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We present 13C NMR spin-lattice relaxation measurements (1/T1) in Na2CsC60and Rb3C60 from 10 to 700K. The large temperature range of this measurementallow to define unambiguously an increase of 1/T1T with increasing temperature,which is anomalous in a simple metallic picture, where the Korringa lawpredicts 1/T1T = cst. We attribute this increase to the existence of anadditional relaxation channel related to singlet-triplet (ST) excitations ofJahn-Teller distorted C60^{2-} and C60^{4-}. These units are formed within themetal on very short time scales (10^{-14} sec) that do not imply static chargesegregation. We show that the amplitude of the ST component depends directly onthe density of states, which indicates an interplay between metallic andmolecular excitations. Such an interaction is also revealed by the hightemperature behavior of Na2CsC60 and CsC60, that we then discuss. A divergencebetween the behaviors of 1/T1, the NMR shift and the ESR susceptibility isinterpreted as the result of a rapid increase of the lifetime of the chargecarriers, signaling a tendency to charge localization. In our analysis, theparticular stability of C60^{2n-} is then a common feature of all knownmetallic fullerides and allow to reconcile apparently contradicting propertiesof these systems.Comment: 10 pages, 7 figure

Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures