1. E.C. Subba Rao, W.E. Wallace, Science and Technology of Rare Earth Materials, Academic Press, New York, 1980.
2. US IBM Bulletin, 675.
3. Mineral Facts and Problems, US Bureau of Mies, Bulletin 675 (1985).
4. T.K.S. Murthy, C.K. Gupta, Science and Technology of Rare Earth Materials, Academic Press, New York, 1980.
5. E.L. Dekalb, V.A. Fassel, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on Physics and Chemistry of Rare Earths, vol. 4, North Holland, Amsterdam, 1979 (Chapter 37D).