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 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.

The clathrates feature large cages of silicon, germanium, or tin, with guest atoms in the cage centers. Sr 8 Ga 16 Ge 30 clathrate is interesting because of its thermoelectric efficiency, and its glasslike thermal conductivity at low temperatures, indicating Sr atom hopping within the cages. We measured 71 Ga NMR with a 9 T superconducting spectrometer down to 1.9 K. Knight shift and T 1 results are consistent with low density metallic behavior. The lineshapes exhibit changes consistent with motional narrowing at low temperatures, and this indicates unusually slow hopping rates. Fitting these line shape changes yielded an activation energy of about 7 K. To further investigate this behavior, we made a series of measurements using the Carr-Purcell-Meiboom-Gill NMR sequence. Fitting the results to a hopping model yielded an activation energy of 4.6 K, consistent with the line shape result. We can understand all of our observations in terms of nonresonant atomic tunneling between asymmetric sites within the cages, in the presence of disorder.

We report the detection of widespread CH 2 OHCHO and HOCH 2 CH 2 OH emission in Galactic center giant molecular cloud Sagittarius B2 using the Shanghai Tianma 65m Radio Telescope. Our observations show for the first time that the spatial distribution of these two important prebiotic molecules extends over 15 arc-minutes, corresponding to a linear size of approximately 36 pc. These two molecules are not just distributed in or near the hot cores. The abundance of these two molecules seems to decrease from the cold outer region to the central region associated with star-formation activity. Results present here suggest that these two molecules are likely to form through a low temperature process. Recent theoretical and experimental studies demonstrated that prebiotic molecules can be efficiently formed in icy grain mantles through several pathways. However, these complex ice features cannot be directly observed, and most constraints on the ice compositions come from millimeter observations of desorbed ice chemistry products. These results, combined with laboratory studies, strongly support the existence of abundant prebiotic molecules in ices.

We describe 27 Al NMR experiments on Ba 8 Al x Ge 46−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 E F . We also address the static distortions from Pm3n symmetry in these structures.