1. M. A. Bredig, Colloq. Int. CNRS, No. 205, 183 (1972).
2. D. J. M. Bevan, O. Greis, and J. Strahle, Acta Crystallogr. A 36, 889 (1980).
3. J. Oberschmidt, Phys. Rev. B 23, 5038 (1981).
4. O. Greis and J. M. Haschke, in Handbook on the Physics and Chemistry of Rare Earth, Ed. by K. A. Gscheidner and L. Eyring (North-Holland, Amsterdam, 1982), Vol. 5, p. 387.
5. S. A. Kazanskii, Pis’ma Zh. Éksp. Teor. Fiz. 38, 430 (1983) [JETP Lett. 38, 521 (1983)].