1. 1) N. Ishigaki and A. Oota: An Overview of Functions and Applications of Rare Earths, ed. by G. Adachi, (CMC Publishing Co., Ltd., Tokyo, 2006) p. 13.
2. 2) E. Nakamura: Technology of Alternatives and Recycling of Rare Metals, ed. by K. Halada and T. Nakamura, (CMC Publishing Co., Ltd., Tokyo, 2006) p. 296.
3. 3) K. Machida: Kinzoku 74 (2004) 385-395.
4. 4) T.H. Okabe: Kinzoku 77 (2007) 598-604.
5. 5) S. Shirayama and T.H. Okabe: Molten Salts 52 (2009) 71-82.