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 2-steps thermal decomposition of barium titanyl oxalate and post-heating treatment. The crystal structures of these particles were investigated as a function of the size using synchrotron radiation XRD measurement and Raman scattering measurement. As a result, the ferroelectric phase transition observed around 30 nm. Moreover, the temperature dependences of the crystal structures were investigated. As a result, the temperature of tetragonal-cubic phase transition was constant despite particle sizes, but the volume change temperature shifted to low temperature with decreasing particle sizes.
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