Investigations of impurity centers, electrical resistivity and microstructure of BaTiO 3 ceramics doped with rareearth ions Y, La, Nd, Sm, Dy and Lu at concentrations x~0.001±0.005 were carried out. Electron paramagnetic resonance, X-ray diffraction and electron microscopy were used for measurements. The most intense EPR lines were shown to belong to paramagnetic complexes Fe 3z ±V O and Ti 3z ±Ln 3z (Ln~rare-earth ion, V O~o xygen vacancy). A change in symmetry of the center Fe 3z ±V O at the transition temperature from the ferroelectric to paraelectric phase has been revealed for the ®rst time. Measurements of the dependence of EPR line intensities and electrical resistivity with rare-earth ion concentrations were performed. The observed correlation in their behaviour showed an essential role of the identi®ed paramagnetic complexes in the appearance of BaTiO 3 ceramic semiconducting properties and the positive temperature coef®cient of resistance (PTCR) effect. The latter effect was at a maximum for x#x c where x c #0.002±0.003 is the critical rare-earth ion concentration which determines the excess charge compensation mechanism. Up to x c , the rare earths investigated, (except for the small ion Lu), substitute for barium, and the main compensation mechanism is an electronic mechanism. At high concentrations (xwx c ) in the case of large ions (e.g. La), substitution is at barium sites, with the creation of titanium vacancies, whereas intermediate ions (e.g. Y) begin to substitute for titanium. The in¯uence of impurities on the BaTiO 3 microstructure, including the grain sizes, is discussed.

The Ti3+ center, based on a regular Ti site perturbed by an oxygen vacancy (VO), is identified by electron spin resonance (ESR) in textured BaTiO3 films. The center shows tetragonal symmetry along cubic ⟨100⟩ axes with g-factors: g‖=1.997, g⊥=1.904. The spectrum of this defect disappeared after the film annealing at 700°C in an O2 atmosphere. We describe the observed spectrum as Ti3+–VO couple defects or F+ center, which have never been observed in bulk BaTiO3. ESR is thus a unique tool to identify oxygen-vacancy-related defects, which have a large effect on the performance of ferroelectric films.

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