PHASE RELATION AND THERMODYNAMIC PROPERTIES OF CUBIC FLUORITE-TYPE SOLID-SOLUTION, BAY/2YY/2U1-YO2+X (X-LESS-THAN=GREATER-THAN-0)

The solubility of barium (large crystal radius) in UO2 was studied under the coexistence of yttrium (smaller crystal radius). After the phase relation and the change of lattice parameter were examined on the fcc fluorite phase of the solid solution, oxygen potentials were measured for Ba0.05Y0.05U0.9O2 + x. It was found that the Ba(y/2)Y(y/2)U1-yO2+x solid solution was formed in single phase in a range 0 less-than-or-equal-to y less-than-or-equal-to 0.1 by heating in helium, vacuum and hydrogen at temperatures between 1000 and 1400-degrees-C. The change rate of the lattice parameter of the solid solution with barium concentration was found to be markedly smaller than the calculated value in contrast to the case of yttrium for which agreement between the observed and calculated rates was reasonably good. Oxygen potential measurements of this solid solutions revealed higher DELTA-G(O2)BAR than those of the solid solutions containing tri- or tretravalent foreign metals. The ratio of oxygen to metal atom giving a very steep change in DELTA-G(O2)BAR was seen to exist at O/(Ba + Y + U) = 1.917. This O/M value is significantly lower than 2.0 for most other solid solutions. Moreover, DELTA-G(O2)BAR of the present solid solution does not vary linearly with temperature between 900 and 1300-degrees-C leading to give temperature dependent DELTA-S(O2)BAR and DELTA-H(O2)BAR. Thus, the DELTA-S(O2)BAR and DELTA-H(O2)BAR values at 912-degrees-C were found to be much lower than those at higher temperatures.