NMR study of slow atomic motion in ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ clathrate

The clathrates feature large cages of silicon, germanium, or tin, with guest atoms in the cage centers. ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ 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 ${}^{71}\mathrm{Ga}$ NMR with a 9 T superconducting spectrometer down to 1.9 K. Knight shift and $T_1$ 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.

Figure 1

Line shapes at room temperature and 4.2 K, offset vertically for clarity.

Figure 2

Temperature dependence of ${}^{71}\mathrm{Ga}{T}_1$; experimental error bars are smaller than the symbol sizes. Dotted curve is a least squares fit to the Korringa Law. Inset: Knight shift of the center of mass, versus $T$.

Figure 3

${}^{71}\mathrm{Ga}$ NMR linewidth (square root of second moment) versus temperature, with activated fit described in text.

Figure 4

Spin echo decay rate at different temperatures, with fits described in text

Figure 5

CPMG measurements at different temperatures. Solid curves for 4.2, 32, and 77 K are fits described in text.

Figure 6

Correlation time versus temperature. (Solid line is the fitting result according to activation energy formula.)