POSITRON-ANNIHILATION STUDIES IN SIXSN1-X AND GEXSN1-X ALLOYS

The angular correlation of positron annihilation radiation (ACPAR) alo ng different crystallographic directions in SixSn1-x and GexSn1-x is c alculated. We observe that the electron-positron momentum density incr eases rapidly with increasing Si and Ge content. The computational tec hnique used here is based on the independent-particle model (IPM) coup led with the use of the electron pseudowave and the virtual crystal ap proximation (VCA) which incorporates compositional disorder as an effe ctive potential. We also present the variation of the positron lifetim e in these alloys.