EXAFS study of local structure at the alkali ions in superconducting K2RbC60, K2CsC60, Rb2CsC60, and Rb3C60

We have studied the local site assignment as well as the structural and dynamic properties of the alkali ions in superconducting K2RbC60, K2CsC60, Rb2CsC60, and Rb3C60 compounds by x-ray absorption fine structure (EXAFS) measurements at the alkali ions K, Rb, and Cs in the temperature range 14 endash 300 K. In the ternary A2BC60 systems a strong preferential site occupation is observed, where the smaller alkali ions (A) are placed in tetrahedral sites and the larger ones (B) in octahedral sites. This site ordering allows a separate structural analysis of tetrahedral and octahedral sites. A quantitative analysis of the tetrahedral site (A) dopants yields the neighbor distances R(A-C), coordination numbers NC and second cumulants σ2. A similar analysis of the tetrahedral sites in Rb3C60 was performed by creating difference spectra with the K2RbC60 system. A multishell analysis of the octahedral sites (B) yields an off-center position of the B dopants. The off-center displacement scales with the size of the octahedral site and inversely with the ionic radius of the B dopant. A preferential displacement in the (1,1,1) direction is discussed in conjunction with possible carbon neighbor configurations at the octahedral sites. copyright 1996 The American Physical Society