Core/shell structure of ferroelectric (Ba_0.94Ca _0.06)TiO₃ grains

The crystal structure of ferroelectric (Ba_0.94Ca _0.06)TiO₃ (BCT) with small grains on an average of 1-μm in size has been investigated by using high-energy synchrotron-radiation powder-diffraction. A core/shell structure model, in which the grain consists of a core with tetragonal symmetry covered all over with a thin tetragonal shell with low tetragonality, is proposed and provides a markedly contrast with the core/shell structure of pure BaTiO₃, which is generally known to have a shell with cubic symmetry. The higher permittivity of BCT is attributed to the characteristic shell structure with tetragonal symmetry. A change in the chemical bonding nature of BCT on the ferroelectric phase transition is clearly demonstrated in the electron charge density map.