1. 1) J. Meija, T.B. Coplen, M. Berglund, W.A. Brand, P. De Bièvre, M. Gröning, N.E. Holden, J. Irrgeher, R.D. Loss, T. Walczyk and T. Prohaska: Pure Appl. Chem., 88 (2016), 265-291.
2. 2) Japan Inst. Metals: Kinzoku databook 4th ed., (Japan Inst. Metals, Sendai, Japan, 2004), 10.
3. 3) I. Barin: Thermochemical data of pure substance, (VCH Verlagsgesellschaft, Weinheim, Germany, 1993) .
4. 4) U.S. Geological Survey: Mineral commodity summaries 2020, (U.S. Geological Survey, Reston, Virginia, USA, 2020) .
5. 5) Alumu Publishing: Industrial rare metals, annual review 2019, (Alumu Publishing, Tokyo, Japan, 2019), 46.