Structural phase transition in the ammoniated alkali C60 compound (NH3)K3C60

X-ray diffraction measurements of (NH3)K3C60 have revealed a structural phase transition at T-s = 150 K, which is attributed to the orientational order-disorder transition of the K-NH3 pair at the octahedral site of the 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 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 C-60 in the crystal.