Low-temperature positron annihilation measurements were performed on single crystals which contained either cation or anion vacancies. The temperature dependence of the S parameter was explained in terms of metastable self-trapped positrons which thermally hopped through the crystal lattice. The experimental results were analyzed by using a 3-state trapping model which assumed that transitions occurred, from both delocalized and self-trapped states, to deep trapped states at vacancies. The energy level of the self-trapped state was deduced to be 0.062eV above the delocalized state. The activation enthalpy for the hopping of self-trapped positrons appeared to depend upon the type of defect which was present.

M.A.Monge, R.Pareja, R.González, Y.Chen: Physical Review B, 1997, 55[1], 243-9