1. 1) Niobium–World market overview from Roskill: http://www.roskill.co.uk/niobium.html.
2. 2) C. Crai. Wojcik: Advanced Materials and Processes, 154 (1998), 27–30.
3. 3) A. Koethe: Ultra High Purity Base Metals, Proc. UHPM ’94, ed. by K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1994), pp. 291–322.
4. 4) H. Schultz: Ultra High Purity Base Metals, Proc. UHPM ’94, ed. by K. Abiko, K. Hirokawa and S. Takaki, The Japan Institute of Metals, (1994), pp. 323–351.
5. 5) B. Fellmuth and D. Elefant: phys. stat. sol. (b), 146 (1988), 207–216.