Positron lifetime spectra and resistivity methods were applied to La-doped material at room temperature. The La content ranged from 0 to 0.8at%. It was found that, with increasing La content, the variations in positron lifetime parameters and resistivity were non-monotonic. The non-monotonic dependence of the positron lifetime parameters upon the La content was explained in terms of an associated-defect model, in which a small critical region of La content was involved. With increasing La content, Ba vacancies which were introduced by La doping appeared mainly in isolated form below the critical region. The associated defect, (2LaBa•VBa) formed quickly above the critical region. It was thought to be possible that the associated defect had no ability to trap positrons.

C.Q.Tang: Physical Review B, 1994, 50[14], 9774-7