NONMONOTONIC DEPENDENCE OF THE POSITRON LIFETIMES ON THE DOPANT CONTENT IN LA-DOPED BATIO3

The positron-lifetime spectra and the resistivity in La-doped BaTiO3 have been measured at room temperature for La content of 0-0.8 at. %. It was found from the experiments that with increasing La content, the variation of both the positron-lifetime parameters and the resistivity are nonmonotonic. The nonmonotonic dependence of positron-lifetime parameters on the La content is explained by an associated-defect model, in which a small critical region of La content is proposed. With an increase of La content Ba vacancies induced by La doping appear mainly in isolated form below the critical region, while above the critical region the associated detect (2La(Ba).V(Ba)'') forms quickly. The associated defect may have no ability to trap positrons.