1. Wyckoff, R. W. G.: Crystal Structures, John Wiley & Sons, New York, 2nd Ed., 1963.
2. Landolt-Börnstein (New Series) Vol. III/6, Springer, Berlin-Heidelberg 1971.
3. Papaconstantopoulos, D. A., Anderson, J. R., McCaffrey, J. W.: Phys. Rev. B 5 (1972) 1214.
4. Christensen, N. E.: Phys. Status Solidi (b) 55 (1973) 117.
5. Feuerbacher, B., Fitton, B.: Phys. Rev. Lett. 30 (1973) 923.