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Publication Detail
Time-resolved in situ synchrotron X-ray diffraction studies of type 1 silicon clathrate formation
  • Publication Type:
    Journal article
  • Publication Sub Type:
    Journal Article
  • Authors:
    Hutchins PT, Leynaud O, O'Dell LA, Smith ME, Barnes P, McMillan PF
  • Publication date:
  • Pagination:
    5160, 5167
  • Journal:
    Chemistry of Materials
  • Volume:
  • Issue:
  • Status:
  • Print ISSN:
Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na 8 Si 46 and Na 2 Ba 6 Si 46 . Although their structure and properties are well described, there is little understanding of the formation mechanism. Na 8 Si 46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si 4 4- polyanions. In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25-500 °C at 8 °C/min) under vacuum conditions (10 -4 bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state 23 Na NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na 8 Si 46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the clathrate phase as Na atoms are removed from the NaSi precursor into the vacuum. © 2011 American Chemical Society.
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