X-ray diffraction studies show that the stable phase of the alkali ful
lerene RbC60 is orthorhombic (o-RbC60) below 350 K. C60 molecules form
chains along a with an unusually short spacing of 9.12 angstrom and m
agnetic properties suggest that o-RbC60 is a quasi-1 D metal with a tr
ansition to a spin density wave ground state at 50 K. The high tempera
ture fcc phase of RbC60 may be stabilized below 300 K by quenching fro
m 500 K; it is paramagnetic above 300 K and transforms into a nonmagne
tic ground state between 300 and 250 K.