Barium titanate (BaTiO_3) crystallites with various particle sizes from 10 to 500 nm were prepared by the 2-step thermal decomposition method of barium titanyl oxalate nanoparticles with particle size of 30 nm under vacuum. Various characterizations revealed that these particles were impurity-free, defect-free, dense BaTiO_3 particles. The powder dielectric measurement clarified that the size dependence of dielectric properties was significantly dependent of the size of intermediate compounds of Ba_2Ti_2O_5CO_3 at the formation of BaTiO_3 nanoparticles from Ba_2Ti_2O_5CO_3 particles. When the size of Ba_2Ti_2O_5CO_3 particles was 30 nm, the dielectric constant of BaTiO_3 particles with a size below 50 nm exhibited a maximum. On the other hand, when the size of Ba_2Ti_2O_5CO_3 particles was 200 μm, the dielectric constant of BaTiO3 particles with a size above 50 nm exhibited a maximum. To explain this size dependence, crystal structure and mesoscopic particle structure were precisely investigated using synchrotron radiation. As the results, the particles were always composed of three layers, i.e., (1) surface paraelectric layer, (2) bulk ferroelectric layer and (3) intermediate layer with gradient lattice structures between bulk and surface, and the thickness of surface cubic layer decreased with increasing the degree of vacuum during the preparation of BaTiO_3 nanoparticles. Thus, it was confirmed that the surface structure was very responsible factor for the dielectric property of the BaTiO_3 particles.
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