Raman scattering studies on nanocrystalline BaTiO3 -: Part II -: consolidated polycrystalline ceramics

BaTiO3 dense ceramics with different grain sizes from 5.6 gm down to 35 nrn were thoroughly studied by Raman spectroscopy. The temperature characteristics of optical phonons were compared with those obtained for powders. The micrograined ceramic revealed the well-known spectrum profiles and transitions, typical for bulk BaTiO3. On the other hand, the Raman spectra obtained for a nanograined ceramic with an average grain size of 35 nm revealed a tetragonally distorted pure BaTiO3 phase showing a diffused phase transition behaviour with respect to temperature. Abnormality of phonon damping characteristics for the nanograined ceramic was demonstrated through comparison with powders with various crystallite sizes and the micrograined ceramic. The Curie temperature of the nartograined ceramic was estimated to be 105 degrees C from the temperature characteristic of a sharp peak at 307 cm(-1), which is one of the most specific tetragonal features for bulk BaTiO3. In the present study, local stabilization of the tetragonal phase in ultra-fine grains was experimentally demonstrated from comparison between the Raman spectroscopic results for powders and ceramics prepared through microemulsion-mediated synthesis. Rather long phonon mean free paths can exist even in such ultra-fine grains, but the phonon characteristics originating from various grains are diffused mainly because of the effect of internal stress. Copyright (c) 2007 John Wiley & Sons, Ltd.