Dielectric properties and defect chemistry of barium titanate ceramics co-doped R and Dy ions (R=Eu, Gd, Tb)

The structure, microstructure, dielectric behavior, point defects, mixed valence states, site occupations, and defect chemistry of nominal (Ba1−xRx)(Ti1−xDyx)O3 (x=0.06, R=Eu, Gd, Tb) (BRTD) ceramics were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, electron paramagnetic resonance (EPR), dielectric and resistivity measurements. BRTD exhibited a tetragonal perovskite structure, a higher ceramic density of ρr=91% of theoretical density, a diffuse phase transition (DPT) behavior, and a nonlinear shift in dielectric-peak temperature (Tm) with the kind of doping ions. The feature of mixed valence states associated with site occupation is responsible for this abnormal Tm-shift rate. BRTD with R=Tb exhibited a lowest dielectric loss of tan δ=0.025 at 1kHz and a nearly frequency-independent stability up to 107Hz at room temperature. A double-site compensation mode of GdBa•-DyTi' could be formed and led to a rapid Tm-shift rate of approximately −10°C/%(Gd-Dy). A symmetrical EPR signal at g=1.985 was designated as Gd3+ (4f7) Kramers ions. The mixed valence states of Ba-site Eu3+/Eu2+ (4f7) and Ba-site Tb3+/Ti-site Tb4+ (4f7) appeared in BRTD owing to the metastable nature of the 4f7 electron configuration. The Raman spectra in a high-wavenumber region of 1500–6700cm−1 were discovered and might be likely linked to the 4f7 doping ions, such as Eu2+, Gd3+, and Tb4+ ions. The defect chemistry is discussed.