Abstract
We report a microscopic investigation of the superconducting clathrates Ag 6 O 8 Ag X ( X =NO 3 and HF 2 ), which are composed of Ag 6 O 8 polyhedral cages and a guest molecule X inside the cages. Site-selective NMR measurements have revealed that the trigonal planar NO 3 molecule in Ag 6 O 8 AgNO 3 is confined in the (001) plane in a low-temperature tetragonal phase, while the uniaxial HF 2 molecule in Ag 6 O 8 AgHF 2 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 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.
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