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 GPa. A superstructure appears at ∼1 GPa as a result of orientational ordering of both the C₆₀ and K⁺_-NH₃ units (orthorhombic symmetry, space group Fddd). The pressure evolution of the lattice constants reveals a highly compressible, strongly anisotropic structure. The centre-to-centre interfullerene separations of ∼10.40 Å in the basal plane are comparable to those in the superconducting cubic analogue, RbCs₂C₆₀ and evidently sufficiently short to drive the system superconducting, despite its low crystal symmetry.