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Hole-doped LiBC has become of great interest as a candidate for hightemperature superconductivity with its structural similarity to MgB 2 and as a possible cathode for rechargeable lithium batteries with the large graphenelike BC hexagons. The limitation on synthesis studies has detracted from the structural and electronic properties and application studies of Li x BC. Here, we present the successful synthesis of hole-doped Li 0.5 BC and its structural and electronic characterization using detailed structural modeling based on a unique stage-2 Daumas−Heŕold-type domain structure consistent with X-ray diffraction data. The strong intralayer motion of the guest Li island induces extraordinary structural properties, such as staging and staging disorder, which are confirmed by the compressibility results obtained from Li 0.85 BC (anisotropic cell compressibility with B 0 ∼ 134 GPa) and Li 0.48 BC (isotropic cell compressibility with B 0 ∼ 190 GPa). Li deficiency increases the conductivity; however, the temperature-dependent conductivity is dominated by the thermal excitation of carriers in a strongly disordered regime with well-defined localized states.

Near-edge x-ray absorption measurements from single-phase Yb 2.75 C 60 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 ͑EXAFS͒ 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 faces͒ 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 YBa 2 Cu 3 O 7 . Additional EXAFS and Raman data from Yb 2.75 C 60 heated above 670°C show the removal of local order around the Yb cations and the production of an amorphous carbidic phase containing Yb͑III͒. Our collective results, along with comparisons to analogous fulleride and organometallic compounds, provide insight into the distinctive structural features of Yb 2.75 C 60 and its unique distribution of anion charge. They also have general implications for a variety of other metal-doped fullerides.

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