1. Yokota, M., Syono, Y., Minomura, S.: J. Solid State Chem. 3 (1971) 520.
2. Donohue, P.C.: J. Solid State Chem. 2 (1970) 6.
3. Collin, G., Flahaut, J.: C. R. Acad. Sci. C 270 (1970) 488.
4. Flahaut, J., Laruelle, P., Collin, G., Guittard, M., Etienne, J., Michelet, A.: Proc. 9th Rare Earth Res. Conf. Virginia Polytechnic Institute and State Univ., Blacksburg, Virginia, 1971, p. 741.
5. Fischer, Ø.: Appl. Phys. 16 (1978) 1.