Impurity centers in a barium titanate ceramic doped with rare-earth ions

The ceramic BaTiO₃ doped with rare-earth ions Y, La, Nd, Sm, Lu, and Dy to 0.1–0.5 at. % was studied in the temperature range 160<T<480 K by ESR. Several ESR spectra were observed. The most intense spectra have g factors close to 5.5 and 1.96. The dependences of their intensity, g factor, and width on the concentration of rare-earth ions and temperature were studied. Analysis of the data obtained made it possible to determine the critical concentration of the rare-earth ions x _c =0.2–0.3 at. %. It is characterized by the fact that for x<x _c or x>x _c all rare-earth ions, except Lu, replace predominantly Ba⁴⁺ or Ti⁴⁺, respectively. Models of paramagnetic centers were established: Fe³⁺-V _O (g≈5.5) and Ti³⁺-Ln³⁺ (g≈1.96), where V _O is an oxygen vacancy and Ln is a rare-earth ion. An abrupt change in axial symmetry to cubic for a Fe₃₊-V _O center at a phase transition from the tetragonal into the cubic phase was observed for the first time. The role of new centers in the appearance of the posistor effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1838–1842 (October 1999)