NovelA15 phase in barium-doped fullerite

A new stable compound Ba₃{C}₆₀$ is reported in the Ba-${C}₆₀$ phase diagram. Rietveld refinement of x-ray powder diffraction data shows that this compound has the A15 structure with a lattice constant of 11.34 {}. The Pm3{}n space group implies a perfect alternating orientational order for the C₆₀ molecules, not previously observed in doped fullerite structures. The relative stability of the A15 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.