Structural phase transition in the ammoniated alkali ${\mathrm{C}}_{60}$ compound $\left({\mathrm{NH}}_3\right){\mathrm{K}}_3{\mathrm{C}}_{60}$

X-ray diffraction measurements of $\left({\mathrm{NH}}_3\right){\mathrm{K}}_3{\mathrm{C}}_{60}$ have revealed a structural phase transition at $T_s=150\mathrm{}\hspace{0.5em}\mathrm{K},$ which is attributed to the orientational order-disorder transition of the $\mathrm{K}-{\mathrm{NH}}_3$ 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 $100\hspace{0.5em}\mathrm{K}<T<\mathrm{}{T}_s,$ 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.