NMR Studies on Orientational Ordering Phase Transition in Na₂CsC₆₀

抄録

Sodium-containing alkali metal fulleride, 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 C₆₀ 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 C₆₀ molecules. It is also suggested that Na₂CsC₆₀ is the first metallic plastic crystal.