1. F. X. Spiegel, D. Bardos, P. A. Beck, Trans. Amer. Inst. Min. (metall) Engrs.227, 575 (1963).
2. Clara B. Shoemaker undD. P. Shoemaker, Acta Cryst.18, 900 (1965).
3. E. Ganglberger, H. Nowotny undF. Benesovsky, Mh. Chem.96, 1206 (1965).; Mh. Chem.97, 220 (1966).