Electron Paramagnetic Resonance Study on Oxygen Vacancies and Site Occupations in Mg-Doped BaTiO₃ Ceramics

Nominal (Ba_1-xMg_x)TiO₃ (x = 0.015) (BM1T) and (Ba_1-xMg_x)TiO₃ (x = 0.03-0.20) (BMT) ceramics were prepared by the mixed-oxide route at sintering temperatures (T_s) of 1200-1400 °C and 1200 °C, respectively. The solubility limit of Mg²⁺ in BMT was determined by XRD to be x = 0.05, and evidence was found for occupation of the A site by Mg²⁺. Electron paramagnetic resonance (EPR) was employed as a key technique to investigate the effect of T_s on oxygen vacancies in BM1T. The structure of BM1T changed from pseudocubic at T_s = 1200 °C to tetragonal at 1300 °C to mixed phases of hexagonal and tetragonal at 1400 °C. When T_s ≥ 1300 °C, a g = 1.956 EPR signal was observed at T = -188 °C and assigned as ionized oxygen vacancies. Mg²⁺ exhibited amphoteric behavior of substituting for the double cation sites. When T_s = 1400 °C, B-site Mg²⁺ and oxygen vacancies mainly existed in the hexagonal phase and A-site Mg²⁺ was dominant in the tetragonal phase. The higher tan δ was attributed to the higher concentrations of oxygen vacancies and Ti³⁺ in the hexagonal phase.