NMR Studies of Ba8AlxGe46-x and Ba8GaxGe46-x Clathrates

We have prepared a series of clathrates of the general form, Ba$_{8}$Al$_{x}$Ge$_{46-x}$. X-ray measurements indicate that Al occupies predominantly the 24$k$ site, similar to previous observations for the Ga analogs. $^{27}$ NMR measurements using wide-line and MAS spectrometers indicate two different local Al sites, one with a relatively small quadrupole splitting, attributed to the 24$k$ site, and one with a larger quadrupole splitting attributed to Al adjacent to a vacancy. We also discuss the results of $ab$-$initio$ calculations supporting these conclusions. $T$-dependent NMR relaxation results show roughly Korringa-type behavior for both series of compounds for $x <$ 16, characteristic of heavily-doped semiconducting materials. The lowest-carrier density Ga$_{16}$ sample shows changes in $T_{1}$ and Knight shift characteristic of incipient localization at low temperatures, however analysis indicates that the carriers remain in the regular band rather than forming an impurity band. For Ga$_{x}$ clathrates synthesized from excess Ga, the NMR $T_{1}$ changes abruptly, indicative of a change in band-edge symmetry for $p$-type material. This work was supported by the Robert A. Welch Foundation (grant A-1526), and the National Science Foundation (DMR-0103455).