Size and temperature induced phase transition behaviors of barium titanate nanoparticles

High density, almost impurity-free and defect-free barium titanate (BaTiO3) 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 degrees C separated into two kinds of phase transition behaviors with decreased particle sizes, i.e., the tetragonal-cubic phase transition temperature was constant at 135 degrees C 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 BaTiO3 nanoparticles was quite different from that of the BaTiO3 single crystals.