ALKALI-METAL-FULLERIDE PHASE-EQUILIBRIA

The binary phase diagrams of the alkali-metal fullerides, AxC60, are discussed in light of recent experimental results concerning the A1C60 phases and their transformations. The Rb1C60 and Cs1C60 phases undergo diffusionless structural transformations upon cooling and their phase diagrams require solutions that differ from the eutectoid transformation of K1C60. X-ray photoelectron spectroscopy has been used to characterize these transformations. Phase diagrams incorporating two invariant reactions are proposed. The equilibrium vapor pressure of C60 has been determined at lower temperatures than prevously examined and the pure C60 phase diagram has been assembled for a large range of temperature and pressure. The predicted effects of solid-solution formation on the fcc to simple cubic transition of α-C60 are discussed in terms of eutectoid and peritectoid phase-diagram topologies. High-temperature decomposition with fractional sublimation has been used to gain insight into the relative thermal stabilities of the various compounds. Using this information and data from the literature, a KxC60 phase diagram extending to vaporization temperatures is proposed. © 1995 The American Physical Society.