Raman spectra and structural phase transition in nanocrystalline lead lanthanum titanate

The effects of particle size on the structure and Raman spectra of lead lanthanum titanate nanocrystalline particles are investigated. Samples with different particle sizes were prepared by the sol–gel process using different calcination temperatures. The sizes of the crystallites of particles were determined by x-ray diffraction with the aid of Scherrer’s equation. Raman vibration modes of samples with various particle sizes were also studied by Raman scattering at room temperature. The results show that the tetragonal distortion of the unit cell apparently decreases with decreasing particle size: the soft A1(1TO) phonon mode which is related to the spontaneous polarization of ferroelectric materials shifts toward lower frequency, and other peaks exhibit broadening and intensity changes with decreasing particle size. These are attributed to a phase transformation from the ferroelectric tetragonal to the paraelectric cubic phase. The above phenomena are related to a grain size-induced structural phase transition.