Pressure evolution of the structure of (NH3)K3C60

The structural features of the ammoniated (NH3)K3C60 fulleride are studied by synchrotron X-ray powder diffraction as a function of pressure to 6.7 GP a. A superstructure appears at similar to1 GPa as a result of orientational ordering of both the C-60 and K+ -NH3 units ( orthorhombic symmetry, space group Fddd). The pressure evolution of the lattice constants reveals a hig hly compressible, strongly anisotropic structure. The centre-to-centre inte rfullerene separations of similar to 10.40 Angstrom in the basal plane are comparable to those in the superconducting cubic analogue, RbCs2C60 and evi dently sufficiently short to drive the system superconducting, despite its low crystal symmetry.