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Composition

Fig. 8.5. Provisional binary phase diagram for K, C60 at 1 atm pressure. Experimentally determined temperatures are indicated, and unknown temperatures of predicted transformations are labeled Tu..., Ta. [8.106],

The experimental temperatures given for the phase boundaries in Fig. 8.5 are for K-doped C60, but the phase diagram is believed to be approximately applicable to the Rb^Q,, compounds also [8.79]. The phases corresponding to I^Qo and K^Qq are also observed for the Cs^C60 compounds. Some underlying assumptions for the phase diagram in Fig. 8.5 are that there is no liquid phase and that dilute amounts of K in Cgg will raise the solid-to-vapor transformation temperature, producing a peritectic-like transformation at Ta. The phase fields of K,C60, K3C60, K4C60, and K^Qq are drawn schematically, since the widths remain to be determined. Areas of two-phase coexistence for equilibrium samples are also shown in Fig. 8.5. It has been reported that the stability of K4C60 exceeds that of the other fullerides, based on decomposition experiments [8.106] and consistent with Fig. 8.5. The effect of C60 addition to a solid-state solution with potassium K(S) or in solution with liquid potassium K(L) is assumed to raise the S-L and L-V transformation temperatures relative to pure K. In the diagram, V, L, and S refer to the vapor, liquid, and solid phases of potassium. Between 425 K < T < T2, Fig. 8.5 shows phase separation into a C60 and a KjC60

Table 8.1

Radii for various metal ions."

Table 8.1

Radii for various metal ions."

Alkali metal

0(A)

Alkaline earth

r, (A)

Li+

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