ELECTRONIC-PROPERTIES AND PHASE-TRANSITIONS OF RBC60 AND CSC60 - INVESTIGATION BY NMR-SPECTROSCOPY

We report the results of Cs-133, Rb-87, and C-13 NMR measurements on t he alkali-metal fullerides CsC60 and RbC60. Measurements of NMR spectr a and spin-lattice relaxation rates provide clear evidence for phase t ransitions near 300 K in both compounds, with substantial changes in t heir electronic properties at the phase transitions. The high-temperat ure phases are paramagnets, with an exchange coupling of roughly 2 cm- 1 between electron spins that are localized primarily on C60- ions. Th e Cs-133 and Rb-87 NMR frequency shifts and relaxation rates are deter mined by Fermi contact hyperfine couplings to the electron spins. The C-13 relaxation rates are determined by dipolar hyperfine couplings. T he electron-spin susceptibility is greatly reduced in the low-temperat ure phases