Structural Studies of (NH_3)K_3C_(60) at Ambient and High Pressure

摘要

The structural features of the ammoniated fulleride (NH_3)K_3C_(60) are studied by neutron and synchrotron X-ray powder diffraction at low temperature and high pressure, respectively. Below 150 K the system adopts an orthorhombic structure in space group Fddd. Rietveld refinements led to the determination of the exact orientation and geometry of both the C_(60) units (antiferrorotative ordering) and the ammonia molecules in the pseudo octahedral sites (antiferroelectric ordering). We also performed synchrotron X-ray powder diffraction measurements at pressures between ambient and 3.9 GPa. Near ～1 GPa, (NH_3)K_3C_(60) undergoes a structural phase transition from orthorhombic Fmmm (isostructural with the ambient pressure high temperature phase) to orthorhombic Fddd (isostructural with the ambient pressure low temperature phase).