Electronic properties and phase transitions of RbC 60 and CsC 60 : Investigation by NMR spectroscopy

We report the results of $^{133}\mathrm{Cs}$, $^{87}\mathrm{Rb}$, and $^{13}\mathrm{C}$ NMR measurements on the alkali-metal fullerides ${\mathrm{CsC}}_{60}$ and ${\mathrm{RbC}}_{60}$. Measurements of NMR spectra and spin-lattice relaxation rates provide clear evidence for phase transitions near 300 K in both compounds, with substantial changes in their electronic properties at the phase transitions. The high-temperature phases are paramagnets, with an exchange coupling of roughly 2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ between electron spins that are localized primarily on ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ ions. The $^{133}\mathrm{Cs}$ and $^{87}\mathrm{Rb}$ NMR frequency shifts and relaxation rates are determined by Fermi contact hyperfine couplings to the electron spins. The $^{13}\mathrm{C}$ relaxation rates are determined by dipolar hyperfine couplings. The electron-spin susceptibility is greatly reduced in the low-temperature phases