Point Defect Characteristics of High-k Double Rare-Earth-Doped BaTiO₃ Ceramics with Diffuse Phase Transition by Electron Spin Resonance

The temperature dependence of the electron spin resonance (ESR) signals of high-k (Ba_1-xLa_x)(Ti_1-x/4-0.05Ce_0.05)O₃ ceramics (BLTC, x = 0.01–0.05), showing a strong diffuse phase transition (DPT) behavior, were examined by the powder ESR technique. Undoped BaTiO₃ (BT) and (Ba_1-xLa_x)Ti_1-x/4O₃ (BL4T, x = 0.04), both of which show a first-order phase transition (FPT), and Ba(Ti_1-yCe_y)O₃ (BTC5, y = 0.05) ceramics with DPT, were also investigated with ESR for comparison. A signal with g = 1.974, which is associated with the intrinsic point defect (V''_Ba) in BT, disappeared in both BLTC and BL4T; whereas, the signal with g = 2.004 was activated markedly and moderately when T > T_C (the Curie point) for BT and BL4T, respectively. BLTC and BL4T ceramics show only a signal with g = 2.004, indexed as Ti-vacancy defects (V_Ti''''). The signal intensity being nearly independent of temperature and La content (except for x = 0.01) is a characteristic of point defects in BLTC with DPT and provides the evidence that Ti vacancies are the predominant compensation mode in BLTC.