NMR and computational modeling of Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates

We report studies of Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates by NMR and modeling in order to better understand the electronic and dynamical properties. For samples exhibiting both the type-I and type-VIII clathrate structures, we have measured $^{71}$Ga quadrupole NMR lineshapes. We used {\it ab initio} modeling to extract structural information, providing an estimate of the site-occupation preferences in these alloys. For the type-I material, the resulting preferred structures are similar to those obtained in type-I Ba$_{8}$Al$_{16}$Ge$_{30}$, and we also find that the calculated formation energies show a preference for local structures that are similar to those of the BaAlGe analog. NMR Knight shifts at high temperatures are constant indicating metallic behavior. {\it Ab initio} results are in partial agreement with the local distribution of this metallic behavior. However the NMR lineshapes and $T_{1}$ relaxation time show low temperature changes in type-I samples attributed to atomic dynamics, which we connect to the prominent rattling behavior in this clathrate. Supported by Robert A. Welch Foundation (Grant A-1526).