Fluorite-related oxide phases

Fluorite-Related Oxide Phases of the Rare Earth and Actinide Elements... 

The fluorite-related oxide phases which are known in the lanthanide and actinide series are displayed in Table II for closer comparison. The most obvious feature is that the oxide systems of Ce, Pr, and Tb reveal greater complexity than any of the actinide elements so far studied. The dioxides of the actinide elements are more easily reduced as one goes from Th02 to Cm02 showing an approach to the behavior of the lanthanides. More complete measurements on CmOx and BkO may well show marked similarity to the rare earths. 

Fluorite-Related Oxide Phases of the Rare Earth and Actinide Elements, L. Eyring. In Advances in Chemistry Series, No. 71, American Chemical Society, Washington, DC, pp. 67-85, 1967. 

Cerium, praseodymium, and terbium oxides display homologous series of ordered phases of narrow composition range, disordered phases of wide composition range, and the phenomenon of chemical hysteresis among phases which are structurally related to the fluorite-type dioxides. Hence they must play an essential role in the satisfactory development of a comprehensive theory of the solid state. All the actinide elements form fluorite-related oxides, and the trend from ThOx to CmOx is toward behavior similar to that of the lanthanides already mentioned. The relationships among all these fluorite-related oxides must be recognized and clarified to provide the broad base on which a satisfactory theory can be built. 

Relationship between the surface structure and the intermediate phase The existence of oxygen deficient fluorite-related intermediate phases of the rare earth higher oxides is due to vacancy ordering. Fig. 3-28 demonstrates the relationship between the surface structure and the bulk superstructure... 

Protactinium oxides can be stabilized in the tetravalent and pentavalent state. The most stable oxide phase obtained by the burning of metal or protactinium compoimds is the white pentoxide, Pa20s. The structme of the pentoxide is related to fluorite and has cubic symmetry. Pa02 is a black solid that crystallizes in the cubic fluorite structure. 

This approach makes it possible to describe several structural features of anion-deficient fluorite-related binary oxides of Ce, Pr and Tb but it is fundamentally descriptive rather than predictive. This limit was pointed out by Khang and Eyring who recently built a model that is also capable of predicting the structural features of unknown phases in the series... 

Fluorite Related Phases in the Rare Earth Oxides... 

Fluorite Related Phases of the Actinide Metal Oxides... 

All the actinide elements whose oxides have been studied can be made as fluorite-type dioxides. For most of them this is their most stable form in air at room temperature. The fluorite-related phases of each actinide element known will be discussed individually before a comparison is made with the rare earth oxides 45). 

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