Phase equilibria and thermal expansion of CaTiO doped with neptunium

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

Phase relationships between NpO and CaTiO or Ca(Ti, Al)O were investigated by X-ray diffraction (XRD) analysis, using specimens prepared at 1773 K in Ar-8%H. Single phase solid solutions were formed 0-7.5 mol%Np and 1-10 mol%Np for CaTiO and Ca(Ti, Al)O, respectively. By substituting Al for Ti in CaTiO, Np solubility in Ca(Ti, Al)O 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 were measured from room temperature to 1273 K in Ar-8%H 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 had practically no effect on stabilization of the crystal lattice. This finding was in a marked contrast to that of Pu doped CaTiO, where pronounced stabilization in the crystal was observed by incorporating Pu into CaTiO.