Locations of Rb ions in A2RbC60 (A=Na,K) superconductors

We have found by extended x-ray-absorption fine structure measurements that Rb ions in Na2RbC60 and K2RbC60 are not centered on octahedral interstitial sites. Interatomic distances between Rb ions in octahedral sites and carbon atoms are the same as the distances between Rb ions in tetrahedral sites and carbon atoms. Simple lattice-energy calculations also show that the ions shift away from the center of the octahedral sites. These results suggest that the interactions between C60 and ions in O sites are important in tuning the orientations of C60 as well as T-site ions.