Negative Thermal Expansion in the Mixed Valence Ytterbium Fulleride, Yb_2.75C₆₀

The powder synchrotron X-ray diffraction technique was used to study the temperature evolution of the structural properties of the mixed valence rare earth fulleride, Yb 2.75 C 60, in the temperature range 5−295 K. The large lattice expansion (negative thermal expansion) on cooling below 60 K provides an unambiguous signature of a temperature-induced valence transition of the Yb atoms. The transformation is of electronic origin and is driven by the coupling of the Yb 4f band and the t 1u band of C 60 . It is analogous to that observed at lower temperatures in Sm 2.75 C 60, but it is absent when the electronically active 4f sublattice is missing in the related alkaline earth fulleride, Ca 2.75 C 60 .