tains its refractory character, with the lowest liquidus temperature being 1930°C (Table 111).The C-type rare earth oxide structure found in europia at < 1050°C exists at lower temperatures to 30 mol% hafnia and at higher temperatures between 35 and 55 mol% hafnia with a very narrow cubic region at 1330°C. A similar phenomenon is found in the system HfOz-G40,,27 where a large high temperature region of the cubic structure was found. V. ConclusionsAlthough this study is not conclusive for structures above 1900°C, the results are very important for determining phase stability in the hafnia-europia system below this temperature. As previously discussed, both hafnia and europia show structural transformations in the pure state, but combinations of the 2 oxides show extensive solid solution in the fluorite and C-type rare earth oxide structures. This gives the structural stability necessary in the design of these materials for high temperature applications as refractory materials and in nuclear applications as control rod materials. Europia is also added to the list of rare earth oxides that will stabilize hafnia in the fluorite structure.

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