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EXAFS analysis of uranium(IV) and thorium(IV) complexes in concentrated CaCl$$_{2}$$ solutions

塩化カルシウム六水和物溶融体中のウラン(IV)及びトリウム(IV)錯体のEXAFS解析

上原 章寛*; 藤井 俊行*; 松浦 治明*; 佐藤 修彰*; 永井 崇之; 湊 和生; 山名 元*; 岡本 芳浩

Uehara, Akihiro*; Fujii, Toshiyuki*; Matsuura, Haruaki*; Sato, Nobuaki*; Nagai, Takayuki; Minato, Kazuo; Yamana, Hajimu*; Okamoto, Yoshihiro

The coordination of U$$^{4+}$$ and Th$$^{4+}$$ in CaCl$$_{2}$$ hydrate melts was studied by U and Th $$L$$$$_{rm III}$$ edge extended X-ray absorption fine structure (EXAFS) spectroscopy. It was confirmed that, with the decrease of concentration from 6.9 M CaCl$$_{2}$$ to 4 M CaCl$$_{2}$$, the Cl$$^{-}$$ ion coordination number $$N$$$$_{rm Cl}$$ in the U$$^{4+}$$ coordination sphere decreased from 3.4 to 1.3, while the hydration number $$N$$$$_{rm O}$$ in it increased from 4.5 to 6.7. The combined coordination number $$N$$$$_{rm O}$$ + $$N$$$$_{rm Cl}$$ of U$$^{4+}$$ in concentrated Cl$$^{-}$$ solution (8.0) was lower than that in 1.5 M perchloric acid aqueous solution (9.0). For Th$$^{4+}$$, with the decrease of concentration from 6.9 to 4 M CaCl$$_{2}$$, $$N$$$$_{rm Cl}$$ in the Th$$^{4+}$$ coordination sphere decreased from 1.9 to 1.5, while $$N$$$$_{rm O}$$ in it increased from 7.6 to 8.8. The $$N$$$$_{rm O}$$ + $$N$$$$_{rm Cl}$$ of Th$$^{4+}$$ in concentrated Cl$$^{-}$$ solution was 9.0 which was similar to that in 1.5 M perchloric acid. It was found that the bond distance of U-Cl was shorter than that of Th-Cl because of low $$N$$$$_{rm O}$$ to U$$^{4+}$$.

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