Controlled thermal decomposition of NaSi to derive silicon clathrate compounds

Formation conditions of two types of sodium containing silicon clathrate compounds were determined by the controlled thermal decomposition of sodium monosilicide NaSi under vacuum. The decomposition began at 360 degrees C. Much higher decomposition temperatures and the presence of sodium metal vapor were favorable for the formation of type I clathrate compound Na8Si46. Type II clathrate compound NaxSi136 Was obtained as a single phase at a decomposition temperature <440 degrees C under the condition without sodium metal vapor. The type 1 clathrate compound was decomposed to crystalline Si above 520 degrees C. The type II clathrate compound was thermally more stable, and retained at least up to 550 degrees C in vacuum. (C) 2008 Elsevier Inc. All rights reserved.