Solid-state behavior, actinides

Interest was rekindled during the 1970s due to advances in solid-state physics and a growing realization of the unusual properties of the lighter actinide metals due to the behavior of their 5/ electrons. 

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. 

Th02—ternary oxides or oxide phases with tetravalent americium are stabilized. The solid-state reaction of Am02 with most group V elements yields compounds with trivalent americium which are isostructural with the analogous rare earth compounds. In the last types of reactions americium exhibits a typical actinide behavior. 

Investigations of the solid-state chemistry of the americium oxides have shown that americium has properties typical of the preceding elements uranium, neptunium, and plutonium as well as properties to be expected of a typical actinide element (preferred stability of the valence state 3-j-). As the production of ternary oxides of trivalent plutonium entails considerable difficulties, it may be justified to speak of a discontinuity in the solid-state chemical behavior in the transition from plutonium to americium. A similar discontinuous change in the solid-state chemical behavior is certainly expected in the transition Am Cm. Americium must be attributed an intermediate position among the neighboring elements which is much more pronounced in the reactions of the oxides than in those of the halides or the behavior in aqueous solution. 

Carbonates. Actinide carbonates have been very thoroughly studied by a variety of solution and solid state techniques. These complexes are of interest because of their fundamental chemistry and environmental behavior, including aspects of actinide mineralology. In addition, separation schemes based on carbonate have been proposed. Coordination numbers are generally quite high, eight to ten carbonate is bound to the metal center in a bidentate fashion and is often hydrogen-bonded to outer sphere waters or counter ions (see Table 9). 

Whereas Ce through Lu are characterized by filling of 4f shell, actinide elements thorium to lawrencium are characterized by filling of 5f shells. The major difference in bonding behavior in the solid state between these two series of elements is that 5f elements have delocalized f electrons whereas 4f electrons are localized in lanthanides and do not readily participate in bonding. This is where pressure plays an important role at room temperature. High pressure can delocalize the 4f shell and it can cause electron transfer between various subbands in the conduction electrons such as sp—and spd f. Participation of f electrons in bonding is accompanied by a volume coUapse in various lanthanides. 

The similarities and differences between the electronic configurations for the lanthanides and actinides (see tables 1 and 23) do provide some insights into the physics and chemistry of these two f series of elements. Beyond plutonium the actinides tend to be more lanthanide-like in behavior, although there are still notable differences between the two series of elements. For the actinide oxides whose solid-state properties have been studied (e.g., through ES2O3), one of the major differences is the greater ease... 

The intra-rare-earth phase diagrams often exhibit total miscibility in the liquid state and in the solid state (except for Eu and Yb). A miscibility gap in the liquid state is present when uranium is alloyed with thorium. The analysis of the phase diagrams of actinides with rare earths shows a particular behavior of uranium and, to a lesser extent, of plutonium. All phase diagrams of uranium alloyed with rare earths have a miscibility gap in the liquid state. This is also observed with plutonium however, with the elements... 

The main features of the phase diagrams of the elements of column IB (Cu, Ag and Au) with rare earths and actinides are presented in figs. 67-69. All the phase diagrams exhibit intermetallic compounds except U-Ag where a miscibility gap in the liquid phase is observed. A unique behavior of uranium with Cu and Au is also observed, that is the presence of one or more intermetallic compounds in the solid state and a miscibility gap in the liquid phase. In the Pu-Ag system a similar behavior is observed. 

Investigations may be carried out on the tracer level, where solutions are handled in ordinary-sized laboratory equipment, but where the substance studied is present in extremely low concentrations. Concentrations of the radioactive species of the order of 10 m or much less are not unusual in tracer work with radioactive nuclides. A much larger amount of a suitably chosen non-radioactive host or carrier is subjected to chemical manipulation, and the behavior of the radioactive species (as monitored by its radioactivity) is determined relative to the carrier. Thus the solubility of an actinide compound can be judged by whether the radioactive ion is carried by a precipitate formed by the non-radioactive carrier. Interpretation of such studies is made difficult by the formation of radiocolloids, and by adsorption on glass surfaces or precipitates. Tracer studies provide information on the oxidation states of ions and complex-ion formation, and are used in the development of liquid-liquid solvent extraction and chromatographic separation procedures. Tracer techniques are not applicable to solid-state and spectroscopic studies. Despite the difficulties inherent in tracer experiments, these methods continue to be used with the heaviest actinide and transactinide elements, where only a few to a few score atoms may be available [11]. 

The 3+ valence of americium is the most stable in solution as well as in solid compounds, as shown by its behavior as a typical actinide element. Because of the similar ionic radii of Am " (r = 0.99 A.) and Nd " r = 0.995 A.), there is a close relationship in the chemical behavior of these elements in the 3- - valence state. 

The actinides are probably more like the lanthanides in their oxide forms than in the solid phases of their other compounds. This is especially true when considering the metallic states, where large deviations are apparent. If an actinide exists in a particular oxide stoichiometry (e.g., a sesquioxide), it is likely to have comparable chemical and physical behavior to that of a lanthanide sesquioxide that has a similar ionic radius. The first important point for these two series of oxides is whether a particular stoichiometry is formed by different members of each series. 

As indicated in Fig. 16.10 for Cf ", / r = 0.47 for emission from an excited (J = 5/2) state to a lower-lying (J = 11/2) state, while / r = 0.14 for emission to the ground state. In the case of the J = 5/2 state, it would be appropriate to monitor for fluorescence near 13000cm as well as near 20 000 cm The identification of the mechanisms of non-radiative relaxation of actinide ions in solution as well as in solids [57] remains an important area for research. Extensive experimental results for lanthanide systems are available for comparison with those obtained for actinide ions. It should be possible to explore sensitively bonding differences between selected actinides and lanthanides by examining their excited-state relaxation behavior. 

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