1. La Placa, S., Post, B.: Planseeber. Pulvermetall. 9 (1961) 109.
2. Matkovich, V. I.: Acta Crystallogr. 14 (1961) 93.
3. Sullenger, B., Kennard, Ch. L.: Sci. Am. 215 No. 7 (1966) 96.
4. Boron 3, T. Niemyski, ed., PWN Warsaw, 1970
5. Will, G.: see [70E], p. 119.