  We have prepared samples of nominal type Ba(8)Cu(x)Ge(46-x) by induction
melting and solid state reaction. Analysis shows that these materials form
type-I clathrates, with copper content between x = 4.9 and 5.3, nearly
independent of the starting composition. We used x-ray powder diffraction and
single-crystal electron diffraction to confirm the cubic type-I clathrate
structure, while electron microprobe measurements confirmed the stability of
the x = 5 composition. This result differs from the corresponding Ag and Au
clathrates, and was not known previously due perhaps to the similar Cu and Ge
form factors in x-ray diffraction. The observed composition adheres very
tightly to a valence-counting scheme, in agreement with a Zintl-type stability
mechanism. This implies a gap in the electronic density of states, also in
contrast to the metallic behavior of the Au and Ag analogs. Magnetization
measurements showed a large diamagnetic response in the Ba-Cu-Ge clathrate.
This behavior is consistent with semiconducting or semimetallic behavior, and
is similar to that of a number of intermetallic semiconductors.
