A thermodynamic model of the Pu-O system was proposed by Kinoshita et a/.27 and Gueneau et a/.28 The calculated phase diagram by Gueneau et a/.28 reproduces the main features of the phase diagram proposed by Wriedt29 in his critical review (Figure 2).

In the Pu-Pu2O3 region of the phase diagram, the experimental data are rare. The existence of

Figure 1 U-O phase diagram (a) calculated using the model derived by Gueneau et a/.8; (b) calculated from 60 to 75at.% O8; the green points come from the critical review by Baichi et a/.9 and Labroche et a/.10 and the blue points show the results of Manara eta/.11; (c) calculated from O/U = 1.9 to 2.4 after Higgs eta/.12 The references of the experimental data are given in Higgs et a/.12 © Elsevier, reprinted with permission.

a miscibility gap in the liquid state was shown by Martin and Mrazek.30 The monotectic reaction was measured at 2098 K.30 There are no data on the oxy­gen solubility limit in liquid plutonium.

More data are available in the region between Pu2O3 and PuO2. The phase relations are complex below 1400 K. PuO2 _ x starts to lose oxygen above
approximately 900 K. A narrow miscibility gap was found to exist in the fluorite phase below approxi­mately 900 K leading to the simultaneous presence of two fcc phases with different stoichiometries in oxygen. Two intermediate oxide phases were found to exist with the formula PuO161 and PuO152. The PuO1.61 phase exhibits a composition range and is

stable between 600 and 1400 K. The PuO152 com­pound only exists at low temperature (T< ^700 K).

Above ^1400 K, the dioxide PuO2 _ x exhibits a large homogeneity range with a minimum O/Pu ratio equal to approximately 1.6 and is in equilibrium with the sesquioxide Pu2O3. The liquidus tempera­tures between Pu2O3 and PuO2 remain uncertain and would need future determinations.

The melting temperature of PuO2 is still a subject of controversy. The recommended value for the melt­ing of PuO2 was for a long time Tm = 2674 ± 20 K, based on measurements from Riley.31 Recent mea­surements are available that suggest higher values. In 2008, Kato et at:2 measured the melting point of PuO2 at 2843 K that is higher by 200 K than the previous measurements. The authors used the same thermal arrest method as in previously published works but paid more attention to the sample/crucible chemical interaction by using rhenium instead of tung­sten for the container. Very recently, a reassessment of the melting temperature of PuO2 was performed by De Bruycker et at.33 using a novel experimental approach used in Manara et at.11 for UO2. The new value of 3017 ± 28 K exceeds the measurement by Kato et at. by 174 K. The noncontact method and the short duration of the experiments undertaken by De Bruycker et at.33 give confidence to their new value which has been very recently taken into account in the thermodynamic modeling ofthe Pu-O system.42 Both studies agree on the fact that the values measured in the past were underestimated.