Size dependence of dielectric properties for nm-sized barium titanate crystallites and its origin

Barium titanate (BaTiO3) crystallites with various particle sizes from 17 to 1,000 nm were prepared by the modified 2-step thermal decomposition method. Investigation of impurities in these particles by using both TG-DTA and FT-IR measurements revealed that no impurity was detected in the BaTiO3 lattice, while hydroxyl and carbonate groups were detected only on the surface. Moreover, their relative densities were always above 99 %. The dielectric constants of these powders were measured by using slurries by a modified powder dielectric-measurement method. As a result, the dielectric constant of BaTiO3 particles with a size of around 140 nm exhibited a maximum of around 5,000. By using a synchrotron-radiation X-ray powder experiment, the crystal structure of the BaTiO3 particles with sizes below 100 nm was investigated. As a result, in the BaTiO3 particles with sizes over 40 nm, it was confirmed that their crystal structure was assigned to 4 mm. In the particle size with a maximum dielectric constant of 5,000, its c/a ratio was smaller than 1.011. Finally, to explain the origin of the high dielectric constant, a model related to 'superparaelectric behavior' was proposed.