POSITRON LIFETIME AND DEFECTS IN Y-DOPED (BA0.6SR0.4)TIO3 CERAMICS

The positron lifetime spectra in Y-doped (Ba0.6Sr0.4)TiO3 ceramics have been measured at room temperature for Y contents of 0-0.8 at. %. It was found from the experiments that with increasing Y content, the variation of the positron lifetime parameters is nonmonotonic, and the Y content corresponding to the knee is about 0.7 at. %, which is much larger than the La content corresponding to the knee in the La-doped BaTiO2 (TANG 1994a). The experimental results show that the Y ions substitute predominantly for Sr ions, and this substitution induces a large number of Sr vacancies; when the Y content increases to about 0.7 at. %, the Y ions begin to substitute for Ba ions, and then the associated defects (2 YBaV ''(Sr)) and (2 YSrV ''(Ba)) form gradually.