Weiping Gou

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.

NMR experiments and electronic structure calculations in Type-I BaAlGe clathrates

We describe 27Al NMR experiments on Ba8AlxGe46-x type-I clathrates coupled with ab initio computational studies. For x=16, calculated spectra determined by the ab initio results gave good agreement with the measurements, with best-fitting configurations also corresponding to the computed lowest-energy atomic arrangements. Analysis of the NMR results showed that a distribution of Knight shifts dominates the central portion of the line. Computational results demonstrate that this stems from the large variation of carrier density on different sites. Al-deficient samples with x=12 and 13 exhibited a split central NMR peak, signaling two main local environments for Al ions, which we connected to the presence of vacancies. Modeling of the wide-line spectrum for x=12 indicates a configuration with more Al on the 24k site than for x=16. The results indicate the importance of nonbonding hybrids adjacent to the vacancies in the electronic structure near EF. We also address the static distortions from Pm-3n symmetry in these structures.

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.