Plutonium oxides phase composition

Chikalla et al. (85), and Gardner et al. (86), whose results were reviewed in the IAEA Panel on the Thermodynamics of Plutonium Oxides at Vienna in 1966 (55). There still remain some ambiguities, however, regarding the stable region of temperature and composition of the PuOi.gi and PuOj 52 phases (83a). 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

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