Guest molecular dynamics in superconducting clathrates Ag₆O ₈AgX (X = NO₃ and HF₂): NMR investigation

We report a microscopic investigation of the superconducting clathrates Ag₆O₈AgX (X = NO₃ and HF₂), which are composed of Ag₆O₈ polyhedral cages and a guest molecule X inside the cages. Site-selective NMR measurements have revealed that the trigonal planar NO₃ molecule in Ag₆O ₈AgNO₃ is confined in the (001) plane in a low-temperature tetragonal phase, while the uniaxial HF₂ molecule in Ag ₆O₈AgHF₂ is preferentially directed along the c-axis in a lowtemperature phase. These results point to the fact that the distortion of the cages takes place so as to minimize the entropy of the molecular motions of the guest ions inside the cages as the cage volume contracts thermally upon cooling. Note that the difference in Tc between these compounds may be primarily ascribed to a difference in their Debye frequencies, but neither to anharmonic vibrations of the guest molecules called rattling nor to other possible remaining degrees of freedom in the molecular dynamics.