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Phase equilibria and thermal expansion of CaTiO$$_{3}$$ doped with neptunium

Sato, Tsuyoshi*; Yamashita, Toshiyuki; Matsui, Tsuneo*

Phase relationships between NpO$$_{2}$$ and CaTiO$$_{3}$$ or Ca(Ti, Al)O$$_{3}$$ were investigated by X-ray diffraction (XRD) analysis, using specimens prepared at 1773 K in Ar-8%H$$_{2}$$. Single phase solid solutions were formed 0-7.5 mol%Np and 1-10 mol%Np for CaTiO$$_{3}$$ and Ca(Ti, Al)O$$_{3}$$, respectively. By substituting Al for Ti in CaTiO$$_{3}$$, Np solubility in Ca(Ti, Al)O$$_{3}$$ increased. Solubility of Np was compared with those of U and Pu, and was discussed with oxidation states and ionic radii of dopants. Thermal expansions of (Ca,Np)TiO$$_{3}$$ were measured from room temperature to 1273 K in Ar-8%H$$_{2}$$ using high-temperature XRD technique. These specimens showed nearly the same value of volumetric thermal expansion coefficients, suggesting that the incorporation of tetravalent Np into CaTiO$$_{3}$$ had practically no effect on stabilization of the crystal lattice. This finding was in a marked contrast to that of Pu doped CaTiO$$_{3}$$, where pronounced stabilization in the crystal was observed by incorporating Pu into CaTiO$$_{3}$$.

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Category:Materials Science, Multidisciplinary

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