A nuclear magnetic resonance probe of group IV clathrates

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Title: A nuclear magnetic resonance probe of group IV clathrates
Author: Gou, Weiping
Abstract: 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 .
URI: http : / /hdl .handle .net /1969 .1 /86058
Date: 2008-10-10

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A nuclear magnetic resonance probe of group IV clathrates. Available electronically from http : / /hdl .handle .net /1969 .1 /86058 .

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