1. Landoldt-Börn;Werheit,1984
2. Proc. IX. Internat. Symp. on Boron, Borides and Related Compounds, Ed. , Duisburg 1987, University of Duisburg (p. 213).
3. and Proc. Internat. Conf. Powder Metallurgy, Kiev, October 1991, in the press.
4. see [2] (p. 154).
5. The Physics and Chemistry of Carbides, Nitrides and Borides, Ed. Kluver Academic Publishers, Dordrecht 1990 (p. 777).