Effect of Ho/Mg ratio on formation of core-shell structure in BaTiO3 and on dielectric properties of BaTiO3 ceramics

Ceramics were prepared from hydrothermally-synthesized BaTiO3 (100 mol%), BaSiO3 (1.5 mol%), MnO (0.1 mol%) with Ho2O3 (0.5 mol%) and MgO (0.25-1 mol%). The compositions with Ho/Mg molar ratios of 4: 4,4 : 2, 4 : 1 were selected for examination. The ceramic samples were sintered at 1340degreesC (P-O2 < 10(-9) Pa) and, then, annealed at 1000degreesC (P-O2 < 30 Pa). The distribution of Ho and Mg in the samples was examined by transmission electron microscope (TEM) equipped with an element analyzer (EDS). Measurements of temperature dependence of dielectric permittivity over -55-155degreesC showed that the sample with a Ho/Mg ratio of 4/4 showed the least change of dielectric permittivity with temperature compared to the sample with a high Ho/Mg ratio of 4/1. The material with a Ho/Mg ratio of 4/1 is characterized by the deepest Ho diffusion into BaTiO3 and the slowest decrease of Ho concentration that is considered to result in a formation of a shell with considerable amounts of low-concentrated Ho-containing solid solutions.