Double clathrate hydrates with helium and hydrogen

Double cubic structure II clathrate hydrates of tetrahydrofuran (THF) with helium (space group Fd3m, a = 17.155(5) A, 120 K) or hydrogen and those of hexafluorophosphoric acid with hydrogen have been obtained. These hydrates are solid solutions due to the variable degree of the filling of the small cavities by helium and hydrogen molecules. The degree of the filling of the small cavities of the hydrate of THF by helium at 3.5 kbar is 24.5%; hydrogen can occupy 100% of the cavities, and at 7.0 kbar it is possible for two molecules to be accommodated in the small cavity.