NMR Studies on Orientational Ordering Phase Transition in Na₂CsC₆₀

Sodium-containing alkali metal fullende, Na₂CsC₆₀, was studied by ¹³C and ²³Na NMR below and above the face-centered cubic (fcc) to simple cubic (sc) phase transition temperature, T_m. It is shown that the molecules rotate faster than the rotational correlation time of 1.6 × 10⁻⁵ sec above ∼ 140 K and 2.4 × 10⁻⁹ sec around 300 K. Using ²³Na NMR spectra broadened by the second-order quadrupole interaction, the electric field gradient at Na sites, e²qQ/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 molecules. It is also suggested that Na₂CsC₆₀ is the first metallic plastic crystal.