Influence of impurities on the properties of rare-earth-doped barium-titanate ceramics

Investigations of impurity centers, electrical resistivity and microstructure of BaTiO3 ceramics doped with rare-earth 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 Fe3+-V-O and Ti3+-Ln(3+) (Ln = rare-earth ion, V-O = oxygen vacancy). A change in symmetry of the center Fe3+-V-O at the transition temperature from the ferroelectric to paraelectric phase has been revealed for the first 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 identified paramagnetic complexes in the appearance of BaTiO3 ceramic semiconducting properties and the positive temperature coefficient of resistance (PTCR) effect. The latter effect was at a maximum for x approximate to x(c) where x(c) approximate to 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 (x > x(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 influence of impurities on the BaTiO3 microstructure, including the grain sizes, is discussed.