Muonium in boron

A paramagnetic muonium centre is detected in elemental boron at cryogenic temperatures and characterized by hyperfine decoupling in longitudinal magnetic fields. The contact term is only 0.52 GHz or 12% of the free muonium value. An effective ionization energy of 0.1 eV is estimated from polarization transfer to the muon Larmor precession signal in transverse field, beginning around 100 K and complete by room temperature. AIMPRO density functional calculations identify candidate sites for monatomic hydrogen defect centres in the rather complex boron lattice and predict the local distribution of unpaired electron spin density. One of these, bridging between two boron atoms in the inter-icosahedral space, provides an adequate match to the muonium hyperfine parameters, as well as its relatively weak binding energy. Negative-U behaviour is predicted, implying that H0 centres are not thermodynamically stable. Identification of the high diamagnetic muon fraction found at all temperatures is also discussed.