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Description:
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The clathrates feature large cages of silicon , germanium , or tin , with guest atoms
in the cage centers . The group IV clathrates are interesting because of their thermoelectric efficiency , and their glasslike thermal conductivity at low temperatures .
Clathrates show a variety of properties , and the motion of cage center atoms is not
well understood .
In Sr8Ga16Ge30 , we found that the slow atomic motion in the order 10 -5 s is
present in this system , which is much slower than what would be expected for standard
atomic dynamics . NMR studies of Sr8Ga16Ge30 showed that 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 . 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 . We can understand
all of our observations in terms of non -resonant atomic tunneling between asymmetric
sites within the cages , in the presence of disorder .
For Ba8Ga16Ge30 , the relaxation behavior (T1 ) deviates from the Korringa relation , and the Knight shift and linewidth change with temperature . Those results could be explained by carrier freezout , and the development of a dilute set of magnetic moments due to these localized carriers . For Ba8Ga16Ge30 samples made from
Ga flux , we observed different T1 and Knight shift behavior as compared to n type
material . This is due to the differences in carrier type among these different samples .
The p type sample has a smaller Knight shift and a slower relaxation rate than n
type samples made with the stoichiometric ratio , which is consistent with a change
in orbital symmetry between the conduction and valence bands .
WDS study for Ba8Al10Ge36 showed the existence of vacancies in the Al -deficient
samples , which results in some degree of ordering of Al occupation on the framework
sites . In Al NMR measurements on Ba8AlxGe40 -x with x = 12 to 16 , we found that
T1 of all Al samples follows the Korringa relation . The broadening of the single NMR
central peak of Ba8Al16Ge30 is due to the inhomogeneous Knight shifts for occupation
of different framework sites . For Ba8Al12Ge34 and Ba8Al13Ge33 , we observed two
peaks , and NMR results show that they are from distinct Al sites , while for each
peak , the inhomogeneous broadening is much smaller . The difference in lineshapes
we attributed to the existence of vacancies which we detected in the Al -deficient
materials , and we assign one of the two Al peaks to Al adjacent to a vacancy . |