Novel A15 phase in barium-doped fullerite

A new stable compound ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ is reported in the Ba-${\mathrm{C}}_{60}$ 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 Å. The Pm3¯n space group implies a perfect alternating orientational order for the ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$-cation interaction energy, and distortion-induced relaxational energy gains.