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A guest-free germanium clathrate


The challenges associated with synthesizing expanded semiconductor frameworks with cage-like crystal structures continue to be of interest1,2. Filled low-density germanium and silicon framework structures have distinct properties that address important issues in thermoelectric phonon glass–electron crystals3, superconductivity4 and the possibility of Kondo insulators5. Interest in empty framework structures of silicon and germanium is motivated by their predicted wide optical bandgaps of the same magnitude as quantum dots and porous silicon, making them and their alloys promising materials for silicon-based optoelectronic devices6,7. Although almost-empty Na1-xSi136 has already been reported8,9, the synthesis of guest-free germanium clathrate has so far been unsuccessful. Here we report the high-yield synthesis and characteristics of germanium with the empty clathrate-II structure through the oxidation of Zintl anions in ionic liquids under ambient conditions. The approach demonstrates the potential of ionic liquids as media for the reactions of polar intermetallic phases.

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Figure 1: Crystal structure determination of □24Ge136.
Figure 2: High-resolution transmission electron microscopy of □24Ge136.
Figure 3: Chemical analysis of □ 24 Ge 136 by EELS.
Figure 4: Structure of empty clathrate-II □24Ge136.


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We thank R. Kniep and U. Schwarz for comments, R. Cardoso-Gil for XRPD measurements, G. Auffermann and U. Schmidt for chemical analysis, S. Müller for differential scanning calorimetry measurements, R. Koban for resistivity and susceptibility measurements, and P. Simon and W. Carrillo-Cabrera for discussion. A.M.G. thanks the PRF-ACS and the R. A. Welch Foundation for financial support. Author Contributions All authors contributed equally to this work. A.M.G., Z.T. and M.B. developed the synthetic method and prepared the clathrate material. Yu.G. solved the structure from X-ray diffraction data. R.R. performed electron microscopy investigations. W.S. made measurements of electrical conductivity and magnetic susceptibility, Z.T. performed diffuse reflectance measurements. All authors discussed the results and contributed to the final manuscript.

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Correspondence to Arnold M. Guloy or Yuri Grin.

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Reprints and permissions information is available at The authors declare no competing financial interests.

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Supplementary Notes

This file contains schematic representation of the synthesis of empty clathrate, crystallographic information, results of the measurements of electrical resistivity, magnetic susceptibility, optical absorption spectra, and additional references for used data evaluation methods and software. (DOC 956 kb)

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Guloy, A., Ramlau, R., Tang, Z. et al. A guest-free germanium clathrate. Nature 443, 320–323 (2006).

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