NMR study of slow atomic motion in Sr₈Ga₁₆Ge₃₀ clathrate

The clathrates feature large cages of silicon, germanium, or tin, with guest atoms in the cage centers. Sr₈Ga₁₆Ge₃₀ clathrate is interesting because of its thermoelectric efficiency, and its glasslike thermal conductivity at low temperatures, indicating Sr atom hopping within the cages. We measured ⁷¹Ga NMR with a 9 T superconducting spectrometer down to 1.9 K. Knight shift and T₁ results are consistent with low density metallic behavior. The lineshapes exhibit changes consistent with motional narrowing at low temperatures, and this indicates unusually slow hopping rates. Fitting these line shape changes yielded an activation energy of about 7 K. To further investigate this behavior, we made a series of measurements using the Carr-Purcell-Meiboom-Gill NMR sequence. Fitting the results to a hopping model yielded an activation energy of 4.6 K, consistent with the line shape result. We can understand all of our observations in terms of nonresonant atomic tunneling between asymmetric sites within the cages, in the presence of disorder.