NMR Evidence for C60 Configurational Fluctuations Around Na Sites in Na2CsC60

Abstract

The 23Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin–spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na2CsC60 in the temperature range of 10–300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T′ lines) similarly to Rb3C60 although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T′ sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C60 molecules. The exchange dynamics inferred from the spectra, spin–spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T′ splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.