1. K. M. Minachev in “Internat. Cong, on Catalysis, 5th Palm Beach, 1975, ” J. W. Hightower, ed., Elsevier, N.Y., 1973.
2. R. L. Koffler, “Proc., Rare Earth Res. Conf.,” 7th, (1968) 697.
3. J. F. Read and A. L. Crandelmire, J. Cat., 38, 54 (1975).
4. J. H. Forsberg, T. M. Balasubramanian and V. T. Spaziano, J.C.S. Chem. Comm., 1060 (1976).
5. Based on relative gram atomic abundance in the earth’s crust. J. E. Huheey, “Inorganic Chemistry,” 2nd Ed., Harper & Row, New York, N.Y. 1978.