1. A. van Wberingen and N. Warmoltz, Physical Grav. 22, 849 (1956), proved that at high temperature helium diffuses through single-crystal walls of the non-metals germanium and silicon. Their experiments show that helium has a noticeable solubility in these elements which crystallize with the loosely-packed diamond structure.
2. See J. D. Fast, Interaction of Metals and Gases, Vol. 1, Thermodynamics and Phase Relations, Philips Technical Library, Eindhoven (1965), Chapter 7.
3. A. Coehn et al, Z. Phys. 83, 291 (1933).
4. J. Knaak and W. Eichenauer, Z. Naturf 23A, 1783 (1968).
5. J. Wesolowski, J. Jarmula and B. Rozenfeld, Bull Acad. Poi. Sci. Sér. Sci. chim. 9, 651 (1961).