Muonium in Sulphur: μSR's Oldest Puzzle

New level–crossing resonance data shows the muonium defect centre in solid sulphur to have axial symmetry with hugedipolar anisotropyatlow cryogenic temperatures.Above100K,thetwo principalvaluesofthehyperfine tensor fall dramaticallyand pro rata, both apparently collapsingto zeroatthe melting point. Thefallis accompaniedby the onset of muon spin–lattice relaxation, visible on the microsecond timescale with low–field rates peaking around room temperature. In conjuction with old zero–field data, the low–T hyperfine parameters are determined accurately. New supercell density–functional calculations suggest their assignment to muonium at a bond–centre (BC) site in a closed–ringS8MuBC complex. The striking decrease of time–average parameters and the appearance of fluctuations causing relaxation are attributed to a dynamic equilibrium or chemical exchange with neutral configurations having muchlowerhyperfine coupling, accessedby smallcyclic displacements fromtheBC site.Time–average occupancy ofthissitefallswith temperatureandvanishesatthemeltingpoint.