Structural phase transition in the ammoniated alkaliC60compound(NH3)K3C60

Abstract

X-ray diffraction measurements of $({\mathrm{NH}}_{3}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$ have revealed a structural phase transition at ${T}_{s}=150 \mathrm{K},$ which is attributed to the orientational order-disorder transition of the $\mathrm{K}\ensuremath{-}{\mathrm{NH}}_{3}$ pair at the octahedral site of the ${\mathrm{C}}_{60}$ lattice. The low-temperature phase has a face-centered-orthorhombic structure derived by doubling the unit lattice vectors of the high-temperature phase along three axes. The superlattice intensity increases continuously below ${T}_{s},$ which means that the transition is second order. At $100 \mathrm{K}<T<{T}_{s},$ a negative thermal expansion is observed along the a and b axes. This is closely related to the contraction of K-N interatomic distance. We also discuss the relation between the superconductivity and the local symmetry of ${\mathrm{C}}_{60}$ in the crystal.