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 (H=2.50.5 J/g) for Na2CsC60, 313 K (H=2.70.5 J/g) for Na2RbC60, and 305 K (H=3.10.5 J/g) for Na2KC60. Orientational ordering of the fullerene unit is responsible for the observed phase transitions (Tsc-fcc) from a high-temperature face-centered-cubic (fcc) to a low-temperature simple-cubic (sc) phase. The observed Tsc-fcc and enthalpy can be compared to those for pristine C60 which has Tsc-fcc=256.5 K (H=80.5 J/g). The observed behavior is discussed in terms of the balance between the C603 - C603- intermolecular interaction and the C603 - A+ Coulombic interaction as a function of lattice parameter.
ASJC Scopus subject areas