Magnetic resonance of fullerene solids and their compounds

Abstract

Nuclear magnetic resonance (NMR) measurements on the novel materials C 60 and C 70, and their compounds with the alkali metals, are reported and reviewed. A sharp change in the NMR relaxation time t 1 at 263 K with a hysteresis of 1 K corresponding to an orientational phase transition was found in sublimed C 60 powder. A similar abrupt change in T 1 −1 at 280 K was also found in sublimed C 70 powder, which is ascribed to the same kind of phase transition as found in C 60, although here it is restricted to uniaxial rotation around the long ellipsoidal axis and unlike free rotation in the case of C 60. At temperatures above 340 K however, three-dimensional free rotation can take place. A curious behavior of the ESR intensity was observed for the pure C 60 powder. The ESR intensity strongly decreased upon exposure of air instead of the expected increase upon exposure to oxygen and light. 13C NMR T 1, measurements on the three superconducting compounds were analyzed in terms of a relation between Tinc and the density-of-states at the Fermi energy.