The quantum diffusion of muonium was studied, in crystals which had been doped with small amounts of Na, by using the muon spin relaxation technique. At low temperatures, the crystal volume could be divided into 2 parts. In one part, the muonium diffused rapidly as in pure KCl. In the other part, the muonium was effectively localized. The results were in accord with recent theories, on quantum diffusion in inhomogeneous media, which predicted that a volume existed around an impurity: inside which localization occurred, and outside of which the particle diffused freely.

R.Kadono, R.E.Kiefl, W.A.MacFarlane, S.R.Dunsiger: Physical Review B, 1996, 53[6], 3177-82