Ji Chi

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.

Transition‐Metal Substitution in Semiconducting Ba8Ga16Ge30 Clathrates

We report magnetization, transport, and 71Ga NMR properties of Ni‐ and Fe‐substituted type‐I Ba6Ge30Ga16 clathrates. While Fe substitutes partially into the lattice, Ni substitutes readily for Ga, and we prepared samples with composition up to Ba8Ge30Ga10Ni6. Results indicate low‐density metallic behavior for all samples, with little change in electronic structure caused by Fe or Ni substitution. NMR relaxation behavior (T1) and Knight shifts in all samples exhibited a constant Korringa product (K2T1T) characteristic of metallic behavior with small exchange enhancement.