Abstract
The nature of the elusive muonium centre in sulphur is re-examined in the light of newdata on its level-crossing resonance and spin–lattice relaxation. The aim is to providea model for the solid-state chemistry of interstitial hydrogen in this element,which is as yet unknown, as well as to solve one of the longest standing puzzles inμSR spectroscopy, namely the surprisingly strong depolarization of muons mimickingion-implanted protons in this innocuous non-magnetic material. The paramagneticmuonium (and by inference hydrogen) centre is confirmed to have the characterof a molecular radical, but with huge anisotropy at cryogenic temperaturesand a striking shift of the resonance at ordinary temperatures, the hyperfineparameters appearing to collapse and vanish towards the melting point. Newdensity-functional supercell calculations identify a number of possible structures for thedefect centre, including a novel form of bond-centred muonium in a closed-ringS8Mu complex. Simulations of the spin dynamics and fits to the spectra suggest a dynamicalequilibrium or chemical exchange between several configurations, with occupancy of thebond-centre site falling from unity at low cryogenic temperatures to zero near the meltingpoint.
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