The temperature dependence of positron annihilation in α-Hf

The equilibrium temperature dependence of the Doppler-broadened positron annihilation line-shape has been measured for α-Hf (6.8 at% Zr) in the temperature interval 298-1945 K. The data, in terms of characteristic peak (S) and wing (W) line-shape parameters, show three regions of behaviour. Region I, 298-750 K, shows a non-linear temperature response of W and, more obviously S. This associated with positron interactions with impurity-related defects. Region II, 750-1750 K, exhibits a well-behaved linear temperature dependence of both S and W: it is associated with thermal expansion effects. Region III, 1750-1945 K, shows an increasing temperature dependence, with respect to Region II, which is suggestive of positron-interactions with equilibrium vacancies. A relatively high vacancy formation energy (∼ 2.5 eV) is implied. (author). 21 refs., 2 tabs., 3 figs