Crystal chemistry and thermal expansion studies of [(U0.1Th0.9)1−yGdy]O2+x, (y = 0.1–0.4)

Phase relations and defect structures in (U0.1Th0.9)O2–GdO1.5 quaternary system were studied at 1673 K. Different phases and their compositions, obtained on heating the oxides in different atmospheres were examined by powder X-ray diffraction and chemical analysis, respectively. The region of existence of fluorite type solid solutions was determined. The cubic lattice parameter of the solid solution, [(U0.1Th0.9)1−yGdy]O2+x, in single phase region (0 ⩽ y ⩽ 0.4) could be expressed in a linear equation of x and y as:a0 (Å) = 5.581 − 0.228x − 0.198y for x < 0O/U analysis and simple ionic model of the quaternary solid solutions indicate the oxidation of U+4 to a mixture of U+5 and U+6. High temperature XRD studies of FCC structured [(U0.1Th0.9)1−yGdy]O2+x showed positive thermal expansion between 298 K and 1273 K.