Quasi-one-dimensional electronic structure in orthorhombic ${\mathrm{RbC}}_{60}$

X-ray diffraction studies show that the stable phase of the alkali fullerene ${\mathrm{RbC}}_{60}$ is orthorhombic (o-${\mathrm{RbC}}_{60}$) below 350 K. ${\mathrm{C}}_{60}$ molecules form chains along a with an unusually short spacing of 9.12 Å and magnetic properties suggest that o-${\mathrm{RbC}}_{60}$ is a quasi-1D metal with a transition to a spin density wave ground state at 50 K. The high temperature fcc phase of ${\mathrm{RbC}}_{60}$ may be stabilized below 300 K by quenching from 500 K; it is paramagnetic above 300 K and transforms into a nonmagnetic ground state beween 300 and 250 K.