Actinide compounds contraction

Crystal Structure and Ionic Radii. Crystal structure data have provided the basis for the ionic radii (coordination number — C-N — 6). For both M3+ and M4 10ns there is an actinide contraction, analogous to the lanthanide contraction, with increasing positive charge on the nucleus. As a consequence of the ionic character of most actinide compounds and of the similarity of the ionic radii for a given oxidation state, analogous compounds are generally isostmctural. 

Speciation and reactivity of actinide compounds comprise an important area for quantum chemical research. Even more so than in the case of lanthanides, f-type atomic orbitals of actinides can affect the chemistry of these elements [185,186] the more diffuse 5f-orbitals [187] lead to a larger number of accessible oxidation states and to a richer chemistry [188]. The obvious importance of relativistic effects for a proper description of actinides is often stressed [189-192]. A major differences in chemical behavior predicted by relativistic models in comparison to nonrelativistic models are bond contraction and changes in valency. The relativistic contribution to the actinide contraction [189,190] is more pronounced than in the case of the lanthanides [191,192]. For the 5f elements, the stabilization of valence s and p orbitals and the destabilization of d and f orbitals due to relativity as well as the spin-orbit interaction are directly reflected in the different chemical properties of this family of elements as compared with their lighter 4f congeners. Aside from a fundamental interest, radioactivity and toxicity of actinide compounds as well as associated experimental difficulties motivate theoretical studies as an independent or complementary tool, capable of providing useful chemical information. 

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. 

Thus the rather easily obtained atomic sizes are the best indicator of what the f-electrons are doing. It has been noted that for all metallic compounds in the literature where an f-band is believed not to occur, that the lanthanide and actinide lattice parameters appear to be identical within experimental error (12). This actually raises the question as to why the lanthanide and actinide contractions (no f-bands) for the pure elements are different. Analogies to the compounds and to the identical sizes of the 4d- and 5d- electron metals would suggest otherwise. The useful point here is that since the 4f- and 5f-compounds have the same lattice parameters when f-bands are not present, it simplifies following the systematics and clearly demonstrates that actinides are worthy of that name. 

Actin, role in heart excitation and contraction coupling, 5 81 Actinide carbides, 4 689 Actinide carbonate, 25 430-431 Actinide-gallium compounds, 22 355 Actinide oxides, 24 761 Actinide peroxides, 28 410 Actinides, 23 569. See also Actinides and transactinides Actinide series absorption and fluorescence spectra, 2 490... 

The contraction of the actinides, as measuredby changes, with atomic number, of the unit cell volume of their compounds in oxidation states III, IV, and VI, exhibits the same tetrad effect as that observed in the corresponding lanthanides. 

In Fig. 5 a minimum as in the metals is observed in the curves of the compounds with more metalloidic elements. For AnAs and AnSh this minimum tends to disappear. After the minimum (see AnN), there is a decreasing trend, which can be explained in terms of actinide contraction. Between PuN and AmN, a jump is seen, which is similar to the one met in metals (see Fig. 2). 

Spin-polarization sets up in the second part of the series, as in the case of metals, and, correspondingly, Eq. (22) should be modified with the use of spin-polarized terms. This explains the onset of an actinide contraction trend in heavier actinide NaCl compounds, as shown in Fig. 5 of Chap. A for the AnN system. 

ACTINIDE CONTRACTION. An effect analogous to the Lanthanide contraction, which lias been found in certain elements of the Actinide series. Those elements from thorium (atomic number 90) to curium (atomic number 96) exhibit a decreasing molecular volume in certain compounds, such as those which the actinide tetrafluoiides form with alkali metal fluorides, plotted in Eig. 1. The effect here is due to the decreasing crystal radius of the tetrapositive actinide ions as the atomic number increases. Note that in the Actinides the tetravalent ions are compared instead of the trivalent ones as in the case of the Lanthanides, in which the trivalent state is by far the most common. 

The ionic radii of the M3+ and M4+ ions of the actinides decrease with increasing positive charge of the nucleus (the actinide contraction) (Fig. 15.15). This contraction is due to the successive addition of electrons in an inner f shell where the incomplete screening of the nuclear charge by the added f electron leads to a contraction of the outer valence orbital. Because the ionic radii of ions of the same oxidation state are generally similar (Fig. 15.15), the ionic compounds of the actinides are isostructural. 

The ionic radii for the commonest oxidation states (Table 20-1) are compared with those of the lanthanides in Fig. 20-1. There is clearly an actinide contraction, and the similarities in radii of both series correspond to similarities in their chemical behavior for properties that depend on the ionic radius, such as hydrolysis of halides. It is also generally the case that similar compounds in the same oxidation state have similar crystal structures that differ only metrically. 

One probably can predict some of the crystallographic properties, of the tetrapositive element 104 by extrapolation from those of its homologs zirconium and hafnium. The ionic radii of tetrapositive zirconium (0.74 A) and hafnium (0.75 A) suggest an ionic radius of about 0.78 A for tetrapositive element 104, allowing for the smaller actinide rather than lanthanide contraction. Further one would expect the hydrolytic properties of element 104 and the solubilities of its compounds (such as the fluoride) to be similar to those of hafnium. The sum of... 

Analogous and carbonates can be assumed to be isostructural with the well-characterized uranyl compounds, albeit with slightly shorter bond distances to reflect the actinide contraction. For the triscarbonato complexes of Np this has been confirmed by the structures of K4Np02(C03)3 by single-crystal structure determinationof [(CH3)4N]Np02(C03)3. " ... 

Crystal structure data obtained by X-ray diffraction methods for the actinide element halides are collected in Table IV. Crystal structure determinations have been most important in identifying new compounds of the actinide elements the data are sufficiently extensive now for use in drawing conclusions regarding systematic trends and relations among the actinide elements. The tetrafluorides, for instance, supply one of the best illustrations of an actinide contraction that is entirely similar to the well-known lanthanide contraction (Table V). 

The compounds formed are normally quite ionic. The ionic radii of the actinide elements of the different valency states decrease with increasing atomic number (the actinide contraction. Table 16.1). Consequently the charge density of the actinide ions increases with increasing atomic number and, therefore, the probability of formation of conq>lexes and of hydrolysis increases with atomic number. This is illustrated in Figure 16.7, where the heavier actinides are eluted before the lighter ones because the a-hydroxy-isobutyrate eluant forms stronger complexes as the cation radius decreases. 

O Table 18.10 shows ionic radii of actinide elements together with those of lanthanide elements (Seaborg and Loveland 1990). The usefiil data on the ionic radii and coordination number are given by Shannon and Prewitt (1969). They carried out comprehensive study of crystal, or ionic, radii by analyzing the crystal structures of many fluoride, oxide, chloride, and sulfide compounds. Marcus published a data book on the properties of ions (Marcus 1997). The book covers a wide range of information on ionic radii of the actinide elements and other ions. Ionic radii of actinide elements decrease with increasing atomic number. This behavior is called actinide contraction and is one of the important examples of the actinide concept. 

Figure 4 presents the change of the crystall( raphic unit cell volume for the (R, An)T4 Alg-type compounds for different T elements (Buschow et al. 1976, Baran et al. 1987). This change is a monotonically decreasing function of increasing atomic number for actinides (only light ones) but for landianides the known lanthanide contraction has some exceptions, which is most pronounced for cerium compounds, and is probably due to the valence of cerium being different from 3+, as has been documented by X-ray spectroscopy experiments (Shcherba et al. 1992) table 3 lists the valence values. 

The importance of Mossbauer spectroscopy in probing electronic structures of intermetallic compounds, especially those reflected in the magnetic properties, is discussed by W. Potzel, G.M. Kalvius and J. Gal (chapter 116). The authors have focused their discussions on selected samples where the differences between lanthanides and actinides can be best contracted. As the authors note, the two most interesting elements, at least with respect to anomalous f-electron behaviors, Ce and U, cannot be studied by Mossbauer spectroscopy because of the lack of a suitable isotope in the case of Ce, and just the experimental difficulty in getting Mossbauer spectra for U. 

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