Ammoniated alkali fullerides (ND₃)_xNaA₂C₆₀: Ammonia specific effects and superconductivity

The crystal structure of the superconducting (ND₃)_xNaA₂C₆₀ (0.7 ≤ × ≤ 1, A= K, Rb) fullerides (T_c= 6-15 K) has been studied by synchrotron X-ray and neutron powder diffraction. It is face-centered cubic (fcc) to low temperatures with Na⁺-ND₃ pairs residing in the octahedral interstices. These are disordered over the corners of two "interpenetrating" cubes with the Na⁺ ions and the N atoms displaced by ∼2.0 Å and ∼0.5 Å from the center of the site and statically disordered over the corners of the inner and outer cube, respectively. Close contacts between the D atoms of the ND₃ molecules and electron rich 6:6 C-C bonds of neighboring C₆₀ units provide the signature of weak N-D⋯π hydrogen-bonding interactions, which control the intermolecular packing in the crystal and may determine the unusual superconducting properties.