A reversible transformation has been observed in the ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ system, for 0<x<3, which is characterized by a redistribution of K ions within the fcc ${\mathrm{C}}_{60}$ lattice. Below ∼150 °C the tetrahedral and octahedral sites are occupied in the ratio found for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, 2 to 1, and C 1s photoemission spectra indicate phase separation into α-${\mathrm{C}}_{60}$ and ${\mathrm{K}}_3$${\mathrm{C}}_{60}$. Above ∼150 °C, only octahedral sites are occupied for x<1, which results in ${\mathrm{KC}}_{60}$ in the NaCl structure. Tetrahedral sites are then successively filled for x>1, reflecting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ formation and coexistence with ${\mathrm{KC}}_{60}$. These results suggest a eutectoid reaction with ${\mathrm{KC}}_{60}$ transforming into α-${\mathrm{C}}_{60}$ plus ${\mathrm{K}}_3$${\mathrm{C}}_{60}$.

• Received 17 February 1993

DOI:https://doi.org/10.1103/PhysRevB.47.10959