NMR studies on orientational ordering phase transition in Na2CsC60

Sodium-containing alkali metal fulleride, Na2CsC60, was studied by C-13 and Na-23 NMR below and above the face-centered cubic (fee) to simple cubic (sc) phase transition temperature, T-m. It is shown that the C-60 molecules rotate faster than the rotational correlation time of 1.6 x 10(-5) sec above similar to 140 K and 2.4 x 10(-9) sec around 300 K. Using Na-23 NMR spectra broadened by the second-order quadrupole interaction, the electric field gradient at Na sites, e(2)qQ/h, was estimated to be 940+/-20 kHz and less than 0.1 kHz below and above T-m, respectively. The detailed analysis suggests that Na atoms in sc phase have a covalent character with C-60 molecules. It is also suggested that Na2CsC60 is the first metallic plastic crystal.