Dielectric properties and defect chemistry of La and Tb co-doped BaTiO3 ceramics

(Ba1−xLax)(Ti1−xTbx)O3 (0.03 ≤ x ≤ 0.20) (BLTT) and (Ba1−xLax)(Ti1−xTbx)O3-0.03nTb (x = 0.03, n = 0, 1, 3, 5, 7) (BL3TT-nT) ceramics were prepared using a mixed oxides method. The structure, microstructure, dielectric properties, valence state, and defect chemistry of BLTT and BL3TT-nT were investigated using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and dielectric temperature and frequency measurements. The solid solution limits of Tb in BLTT and BL3TT-nT were determined by XRD to be x = 0.15 and n = 3, respectively. The incorporation of the extra Tb ions in BLTT enhanced the relative density of the ceramic from 84% to 96%, accompanied by an improvement in dielectric permittivity. The defect chemistry of BLTT is discussed. Tb ions coexist in mixed-valence states, at the Ba-site as Tb3+, and at the Ti-site as Tb3+/Tb4+. The dielectric-peak temperature (Tm) decreased linearly at a rate of −19 °C/%(LaTb) for BLTT. Due to its higher room-temperature permittivity (ε′ = 2450) and lower loss (tan δ = 0.029), BLTT with x = 0.07 can be considered as a promising dielectric for X7U applications. The existence of a small number of metastable Tb4+ ions at Ti-sites and the predominant presence of La3+Tb3+ complexes at double sites are both responsible for the lower tan δ of BLTT.