Phase relations and crystal chemistry in the ternary PrO1.5-UO2-O2 system

Abstract

Phase relations and defect structures were studied in a PrO1.5-UO2-O2 ternary system in the temperature range from 1200 to 1500°C. Phases and compositions of the products heated in either air, helium or vacuum were examined by X-ray diffraction and chemical analysis, respectively. The regions of existence of solid solution having fluorite type structure, of rhombohedral phase and of A-type rare earth sesquioxide phase were determined. It was found that these phase relations could be classified by the mean valency of uranium and the type of oxygen defects. The change of cubic lattice parameter of the solid solution, PryU1−yO2+x, in single phase region (0 ≤ y ≤ 0.7) was expressed as linear equations of x and y: a = 5.4704−0.127x −0.007y, for x ≥ 0 and a = 5.4704−0.397x−0.007y, for x < 0. The change of lattice parameters was discussed in some detail.