Structure and stability of Ba-Cu-Ge type-I clathrates

We have prepared samples of nominal type Ba8CuxGe46-x by induction melting and solid state reaction. Analysis shows that these materials form type-I clathrates, with a 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 behaviour of the Au and Ag analogues. Magnetization measurements showed a large diamagnetic response in the Ba-Cu-Ge clathrate. This behaviour is consistent with semiconducting or semimetallic behaviour and is similar to that of a number of intermetallic semiconductors.