Thermal ionisation of bond-centred muonium in diamond?

A positive muon implanted into an insulator or semiconductor often binds an electron to form muonium (μ+e-≡Mu), a paramagnetic “atom” electronically similar to hydrogen. Evidence was sought for thermal ionisation of bond-centred muonium (MuBC) in natural type Ia single crystal diamond using the longitudinal magnetic field muon spin relaxation (LF-μSR) and a similar study was made of the diamagnetic state and tetrahedral muonium (MuT) using the transverse field muon spin rotation (TF-μSR) techniques. Optical characterisation methods revealed the presence of A, B, P1, and P2 centres in the samples. The MuBC state in diamond is easily observed and there is a good correlation between theoretical and experimental hyperfine parameters [J.W. Schneider, et al., Phys. Rev. Lett. 71(4) (1993) 557 [1]]. Curiously, despite its predicted stability, the bond-centred hydrogen state has not yet been observed in diamond. Following the discovery of hydrogen dopant states in certain wide band gap metal oxides, and the possibility of hydrogen related molecular dopants in diamond, the study of hydrogen in diamond is important. Although it is evident from its hyperfine parameters that MuBC is not a shallow donor, the question still arises as to where the MuBC state in diamond might lie in the band gap. Accordingly, measurements of high temperature stability of MuBC have been performed in a search for its possible ionisation. The longitudinal-field signature of MuBC begins to disappear above 1000 K. The possible onset of diffusion between bond-centre sites has still to be discounted but, interpreted as ionisation, the data place limits of 0.60 and 1.8 eV on the muonium donor depth in diamond.