Structural phase transition in the ammoniated alkali C_<60> compound (NH_3)K_3C_<60>

X-ray diffraction measurements of (NH_3)K_3C_60 have revealed a structural phase transition at T_s=150 K, which is attributed to the orientational order-disorder transition of the K-NH_3 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.