Effect of Na–Sn Flux on the Growth of Type I Na8Si46 Clathrate Crystals

Abstract

In the crystal growth of Na–Si clathrate (type I, Na8Si46) during Na evaporation from a Na–Si–Sn solution at 723 K, the composition of a Na–Sn flux in the starting material strongly influences the morphology and size of the formed clathrate crystals. In this study, the crystals obtained using this flux were larger than the crystals prepared without a flux, and some of them had faceted surfaces. At the Na4Si4 (precursor):4Na–Sn (flux) = 1:4 ratio, multiple dents were observed on crystal surfaces, indicating that the precipitation of a Na9Sn4 solid phase prevented the growth of Na–Si clathrate crystals. In addition, synthesis conditions, under which type I crystals could be obtained by conventional thermal decomposition in vacuum, were established. The results of this work suggest that type I Na–Si clathrate crystals are stable even at temperatures as high as 723 K due to the suppressed evaporation of Na.