Guest Molecular Dynamics in Superconducting Clathrates Ag6O8AgX (X=\text{NO_{3}} and HF2): NMR Investigation

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We report a microscopic investigation of the superconducting clathrates Ag6O8AgX (X=\text{NO_{3}} 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 Ag6O8AgNO3 is confined in the (001) plane in a low-temperature tetragonal phase, while the uniaxial HF2 molecule in Ag6O8AgHF2 is preferentially directed along the c-axis in a low-temperature 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 T_{\text{c}} 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.
2011-07-15