POSITRON-ANNIHILATION IN BI(12)XO(20) (X=GE,SI,TI) STRUCTURES

A positron annihilation study has been carried out on Bi(12)XO(20) (X = Ge, Si, Ti) structures with the aim of identifying the nature of nat ive defects in these structures. The positron lifetimes have been meas ured at room temperature for the three structures, and a long componen t which depends on the type of cation has been obtained. Moreover, the oretical calculations have been carried out for the bulk lifetime in t hese materials and for several types of vacancy-like defect. From both the experimental and the theoretical results the native defects are t entatively identified as cation-vacancies complexes.