THE PHASE-DIAGRAM AND THERMODYNAMICS OF SODIUM FULLERIDE FROM ELECTROCHEMICAL MEASUREMENTS

Phase relations and thermodynamic properties of sodium fulleride (NaxC 60) have been measured up to x = 6 by the electromotive force (EMF) me thod. Na beta-alumina was used as the solid electrolyte and Na was add ed to the cell containing a fixed amount of fullerene by coulometric t itration. The phase diagram is characterized by alternate single-phase regions (EMF varying with composition) and two-phase regions (EMF pla teaus). At 599 K, the Gibbs energies of formation of Na3C60 and Na6C60 are 113.2 and 94.7 kJ/mole of Na, respectively. A model is proposed f or the Na activity (or EMF) as a function of composition based on near est-neighbour interactions, orientational disorder and a random distri bution of Na ions. The ionic size and the hindrance of rotation of C60 accounts for the miscibility gap in alkali fullerides in this model.