We have performed 63Cu, 65Cu, and 137Ba 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 ² x ² 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.
Click here for link to full text of paper.