1. Kuijpers, F. A.: Philips Res. Rep. (Suppl. 2), 1 (1973)
2. Takeshita, T., Wallace, W.E., Craig, R.S.: Inorg. Chem. 13, 2283 (1974)
3. Bechman, C.A., Goudy, A.J., Takeshita, T., Wallace, W.E., Craig, R.S.: Inorg. Chem. 15(9), 2184 (1976)
4. Oesterreicher, H., Clinton, J., Bittner, H.F.: Mater. Res. Bull. 11, 1241 (1976)
5. Tauber, A., Finnegan, A. S. R. D., Rothwarf, F., Wallace, W. E.: In: 12th Rare Earth Research Conference, p. 1073 (1976)