Muon spin-rotation measurements, made at above room temperature, revealed the onset of a substantial paramagnetic shift from the muon Larmor frequency. The fraction of time spent as neutral paramagnetic muonium was close to unity at above 600K, and the capture rate of conduction electrons therefore greatly exceeded the effective ionization rate. These results were consistent with the configuration-coordinate model for the interplay between site and charge state, in which a site change was the bottleneck which impeded electron loss. However, it was difficult to obtain parameters which also accounted for available data on muon spin relaxation. Extensions to the model were suggested, with the aim of determining the site-change energy for neutral muonium between bond-centered and cage-centered sites. This elusive parameter was necessary for the determination of whether interstitial muonium or H atoms constituted negative-U centres.

The Neutral Fraction of Muonium in Silicon at High Temperatures. S.F.J.Cox, M.Charlton, P.Donnelly, A.Amato, A.Schenck: Journal of Physics - Condensed Matter, 2001, 13[10], 2155-62