Solid state 19F NMR study of acceptor-doped fullerenes (MF6)2C60 (M=As, Sb)

We report on temperature-dependent 19F NMR measurements of solid C60 intercalated with AsF5 and SbF5 acceptor species. Structure, molecular dynamics and possible phase transitions in the compounds are studied. The temperature dependence of the second moment, S2, of (AsF6)2C60 is typical for activation of molecular mobility on heating. The reorientation of the (AsF6)- complex is random and lasts at least down to 67 K. The temperature dependence of S2 for (SbF6)2C60 showing significant change in the range of 200-270 K is discussed. Further narrowing of the 19F spectra of (SbF6)2C60 on heating occurs around 320 K. 19F spectra at 7.04 T resolve two resonances corresponding to different fluorine species. Suggesting that these species randomly occupy two different interstitial sites, we give reasons for the absence of superconductivity down to 5 K in the compounds under study