INELASTIC NEUTRON-SCATTERING AND SUPERCONDUCTIVITY OF RB3C60(NH3)(X)

Ammonia addition to Rb3C60 forms compounds Rb3C60(NH3)(x) whose diffra ction signatures indicate structures all derived from a basic body cen tred unit cell with a C-60... C-60 distance averaging 10.42(5) Angstro m. The superconductivity above 10 K of these compounds rapidly decreas es with x but despite this, there are few changes in the C-60 internal mode density of states compared to Rb3C60 and quenching of the H-g an d T-1u modes up to about 80 meV is still strong. The repacking of the face centred cubic Rb3C60 cell and loss of superconductivity occur at ammonia compositions as low as about one mole per C-60, resulting in d isordered or lower symmetry structures. To probe this and the newly in troduced low frequency excitations, the ammonia sorption potential has been studied by high resolution neutron inelastic scattering and the analysis is consistent with a broad range of hindering barriers to rot ation about the Rb-NH3 threefold axis. In this range of potentials the distribution of rotational tunnelling excitations peaks at about +/-2 20 mu eV at 4 K. The densities of librational states has been assigned for both NH3 and ND3 intercalated materials and the threefold hinderi ng potential parameters for the average site determined.