Crystal structure of dense nanocrystalline BaTiO3 ceramics
Introduction
BaTiO3 is one of the most widely used ferroelectric materials and has been extensively studied. It is a typical ABO3 perovskite-type material with a variety of crystal structure modifications. Depending on the transition temperature, BaTiO3 have five kinds of crystal systems, that is, hexagonal, cubic, tetragonal, orthorhombic and rhombohedral. For single-crystal and polycrystalline BaTiO3, the phase transition temperatures are 1432, 130, 5 and −90 °C, respectively. Among them, the tetragonal phase is stable at room temperature. However, as the crystallite size of BaTiO3 powder reduced to the nanoscale, some novel structural characters occurred [1], [2], [3], [4], [5]. For example, at room temperature, the cubic phase was observed and there is even a coexistence of hexagonal and tetragonal phases in BaTiO3 nanoparticle with a size of 40 nm. In nanocrystalline BaTiO3 ceramics, the different multiphase coexistences also occurred at various temperatures [6], [7], [8]. These phenomena indicated that the phase transition of BaTiO3 may be a function of the temperature and the crystallite size. In order to investigate the properties of nanocrystalline BaTiO3 ceramics, it is necessary to determine the crystal structure.
Although the size effects on the phase transition of BaTiO3 were investigated, the structural studies have not been sufficiently carried out even though the structural data including lattice parameters, atomic positions, and phase fraction, are closely related to the ferroelectric properties. In particular, the quantitative structural information is more important to control the ferroelectric properties for nanocrystalline BaTiO3 ceramics.
In this paper, based on the results of the Raman spectroscopy for the phase identification, we described in detail the structural study of nanocrystalline BaTiO3 ceramics by means of the Rietveld refinement using X-ray powder diffraction data.
Section snippets
Experimental
The raw BaTiO3 powder was synthesized by chemical processing [9]. The primary particle shape and size of raw BaTiO3 powder was observed by the transmission electron microscopy (TEM). Fig. 1 showed the TEM micrograph of the raw BaTiO3 powder. The TEM micrograph indicated that the average size of raw BaTiO3 powder was about 10 nm and the overall shape was nearly spherical. In order to densify the sample and inhibit the grain growth, a three-step pressure assisted sintering was adopted. The raw
Result and discussion
Fig. 2 showed the SEM image of BaTiO3 ceramics. From the SEM image, the sample exhibits a uniform grain size distribution. The grain size was estimated to be about 30 nm by the intercept line method. The grain size was also calculated using Scherrer's equation from the full width at half maximum (FWHM) value of the broadening (1 1 1) reflection of XRD pattern taken for BaTiO3 ceramics. The value of FWHM was 0.2874 and the grain size determined by the XRD method was about 28 nm. It was in agreement
Conclusions
Through the combination of the Raman spectrum and the Rietveld refinement using X-ray powder diffraction data, the structural parameters for the nanocrystalline BaTiO3 ceramics, such as mass fractions, lattice parameters, atomic coordinates, and isotropic thermal parameters, all were successfully determined. A multiphase coexistence of the tetragonal and orthorhombic phases was observed in 30 nm BaTiO3 ceramics. The weight fractions were estimated to be 66.3% and 33.7%, respectively. The lattice
Acknowledgements
This work is supported by the Ministry of Sciences and Technology of China through the 973-Project under Grant No. 2002CB613301.
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