NMR study of Ba8Cu5Si(x)Ge(41-x) clathrate semiconductors

We have performed (63)Cu, (65)Cu, and (137)Ba NMR on Ba8Cu5SixGe41-x, a series of intermetallic clathrates known for their potential as thermoelectric materials, in order to investigate the electronic behavior of the samples. The spectra and spin-lattice relaxation times were measured at 77 K and 290 K for the entire composition range 0 ≤ x ≤ 41. Magnetic and quadrupole shifts and relaxation rates of the Cu NMR data were extracted, and thereby carrier-induced metallic contributions identified. The observed shifts change in a nonlinear way with increasing Si substitution: from x = 0 to about 20 the shifts are essentially constant, while approaching x = 41 they increase rapidly. At the same time, Ba NMR data indicate greater Ba-site participation in the conduction band in Ba8Cu5Si41 than in Ba8Cu5Ge41. The results indicate surprisingly little change in electronic features vs. Si content for most of the composition range, while Ba8Cu5Si41 exhibits enhanced hybridization and a more metallic framework than Ba8Cu5Ge41.