Crystal high-oxygen-pressure treatment

The highest oxide of U that is known is the yellow/orange UO3. It can be prepared by different synthetic approaches, one being the treatment of lower oxides with high-pressure oxygen at modest temperatures (e.g., 300°C) another is the thermal decomposition of hydrated uranyl salts at temperatures below 650°C. In air, UO3 loses oxygen when heated above 650°C to form U3O8. There are several different crystal modifications of UO3 (see table 25). 

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