The plutonium-oxygen phase diagram in the 25-900 °C range: Non-existence of the PuO1.515 phase

Evaluation of data for phases formed in the Pu-O system at temperatures below 900 °C shows that the observed oxides are not at equilibrium. Results are consistent with coexistence of a hexagonal solid solution (hex-PuO1.5+z, 0 ⩽ z ⩽ 0.010) and a cubic phase (cub-PuO1.60) in equilibrium at 800 °C, but fail to confirm that the O/Pu ratio of the body-centered-cubic (bcc) oxide formed near the sesquioxide composition is 1.515 (1.52) or that bcc-PuO1.515 is formed by the peritectic reaction of hex-PuO1.510 with cub-PuO1.60. Stable Pu(IV)/Pu(III) ratios observed for products of the Pu-H2O reaction correspond to members of the PunO2n−2 homologous series, but a product is not formed at O/Pu = 1.515. Metastable bcc-PuO1.50 (n = 4) and stable hex-PuO1.5+z coexist below 285 °C, the point at which reversible eutectic decomposition of cubic PuO1.60 (n = 5) produces a non-equilibrium mixture of bcc-PuO1.50 and sub-stoichiometric dioxide (PuO2−y). Transformation of bcc-PuO1.50 to stable hex-PuO1.50 and reactions of the hexagonal oxide to form higher-composition cubic phases are kinetically hindered. An alternative diagram describing non-equilibrium chemical behavior of the Pu-O system is presented.