In this study, we prepared Si clathrate films (Na 8 Si 46 and Na x Si 136) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapor to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10 −2 Pa) resulted in a film of Si clathrates having a thickness of several micrometers. Furthermore, this technique enabled the selective growth of Na 8 Si 46 and Na x Si 136 using the appropriate crystalline orientation of Si substrates.

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