Phase relations and crystal chemistry of solid solution, Sr yU 1− yO 2+ x

Abstract

Solid solutions, Sr y U 1− y O 2+ x , were prepared by heating the mixtures of β-SrUO 4, UO 2, and U 3O 8 in either helium or vacuum. When heated in helium at 1300 or 1400°C, a single-phase cubic fluorite (face centered cubic, FCC) phase was obtained up to strontium concentration ( y of Sr y U 1− y O 2+ x ) ca. 0.3, although it splits into two fluorite phases around y = 0.03, 0.05, and 0.08 during the relatively slow cooling process. Due to larger x values of Sr y U 1− y O 2+ x , for the samples heated at 1200°C, a single rhombohedral phase was formed in the range 0.06 ≤ y ≤ 0.10. The x value and uranium valency in the solids showed steep change at y = 0.03, 0.06, and 0.09, which is presumably related to the stepwise ordering of strontium atoms to form the rhombohedral phase. A statistical thermodynamical calculation was made concerning this transition. When heated in vacuum, only the FCC phase was formed below y = 0.15. For this phase, the lattice parameter change was expressed as linear functions of x and y by least-squares fittings. The coefficient of y of these equations was found to be on a straight line irrespective of the valency of the foreign cation if the net change in crystal radius was taken as the variable.