Temperature dependence of local structure around Rb⁺ and Cs⁺ ions in alkali doped fullerenes

Rb and Cs K-edge EXAFS spectra have been measured in the temperature range from 10 to 290 K, in order to investigate the temperature dependence of local structure around Rb⁺ and Cs⁺ ions in various alkali doped fullerenes. It has been found that the alkali metal ions in Rb₃C₆₀, K₂RbC₆₀ and Na₂CsC₆₀ occupy the center of the tetrahedral and octahedral sites of C₆₀ crystals. The metal ion in the octahedral site exhibits a large fluctuation in comparison with that in the tetrahedral site. The increase in σ(2) of the t-site Rb-C for Rb₃C₆₀ has been observed around T_σ while the increase in σ(2) has not clearly been observed for the o-site Rb-C in K₂RbC₆₀. The local structure around Rb⁺ ion in non-superconducting alkali doped fullerene Rb₆C₆₀ has also been studied to compare with that in superconducting fullerenes.