1. K. Fredenhagen and G. Cadenbach, Z. Anorg. Allgem. Chem., 3: 158 (1926)
2. A. Hérold, Bull. Soc. Chim. Fr., 999 (1955)
3. R.C Asher and S.A. Wilson, Nature, 181: 409 (1958), A. Métrot and A. Hérold, C.R. Acad. Sc. Paris, 2C:883 (1967)
4. R. Juza and V. Wehle, Natw., 52: 560 (1965), D. Guérard and A. Hérold, .C.R Acad. Sc. Paris, 262C:557 (1966) and Carbon, 13:337 (1975)
5. D. Guérard and A. Hérold, C.R. Acad. Sc. Paris, 279C: 455 (1974) and 280C:729 (1975)