Alkali-metal-fulleride phase equilibria

The binary phase diagrams of the alkali-metal fullerides, A_xC₆₀, 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 K_xC₆₀ phase diagram extending to vaporization temperatures is proposed.