Molecular dynamics of ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ radical in solid ${\mathrm{C}}_{60}$

The molecular dynamics and electronic structure of the ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ radical in crystalline ${\mathrm{C}}_{60}$ have been studied using muon spin rotation and relaxation. At room temperature ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ appears to be in a state of quasifree rotation. At the critical temperature ${\mathit{T}}_{\mathit{S}}$=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on ${\mathrm{C}}_{60}$, suggesting that the molecular dynamics of ${\mathrm{\mu }}^{+}$-${\mathrm{C}}_{60}$ are strongly coupled to those of its ${\mathrm{C}}_{60}$ neighbors.