing in the precursor level [11]. The wet chemical technique is capable to produce fine particle size of the powder ranging from nanometers to submicron that require low sintering temperature. Thus, the problem of Bi volatilization could be eliminated. Furthermore, wet chemical technique is also less expensive, better control of stoichiometry and requires simple equipment set up. Hence, BIT as well as BPT have been synthesized in our laboratory by a wet chemical route, which was based on aqueous solutions of nitrate salts and a Ti(IV) isopropoxide. In this paper, the effects of substitution Pr3+ for Bi3+ in BIT structure on crystallization behaviour, surface morphology and electrical properties are reported.

Section snippets

Experimental details

Bi4Ti3O12 and Bi4−xPrxTi3O12 with different compositions of praseodymium (x = 0.5, 0.6, 0.7, 0.8) were synthesized using a wet chemical route. For the preparation of BPT, bismuth nitrate [Bi(NO3)3.5H2O], praseodymium nitrate [Pr(NO3)3·6H2O] and titanium isopropoxide [Ti[OCH(CH3)2]4] were used as starting materials for Bi, Pr and Ti, respectively. Bismuth nitrate and praseodymium nitrate were dissolved at 40 °C in 2-methoxyethanol [CH3OCH2CH2OH]. Separately, titanium isopropoxide was dissolved in

Results and discussion

To critically assess the effect of Pr substitution on the phase stability, both BPT with different compositions of Pr (x = 0.5, 0.6, 0.7, 0.8) and BIT powders were prepared, and their crystal structures were analyzed using XRD method. XRD spectra of the BPT and BIT powders calcined at 800 °C for 3 h are shown in Fig. 1. From the XRD patterns, it is clear that reflection peaks can be indexed as a bismuth-layered perovskite structure. BPT powders do not show any evidence of the formation of

Conclusion

Bismuth titanate (BIT) and Pr-doped bismuth titanate (BPT) with different composition of dopant were successfully synthesized using a wet chemical route. XRD spectra of calcined powders show a single phase bismuth-layered perovskite structure. This observation indicates that the praseodymium ions in the BPT do not form minority phases or segregate from the interior grain but dissolve into the perovskite lattice. BIT single phase possesses an orthorhombic structure. Substitution of Pr in Bi

Acknowledgements

The authors appreciate technical supports from School of Materials and Mineral Resources Engineering, and School of Electrical and Electronic Engineering Universiti Sains Malaysia. This research was supported by Science fund 305/Pbahan/6013357.

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