Cerium systems

Bourcet and Fong (46) in their study of energy transfer between cerium and terbium in lanthanum phosphates performed an analysis of the dependence of the donor luminescence decay with temperature, and found that the diffusion in the cerium system plays an important role in the energy transfer process. 

The composition ranges of the nitride-carbide have been determined. For the cerium system and the praseodymium system, two series of fee nitride-carbide phases were identified. 

McColm et al. (1977) considered that the eomposition ranges seem to be related to the amounts of R(IV) found in the respective nitrides and carbides (Lorrenzelli et al. 1970, Atoji 1962). In the cerium systems, Ce(IV) is present up to 70%, while in the praseodymium case the amount is less and in the lanthanum system the higher oxidation state is absent. This suggests that Ce(IV) and Pr(IV) do assist in preventing the catenation that leads to acetylide ion formation. However, the reviewers suggest that this factor is probably minor while the size factor of the rare earth atom is essential. 

R. Vogel in Germany and G. Canneri in Italy were the principal early pioneers in determining phase diagrams of the rare earth metals. Vogel primarily concentrated on cerium systems - the first studied was the Ce-Sn system (Vogel 1911) - and... 

Lanthanum promoted supports with 7% of copper and higher thermally treated at 773 and 973 K contain a roughly-dispersed copper oxide, while the spinel form is not found probably because of its high dispersity and disorder. At 1273 K, unlike the cerium system, lanthanum exhibits a stabilizing effect only at 15% of Cu, all alumina appears to be in the form of a-Al203. 

Contrary to cerium system, lanthanum modified support exhibits a limited chromium solubility, since a-Cr203 appears already at the deposition of 7% of Cr. At 1273 K, the content of a-Al203 is practically as large as in the cerium system with 2 and 7% of chromium and significantly lower, if Cr content is 15% (Fig. 1). 

In the zinc/cerium system, methanesulfonic acid has been widely used as a supporting electrolyte. It is less corrosive than sulphuric acid, and its sufficient acid strength can dissolve a high concentration of cerium ions. Moreover, researchers suggested that the zinc dendritic growth can be inhibited by the methanesulfonic acid [50]. 

In the salt [K (diglyme)][Ce(CgHg) ] the anion has staggered 0, symmetry. The cerium system is unusual for a lanthanide complex in showing two one-electron oxidation-reduction steps. 

Parks et al. (1984) and Wieliczka et al. (1984b) noticed that the two-peak structure present in cerium systems also appeared in Pr and Nd systems, but the structures there were further removed from Ef. This seems to rule out a Kondo explanation for the upper peak since in that model such a feature is inherently tied to p It is, therefore, of some interest to see whether our approach followed for the Ce compounds would explain the Pr and Nd pnictides in the same manner as it did the Ce pnictides. Comparison will be made with photoemission data on PrSb and NdSb (unfortunately, no resonance data exist). 

Because stable tetravalent rare earth halides are formed only for the fluorides of Ce, Pr and Tb, the number of MX-MX4 complexes is much lower than for the MX-MX3 systems. Early reports of stable complex fluorides of Nd(IV) and Dy(IV) have not been confirmed. The complex tetravalent fluorides are predicted by radius ratio correlations (Thoma, 1962) and are reviewed by Brown (1968). An extensive investigation by Delaigue and Cousseins (1972) has greatly expanded the data for the cerium systems. Seven complex fluorides and the corresponding metal systems are shown in table 32.11. As expected, these phases are similar to the corresponding complexes of the actinides (Brown, 1968). 

Fig. 2. Lattice spacings tor the lanthanum-cerium system. The straight line connecting the end-members is the Vegard s law relationship based on the values listed in table 2.

This approach does at first sight seem limited to cerium systems, because the two body correlations coincide with the dominance of the Hf = 0 and m = 1 configurations which involve precisely two electrons. However, this can be extended to uranium as indicated by preliminary calculations by Cooper, Lin and Sheng (unpublished). These calculations show that the essential features of the above description are preserved if the mixing occurs between the f = 1 and f =2 configurations which involve three electrons. 

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