The temperature dependent structural evolutions of RbxC60
(x = 3, 5, 6) and K4C60
were studied using both in-house andsynchrotron x-ray powder diffraction and thermal analysis techniques over a temperature range of 10K - 673K. The superconducting face centered-cubic (fcc) Rb3C60
and the body centered-tetragonal (bct) M4C60
(M = K, Rb) phases are found to be line compounds in this temperature range, while the body centered-cubic (bcc) phase forms a solid solution in which the solubility of vacant M sites increases with temperature. The orientation of the C60 molecules in the K4C60
phase was analyzed. A crystalline fcc Rb1C60
phase is stable only above room temperature.