NMR-STUDIES ON ORIENTATIONAL ORDERING PHASE-TRANSITION IN NA2CSC60

Sodium-containing alkali metal fulleride, Na2CsC60, was studied by C-1 3 and Na-23 NMR below and above the face-centered cubic (fee) to simpl e 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 in teraction, the electric field gradient at Na sites, e(2)qQ/h, was esti mated to be 940+/-20 kHz and less than 0.1 kHz below and above T-m, re spectively. The detailed analysis suggests that Na atoms in sc phase h ave a covalent character with C-60 molecules. It is also suggested tha t Na2CsC60 is the first metallic plastic crystal.