NOVEL A15 PHASE IN BARIUM-DOPED FULLERITE

A new stable compound Ba3C60 is reported in the Ba-C60 phase diagram. Rietveld refinement of x-ray powder diffraction data shows that this compound has the A 15 structure with a lattice constant of 11.34 angstrom. The Pm3nBAR space group implies a perfect alternating orientational order for the C60 molecules, not previously observed in doped fullerite structures. The relative stability of the A 15 phase over the fully intercalated fcc structure can be explained by a simple model involving the Madelung energy differences, orientation dependence of the C60-cation interaction energy, and distortion-induced relaxational energy gains.