ARTICLE IN PRESS
H.-o. Horie et al. / Journal of Solid State Chemistry 182 (2009) 129–135
135
type I clathrate, excess Na metal should be first removed at
temperatures below 340 1C, where no decomposition of NaSi
begins, then NaSi should be decomposed at temperatures below
440 1C by prolonged heating. The staying of Na vapor should be
avoided; NaSi should be shallowly loaded in a container with a
sufficiently wide diameter, so that Na vapor evolved from NaSi can
be removed as rapidly as possible. The lower the decomposition
temperature, the longer evacuation time is required. The final
concentration of Na in NaxSi136 varies depending on the decom-
position temperature and time. A type II clathrate Na11.76Na20.64
Si136 with less than 1% contamination of the type I clathrate was
prepared at 440 1C by evacuation for 18 h.
The preparation of the type I clathrate free from the type II phase
was difficult. More systematic detailed study on the suitable Na
vapor pressure for the NaSi decomposition is required.
Acknowledgments
This study has also been supported by a Grant-in-Aid for
Scientific Research (Nos. 19105006 and 19051011) of the Ministry
of Education, Culture, Sports, Science, and Technology of Japan.
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˚
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