High density, almost impurity-free and defect-free barium titanate (BaTiO_3) fine particles with various sizes from 20 to 1000 nm were prepared by the two-step thermal decomposition of barium titanyl oxalate and postheating treatment. The crystal structures of these particles were investigated as a function of the size and the temperature using synchrotron radiation x-ray diffraction (XRD) measurement. As a result, the size-induced ferroelectric (tetragonal-cubic) phase transition observed at around 30 nm. Moreover, the temperature dependence of the crystal structures revealed that one phase transition at 135℃ separated into two kinds of phase transition behaviors with decreased particle sizes, i.e., the tetragonal-cubic phase transition temperature was constant at 135℃ despite particle sizes while the cell volume expansion temperature shifted to low temperature with decreasing particle sizes. Moreover, the temperature dependence of Raman scattering spectra clarified that the temperature at a discontinuous change of damping factor was almost consisted with the cell volume expansion temperature from the XRD measurement. These results suggested that the phase transition behavior of the BaTiO_3 nanoparticles was quite different from that of the BaTiO_3 single crystals.
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