POSITRON LIFETIME CALCULATIONS OF HEXAGONAL METALS WITH THE TRUE GEOMETRY

The approaches to get lifetimes of the hexagonal metals have been perf ormed making use of the face centred cubic (f.c.c.) structure, instead of the real hexagonal one. Although some structural reasons allow thi s approximation, it is expected to fail when free volume is created in the solid. In this work, we have used the real hexagonal geometry to study these metals. For the cases of bulk, mono-, di- and trivacancies no appreciable differences have been observed in the calculated lifet imes. However, when studying clusters of vacancies in the real hexagon al structure and in the f.c.c. one, differences between 3.2% and 7.4% appear in lifetimes between these structures. This fact shows that the approximation of using the f.c.c. structure is not good for studies o f extended de defects. Positron lifetimes in these metals have also be en obtained by means of the local density and general gradient approxi mations, using for enhancement factors Arponen-Pajanne and Boronski-Ni eminen approximations. In alkaline earth and early transition metals t he calculations performed with the general gradient approximation do n ot improve the results obtained using Boronski-Nieminen's formula for the enhancement Factor.