A brief review was presented of the theory of positron diffusivity, D+, in condensed matter. Emphasis was placed upon the role which was played by so-called e+-polarons, and upon the interaction of positrons and positronium with optical phonons; including the thermalization of positronium. In the case of D+ in liquid metals, 2 possibilities were predicted. These were either a low effective positron mass and a high positron diffusivity (with a T½ temperature dependence) or a high positron mass and a low positron diffusivity; leading to a stronger Arrhenius-type temperature dependence. Recent slow-positron data indicated that the first possibility occurred in liquid Bi, while the second one occurred in liquid Ga.

A.Seeger: Applied Surface Science, 1995, 85, 8-16