Grain size and grain boundary-related effects on the properties of nanocrystalline barium titanate ceramics

Dense nanocrystalline BaTiO3 ceramics with grain size (GS) down to 50 nm were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), impedance spectroscopy and Raman spectroscopy. A continuous reduction of the tetragonal distortion towards the pseudo-cubic state was obtained when the GS was reduced. Therefore, even the finest structure (ceramic with average GS of 50 nm) is still non-centrosymmetric. The dielectric constant (K) shows relative thermal stability in a large range of temperatures and is strongly depressed in the nanocrystalline ceramics, in comparison with the micrometric ones (K being below 1000 for the ceramic with 50 nm GS). The losses are smaller than 5% in the frequency range of 10(2)-10(6) Hz and temperatures below 200 °C. As the GS decreases, the structural phase transitions assume a more diffuse character. A decrease of the Curie temperature with reducing the GS was confirmed by X-ray, calorimetric and permittivity data. The Raman spectra collected for the range 80-800 K provided evidence for the presence of all the crystalline phases of BaTiO3, as in single-crystal and micrometric ceramics; a few differences can be attributed to GS effects and to the high density of the non-ferroelectric grain boundaries. Evidence for the different phase transitions were provided by the disappearance of some bands and by anomalies in positions and intensities of selected Raman modes. The overall properties of the nanocrystalline BaTiO3 ceramics can be explained as a combination of intrinsic effects, associated with the decrease of tetragonality and heat of transition with reducing GS, and extrinsic contributions due to the non-ferroelectric grain boundaries causing a "dilution" of the ferroelectric properties.