T. Saito et al., NMR-STUDIES ON ORIENTATIONAL ORDERING PHASE-TRANSITION IN NA2CSC60, Journal of the Physical Society of Japan, 64(12), 1995, pp. 4513-4517
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.