Distorted and inequivalently charged C-60 anions in Yb2.75C60

Near-edge x-ray absorption measurements from single-phase Yb2.75C60 establish that all of the Yb cations are divalent, meaning that the average negative formal charge of the C-60 anions is 5.5. Extended x-ray absorption fine structure (EXAFS) and Raman measurements also reveal that the C-60 anions are distorted in shape, despite their well-ordered local structure around the Yb cations. These findings are combined with a model to show that the charge transferred from Yb to C-60 is both anisotropically distributed on the anions and inequivalently partitioned among them within the unit cell. In the model, the inhomogeneously distributed transferred charge (which arises from the comparatively greater electron affinity of the C-60 pentagonal faces) interacts strongly with the divalent cations and leads to significant anion distortions. The redistribution of transferred charge is traceable to the three crystallographically different types of C-60 anions that are surrounded by 8, 10, or 12 Yb cations, a situation similar to the inequivalently coordinated and charged O anions in the high-T-c compound YBa2Cu3O7. Additional EXAFS and Raman data from Yb2.75C60 heated above 670 degrees C show the removal of local order around the Yb cations and the production of an amorphous carbidic phase containing Yb(III). Our collective results, along with comparisons to analogous fulleride and organometallic compounds, provide insight into the distinctive structural features of Yb2.75C60 and its unique distribution of anion charge. They also have general implications for a variety of other metal-doped fullerides.