This work was supported by the Department of Energy, Grants Nos. DE-FC02-86ER45254 and DE-FG05- 90ER75596, and by the NSF MRL Program under Grant No. DMR91-20668.
Physical Review B
X-ray diffraction, Calorimetry, Crystal lattices -- Mathematical models, Fullerenes -- Properties, Organic superconductors
X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60.
Zhu, Q. and Zhou, O. and Fischer, J. E. and McGhie, A. R. and Romanow, W. J. and Strongin, R. M. and Cichy, M. A. and Smith III, A. B. (1993). Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Physical Review B. 47(20): 13948--13951. DOI: 10.1103/PhysRevB.47.13948