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Zusammenfassung:
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 14–300 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.
