Structural properties of the polymeric compounds LixNa(4 − x)C60

We have synthesized the series of nominally isoelectronic intercalated fullerene compounds LixNa(4 − x)C60 and investigated these by X-ray diffraction and Raman spectroscopy. All compounds are two-dimensional polymers, with the Li4C60 structure dominating for x > 1 and the Na4C60 structure for x ⩽ 1. We find almost no shift of the Ag Raman modes with x, indicating that the charge transfer is also practically independent of composition. We conclude that the Li4C60 structure is the lowest energy structure for all these compounds, and that the Na-rich compounds choose the Na4C60 structure because of geometrical factors connected with effective ion radii.