A laser heating approach combined with fast pyrometry in a thermal arrest method was used to provide new data for the melting/solidification phase transition in mixed PuO2 – ThO2 at high temperature. Low addition of ThO2 to PuO2 resulted in a minimum in the solidification temperature in the pseudo binary PuO2 – ThO2. The minimum was found around a composition with 5 mol % ThO2. Phase transition temperatures of other compositions follow better the expected ideal solution behaviour. To detect changes in the material a complete investigation with electron microscopy, Raman spectroscopy and powder X-ray diffraction was done. With these, Raman vibration modes were found, which are characteristic for materials containing PuO2, and high temperature segregation effects during solidification were described. The results obtained in the present work are compared to other mixed actinide dioxides and are discussed together with the ideal solution assumption for this system. The presented results show,together with the encountered experimental difficulties at high temperatures,the importance of the high-temperature oxygen chemistry in this actinide oxide phase.
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