Electronic properties and phase transitions ofRbC60andCsC60: Investigation by NMR spectroscopy

Abstract

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 ${\mathrm{cm}}^{\mathrm{-}1}$ between electron spins that are localized primarily on ${\mathrm{C}}_{60}^{\mathrm{-}}$ 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