1. F. W. Vahldiek and S. A. Mersol (editors), Anisotropy in Single-Crystal Refractory Compounds, Vols, 1 and 2, Plenum Press, New York (1968).
2. R. B. Kotel'nikov, S. N. Bashlykov, et al., Highly Refractory Elements and Compounds [in Russian], Metallurgiya (1969).
3. O. Kalish and E. Clougherty, Am. Ceram. Soc. Bull.,48, 570 (1969).
4. D. W. Lee and J. S. Haggerty, J. Am. Ceram. Soc.,52, 641 (1969).
5. W. Precht and G. E. Hollox, J. Crystal Growth, 3/4, 818 (1968); J. Haggerty, disc., p. 823.