COMPOSITION AND OXYGEN POTENTIAL OF CUBIC FLUORITE-TYPE SOLID-SOLUTION EUYU1-YO2+X(X-LESS-THAN-OR-EQUAL-TO-OR-GREATER-THAN-O) AND RHOMBOHEDRAL EU6UO12+X'(X'-LESS-THAN-O)

A fluorite-type solid solution, EuyU1-yO2+x, was formed in single phase up to y = 0.51 when heated in vacuum at 1400°C. Between y = 0.51 and 0.80, a two-phase mixture of fluorite and rhombohedral phases exists. The rhombohedral phase was of single phase in a range y = 0.8-0.9. The change of lattice parameter and x value in the fcc solid solution were determined as a function of y value in EuyU1-yO2+x. The crystal structure of the nonstoichiometric Eu6UO11.41 is the same as RE6UO12 (R 3 ̄, Z = 3) with Bartram's atom parameters. The measured oxygen potentials (ΔḠO) for fcc EuyU1-yO2+x (y = 0.1 and 0.3) are not only much higher than those of the solid solutions of the other rare-earth elements, but are also shifted to the lower x side yielding a nearly vertical change of ΔḠO in the hypostoichiometric range (x < 0). As a result, the partial molar entropy of oxygen (ΔS̄O) and enthalphy (ΔH̄O) are significantly different from those of the solid solutions of the other rare-earth elements. ΔḠO, ΔS̄O and ΔH̄O for rhombohedral Eu0.8571U0.1429O1.7143+x (x < 0) were measured and discussed. © 1990.