PHASE-TRANSITIONS IN NA2AC60 (A=CS, RB, AND K) FULLERIDES

The ternary system fullerides Na2AC60 (A = Cs, Rb, and K) are studied by differential scanning calorimetry in the temperature range of 100 to 450 K. All show reproducible phase transitions at the transition temperatures of 299 K (DELTAH = 2.5 +/- 0.5 J/g) for Na2CsC60, 313 K (DELTAH = 2.7 +/- 0.5 J/g) for Na2RbC60, and 305 K (DELTAH = 3.1 +/- 0.5 J /g) for Na2KC60. Orientational ordering of the fullerene unit is responsible for the observed phase transitions (T(sc-fcc)) from a high-temperature face-centered-cubic (fcc) to a low-temperature simple-cubic (sc) phase. The observed T(sc-fcc) and enthalpy can be compared to those for pristine C60 which has T(sc-fcc) = 256.5 K (DELTAH = 8 +/- 0.5 J/g). The observed behavior is discussed in terms of the balance between the C60(3-)C60(3-) intermolecular interaction and the C60(3-) - A+ Coulombic interaction as a function of lattice parameter.