1. C. A. Hampel, ed.:Rare Metals Handbook, 2nd ed., p. 480, Reinhold, London, 1961.
2. M. C. Marignac:nn. Chim. (Paris), 1866, vol. 8, pp.5–21;id, 1866, vol. 9, pp. 249-61.
3. K. B. Higbie and J. R. Werning: U.S. Bur. Mines Rep. Invest., RI 5239, 1956.
4. S. Fisel, N. Topal, and V. Tintaru:Lucr. Conf. Nat. Chim. nal., 1971, vol. 4, pp. 425–30.
5. R. Monnier, P. Grandjean, and J. Zahler:Helv. Chim. Acta, 1963, vol. 46, pp. 2966–70.