Unusual Dielectric Relaxation in Lightly Doped n-Type Rhombohedral BaTi0.85Zr0.15O3:Ta Ferroelectric Ceramics

The dielectric properties of tantalum-doped BaTi 0.85 Zr 0.15 O 3 were investigated. The conventional solid-state reaction method was used to make the ceramic samples. The composition was close to the pinch point of the three structure phase transitions in BaTiO 3 . Powder X-ray diffraction results indicated that the samples were in a slightly distorted rhombohedral structure at room temperature, and the dielectric measurement showed that only a cubic-to-rhombohedral phase transition occurred. Instead of a sharp peak characteristic of a ferroelectric (FE) transition, the large dielectric response was similar to those in nonferroelectric perovskites CaCu 3 Ti 4 O 12, AFe 1/2 B 1/2 O 3 (A = Ba, Sr, Ca; B = Nb, Ta, Sb), and IBLC (internal barrier layer capacitance) BaTiO 3 -based FE ceramics. However, unlike its FE analogue, the conventional BaTiO 3 -based IBLC ceramics, our samples showed weaker temperature dependence of relative permittivity which spans over a wide temperature range. A Maxwell−Wagner-type relaxation in the FE state was observed. Moreover, another relaxation mode appears from the vicinity of the FE phase transition temperature T C and extends over a narrow temperature range, with unusual slowing down of relaxation rate as temperature rises, which is contrary to what is commonly observed. We associate these unusual phenomena with the slightly distorted rhombohedral structure of our samples and the presence of the grain boundary barrier layer where oxygen vacancies distribute nonhomogeneously.