BaTiO 3 powders with various crystallite sizes, which were prepared through microemulsion-mediated synthesis, were thoroughly studied by Raman spectroscopy. Clear evidence for the presence of the tetragonal phase was found for ultrafine powders with an average crystallite size above 30 nm. The lifetime of phonons that are specifically assigned to the tetragonal phase decreased with decreasing crystallite size below a critical size of 100 nm. In particles as fine as 100 nm, the short mean free path of phonons, mainly due to internal pressure, causes decoupling of the coupled A 1 (TO) phonons and a diffuse phase transition behaviour (T C = 115°C). Coupled A 1 (TO) phonons, which give a spectral dip at around 180 cm −1 and a lesser extent of diffuseness, were revealed for powders consisting of particles as large as 0.17 µm (T C = 123°C). Further coarsening upon annealing induced the formation of aggregates, resulting in the shift of phase transition points to higher temperatures for the rhombohedral to orthorhombic and the orthorhombic to tetragonal transitions and to lower temperatures for the tetragonal to cubic transition, respectively. Phase stability in powders is discussed by considering possible factors such as internal pressure in isolated particles and internal stress in aggregates.

Future improvements in the development of highly volume‐efficient multilayer ceramic capacitors (MLCC) require manufacturing processes that allow for a reduced dielectric thickness well below 1 μm. Obviously, such thin dielectric layers can only be produced if nanosized dielectric powders are applied and deposited by techniques more advanced than tape casting and screen printing. These processes require high‐purity, homogeneous, weakly agglomerated ultrafine powders. Over the past few years, significant progress has been made in the synthesis of nanocrystalline BaTiO3‐based powders for use in MLCC. The present review article aims to summarize the present state of the art with respect to some of the technically and industrially most relevant preparation routes, including methods based on solid‐state reactions and, more importantly, solution‐based approaches.

NaNbO(3) powders with various particle sizes (ranging from 30 nm to several microns) and well-controlled stoichiometry were obtained through microemulsion-mediated synthesis. The effect of particle size on the phase transformation of the prepared NaNbO(3) powders was studied using X-ray powder diffraction, Raman spectroscopy, and nuclear site group analysis based on these spectroscopic data. Coarsened particles exhibit an orthorhombic Pbcm (D(2h)(11), no. 57) structure corresponding to the bulk structure, as observed for single crystals or powders prepared by conventional solid-state reaction. The crystal symmetry of submicron powders was refined with the space group Pmc2(1) (C(2v)(2), no. 26). The reduced perovskite cell volumes of these submicron powders were most expanded compared to all the other structures. Fine particles with a diameter of less than 70 nm as measured from SEM observations showed an orthorhombic Pmma (D(2h)(5), no. 51) crystal symmetry. The perovskite formula cell of this structure was pseudocubic and was the most compact one. A possible mechanism of the phase transformation is suggested.

BaTiO 3 dense ceramics with different grain sizes from 5.6 µm down to 35 nm 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 BaTiO 3 . 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 BaTiO 3 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 nanograined ceramic was estimated to be 105°C from the temperature characteristic of a sharp peak at 307 cm −1 , which is one of the most specific tetragonal features for bulk BaTiO 3 . 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.