It was noted that the presence of Si donors had a marked effect upon the charge state and diffusion of muonium at the tetrahedral interstitial site (MuTo), while it had a relatively weak effect upon bond-center muonium (MuBCo). In metallic Si-doped material, the highly mobile MuTo center which was observed in non-metallic material was replaced by a charged species (probably Mu-) which had a diffusion rate that was smaller than that of the MuTo center. The MuBCo center was metastable, and its transition to MuTo or Mu- centers (depending upon dopant concentration) was observed at 50 to 150K. The results also suggested that the MuTo state underwent fast spin-exchange interaction with conductive carriers before the transition: MuTo  Mu-.

R.Kadono, A.Matsushita, K.Nagamine, K.Nishiyama, K.H.Chow, R.F.Kiefl, A.MacFarlane, D.Schumann, S.Fujii, S.Tanigawa: Physical Review B, 1994, 50[3], 1999-2002