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

The positron lifetime spectra in 0.25 at.% La-doped BaTiO3 with different 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 significantly similar changes for Ba/Ti ratio larger than stoichiometric. The non-monotonic dependence of positron lifetime parameters on Ba/Ti ratio is regarded as that the tau(2) component is contributed together by positrons annihilating at the grain boundaries and Ba vacancies and/or Ti vacancies, and small deviations from exact stoichiometry would be expected to increase the concentration of defects such as Ba vacancies for Ba/Ti < 1 and Ti vacancies for Ba/Ti > 1.