Inelastic neutron scattering and superconductivity of Rb3C60(NH3)x

Abstract

Ammonia addition to Rb3C60 forms compounds Rb3C60(NH3)x whose diffraction signatures indicate structures all derived from a basic body centred unit cell with a C60…C60 distance averaging 10·42(5) Å. The superconductivity above 10 K of these compounds rapidly decreases with x but despite this, there are few changes in the C60 internal mode density of states compared to Rb3C60 and quenching of the Hg and T1u 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 C60, resulting in disordered or lower symmetry structures. To probe this and the newly introduced 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 rotation about the Rb-NH3 threefold axis. In this range of potentials the distribution of rotational tunnelling excitations peaks at about ± 220 μeV at 4 K. The densities of librational states has been assigned for both NH3 and ND3 intercalated materials and the threefold hindering potential parameters for the average site determined. Show more