A study was made of the motional behavior of the tetrahedral interstitial muonium (MuT) the chemical analogue, and light isotope of neutral atomic H, in a 90% enriched 13C diamond. Muonium formed after the implantation of positive muons into diamond. In the 13C diamond, the muon spin relaxes due to the motion of MuT against the background of randomly oriented spins of the host nuclei. Spin-lattice relaxation of the initially spin-polarized muons, in their capacity as nuclei of the MuT atoms, was studied as a function of magnetic field (20mT to 0.4T) and temperature (10 to 400K). The extracted hop rate was about 1011Hz; indicating fast diffusion of MuT, as expected for such a light interstitial atom. Its approximately constant value over the entire measured temperature range suggested weakly scattered coherent tunnelling rather than incoherent or phonon-assisted tunnelling, consistent with the high Debye temperature of diamond and the shallow interstitial potential for neutral H or muonium. There were indications of increasing localization of the wave-packet, i.e., of an incipient change to incoherent tunnelling, towards higher temperatures. Thermal conversion of the mobile MuT state to the stable bond-centered muonium (MuBC) state prevented the diffusion of MuT from being explored beyond 400K.

Diffusion of Interstitial Muonium, MuT, in a 13C Diamond. D.Gxawu, I.Z.Machi, S.H.Connell, K.Bharuth-Ram, M.J.Sithole, S.F.J.Cox: Diamond and Related Materials, 2005, 14[3-7], 375-9