Unusual thermal stability of a site-orderedMC60rocksalt structure (M=K, Rb, or Cs)

Abstract

X-ray diffraction and differential scanning calorimetry of ${\mathit{M}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$, 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 ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ 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 ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$.