Fluorite-related lanthanide oxides

The fluorite-related lanthanide oxides exhibit unusual diffusional properties. The conventional rule-of-thumb is that atomic mobility in a solid does not become significant until one-half of the melting point temperature (the Tammann temperature) is reached. In these oxides this value is about 1200°C. At the Tammann temperature the metal atoms in lanthanide oxides just begin to become mobile as confirmed by the temperatures required for solid-state reactions. The oxygen substructure, to the contrary, is mobile below 300°C. This leads to a situation where equilibration and reaction must be considered for each substructure separately (see Bevan and Summerville 1979). This places the lanthanide oxides with fluorite-related structures in the category of fast-ion conductors along with, e.g., calcia-stabilized zirconia as indicated in table 18. 

TABLE 1 Summary of the oxygen-deficient fluorite-related lanthanide higher oxides of the Rn02n-2m series... 

Nearly all of the information available on the kinetics of heterogeneous reactions with lanthanide oxides concerns the C-type sesquioxides or the fluorite-related higher oxides. As stated in the section above, in these materials oxygen mobility is very high, whereas, metal-atom movement is extremely limited below 1200°C. Table 19 suggests the type of experiments that were done and the phenomenological mechanisms proposed before 1980 (Eyring 1979). 

The lanthanide higher oxides have not only peculiar thermodynamic properties, but also unique physical and chemical properties. The physical and chemical properties are presented as a macroscopic parameter, such as the electrical conductivity, the coefficient of expansion, and the conversion rate of a catalysis process. Due to the lack of knowledge of the wide range of non-stoichiometry of the oxygen-deficient fluorite-related homologous series of the lanthanide higher oxides, the macroscopically measured data of the physical and chemical properties are scattered, and therefore, based on the structural principle of the module ideas a deep understanding the relationship between the properties and structures is needed. 

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. 

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. 

Figure 6. Lattice parameters of fluorite-related oxides of some lanthanide and actinide elements as a function of composition...

Most of LeRoy s research at Arizona State University was involved with the higher, predominantly non-stoichiometric lanthanide oxides, of Ce, Pr, and Tb (see below), but it also included non-rare earth oxides of Ti, Zr, Pb, Cm and Bk, mixed rare earth-non-rare earth oxides (especially with fluorite related structures) and rare earth carbonates. A complete list of his publications, awards and other notable achievements is given in the Appendix of the Dedication. 

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