Crystal Growth Conditions of Types I and II Na–Si Clathrates by Evaporation of Na from a Na–Si–Sn Solution

Single crystals of the type I clathrate Na8Si46 were synthesized by the evaporation of Na from a Na–Si–Sn solution (Na:Si:Sn = 6:2:1 molar ratio) at 723–873 K under an Ar atmosphere of 105 Pa. In the mixture of type I and type II clathrates prepared by heating at 773 K for 72 h, type I single crystals with sizes of up to 5 mm were found. A single phase of single crystals of the Na24Si136 type-II clathrate having a {111} crystal facet of about 2 mm on one side was obtained by heating at 873 K for 9 h.