POSITRON LIFETIME STUDY IN BATIO3 CERAMICS WITH DIFFERENT BA TI RATIOS/

The positron lifetime spectra in 0.25 at.% La-doped BaTiO3 with differ ent Ba/Ti ratios have been measured at room temperature. It was found from the experiments that with deviating from exact stoichiometry the positron lifetime tau(2) decreases and the intensity I-2 increases for Ba/Ti ratio smaller than stoichiometric and they have more significan tly similar changes for Ba/Ti ratio larger than stoichiometric. The no n-monotonic dependence of positron lifetime parameters on Ba/Ti ratio is regarded as that the tau(2) component is contributed together by po sitrons annihilating at the grain boundaries and Ba vacancies and/or T i vacancies, and small deviations from exact stoichiometry would be ex pected to increase the concentration of defects such as Ba vacancies f or Ba/Ti < 1 and Ti vacancies for Ba/Ti > 1.