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

Investigations of impurity centers, electrical resistivity and microstructu re of BaTiO3 ceramics doped with rare-earth ions Y, La, Nd, Sm, Dy and Lu a t concentrations x = 0.001-0.005 were carried out. Electron paramagnetic re sonance, X-ray diffraction and electron microscopy were used for measuremen ts. The most intense EPR lines were shown to belong to paramagnetic complex es 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 fro m the ferroelectric to paraelectric phase has been revealed for the first t ime. 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 p roperties and the positive temperature coefficient of resistance (PTCR) eff ect. 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 w hich determines the excess charge compensation mechanism. Up to x(c), the r are earths investigated, (except for the small ion Lu), substitute for bari um, and the main compensation mechanism is an electronic mechanism. At high concentrations (x > x(c)) in the case of large ions (e.g. La), substitutio n is at barium sites, with the creation of titanium vacancies, whereas inte rmediate ions (e.g. Y) begin to substitute for titanium. The influence of i mpurities on the BaTiO3 microstructure, including the grain sizes, is discu ssed.