Actinide metal series

A simple application of Hubbard s criteria to the actinide metals series, with a non-hybridized Wf, would tend to indicate Pu as the first localized metal. Two complications, however, occur for the actinide metals ... 

Table 4. Comparison of Ua Uh, Whyb in the actinide metals series from Th to Am. In the last row, only the qualitative departure from 1, rather than a well defined value for W/Uh is indicated, since evaluations different from those selected, are reported in the literature for Uh and W. Notice the peculiar position of Pu metal and the localized WAJ value for Am metal...

The Mott-like transition, a central concept for the description of the actinide metal series, causes the sudden increase of the atomic volumes, encountered when between Pu and Am (Fig. 3). All other properties indicate the onset of a 5f localized behaviour at Am (see Part V) the 5 f pressure, which had contained to smaller values the equilibrium interactinide distance, suddenly gives in, with the withdrawal of the 5f s within the atomic core. The occurrence of such a transition within a series characterized by an unsaturated shell, is a unique phenomenon of the actinide series. In lanthanides, it does not occur except perhaps under pressure in cerium metal the approaching of cerium atoms induces suddenly the itineracy of 4f orbitals and a sudden volume collapse - see Chap. C. Neither it occurs in d-transition metal series, where the atomic volumes have an almost parabolic behaviour when plotted vs. Z (see Fig. 3 and Chap. C). The current... 

Examples of Semi-Theoretical Thermodynamic Treatments for the Actinide Metal Series... 

This is the Mott-like transition in the actinide metals series, introduced by Johansson and already discussed in Chap. A. 

No photoemission spectra are unfortunately available for Np metal, in the tight actinide 5 f itinerant side, and on Cm, Bk, Cf, etc. on the heavy actinide 5 f localized side. It is worthwhile to stress the need for good photoelectron evidence on these systems in order to shed more light on the elemental actinide metals series. 

Partial localization of the 5 f states in the light actinides (line III of subsection b) might cause the appearance of satellite structures at energies not very far from Ep in their valence band photoemission spectra. If such structures could be convincingly demonstrated, important information would be added to the theoretical analysis of the locahza-tion vs. itineracy problem of the actinide metal series. 

Transitions from a localized to an itinerant state of an unfilled shell are not a special property of actinides they can, for instance, be induced by pressure as they rue in Ce and in other lanthanides or heavy actinides under pressure (see Chap. C). The uniqueness for the actinide metals series lies in the fact that the transition occurs naturally almost as a pure consequence of the increase of the magnetic moment due to unpaired spins, which is maximum at the half-filled shell. The concept has resulted in re-writing the Periodic Chart in such a way as to make the onset of an atomic magnetic moment the ordering rule (see Fig. 1 of Chap. E). Whether the spin-polarisation model is the only way to explain the transition remains an open question. In a very recent article by Harrison an Ander-... 

The Van Arkel process can also be used to prepare actinide metals if the starting compound reacts easily with the transporting agent (I2). The thorium and protactinium carbides react with I2 to give volatile iodides above 350°C these are unstable above 1200°C and decompose into the actinide metals and iodine. Attempts to prepare other actinides, such as U and Pu, through the process were not successful, because from Th to Pu along the actinide series, the vapour pressure of the iodide decreases and the thermal stability increases. 

Figure 5.11. Connected binary phase diagrams of the actinides. The binary phase diagrams (temperature vs. composition) for adjacent actinide metals are connected across the entire series (two-phase regions are in black, uncertain regions in grey). The transition from typical metallic behaviour at thorium to complex behaviour at plutonium and back to typical metallic behaviour past americium can be noticed (adapted from Hecker 2000).

Uranium is the fourth metal in the actinide series. It looks much like other actinide metallic elements with a silvery luster. It is comparatively heavy, yet malleable and ductile. It reacts with air to form an oxide of uranium. It is one of the few naturally radioactive elements that is fissionable, meaning that as it absorbs more neutrons, it splits into a series of other lighter elements (lower atomic weights) through a process of alpha decay and beta emission that is known as the uranium decay series, as follows U-238—> Th-234—>Pa-234—>U-234—> Th-230 Ra-226 Rn-222 Po-218 Pb-2l4 At-218 Bi-2l4 Rn-218 Po-2l4 Ti-210—>Pb-210—>Bi-210 Ti-206—>Pb-206 (stable isotope of lead,... 

The first actinide metals to be prepared were those of the three members of the actinide series present in nature in macro amounts, namely, thorium (Th), protactinium (Pa), and uranium (U). Until the discovery of neptunium (Np) and plutonium (Pu) and the subsequent manufacture of milligram amounts of these metals during the hectic World War II years (i.e., the early 1940s), no other actinide element was known. The demand for Pu metal for military purposes resulted in rapid development of preparative methods and considerable study of the chemical and physical properties of the other actinide metals in order to obtain basic knowledge of these unusual metallic elements. 

All considerations in the following development of this chapter answer this question affirmatively. This affirmative answer means that the light actinides (up to Pu) constitute a new kind of transition series, as yet unknown, where the 5 f wave functions play the role that d-wave functions have in the d-transition metal series. After Pu (with Am (and Cm) occupying an intermediate and particularly interesting position) heavy actinides may on the contrary be described as a new lanthanide-type series. 

Contrary to the lanthanide metals, at least in the first half of the series, the conduction band of the actinide metals (bonding band of the metal) will be very complex. It will consist of 6 d, 7 s and 5 f admixtures. The physical properties, even the magnetic ones will be determined by this complex conduction band. 

Table 5. Ground state properties of heavy actinide metals (from Am to Es), emphasizing the lanthanide-like character of this part of the series...

This treatment aiming to evaluate thermodynamically the orbital character of the bond in actinide metals, follows closely the general features illustrated above and has a particular value inasmuch as it is accompanied by a fairly comprehensive survey of the chemical and physical properties of actinide metals known at that time. In it, the metallic radius and the crystal structures are taken as valence indicators AH nd Tm as the bonding indicators . The metallic valence, however, is not taken as constant throughout the actinide series, but rather allowed to vary. The particular choices are justified by physical and chemical arguments, which are taken in support of the hypothesis chosen. 

These quantities have been measured for most lanthanide and actinide metals (see Nugent ). For trivalent lanthanide metals, they correspond, invariably, to the destruction of the (s, d) metallic bond, without damage to the 4f shell. Therefore, AHf vs. Z is a slowly var5dng curve across the lanthanide series. 

It is possible to characterize f-electron states in the actinides in quite a simple manner and to compare them with the states of other transition metal series. To this, we employ some simple concepts from energy band theory. Firstly, it is possible to express the real bandwidth in a simple elose-packed metal as the product of two parts . One factor depends only upon the angular momentum character of the band and the structure of the solid but not upon its scale. Therefore, since we shall use the fee structure throughout, the scaling factor X is known once and for all. 

From Table 1 the scheme for the actinide metals shown in Fig. 4 is arrived at. The valence band structure is evidently more complicated in detail than that of the d-transi-tion metals because there are now four different angular momentum states to deal with. However, the d bands are now broad conduction bands. This is not surprising since the broadening of d-bands is a systematic trend from the 3rd to the 5th transition metal series and has now passed a stage further. The reason for this is that the wave functions of each new d-series must be orthogonal to those of the earlier series. The necessary additional orthogonality mode extends the wave functions spatially and broadens the bands. Precisely the same phenomenon occurs between the 4f and 5f series. Thus d-electrons play the role of the major conduction electrons in the actinides and the relative population of the sp conduction bands is reduced. The narrow f-bands are pinned to the Fermi level... 

Self-consistent energy band calculations for the actinide metals have been made by Skriver et al. for the metals Ac-Am. The modified Pauli equation was used for this series of calculations but the corrections arising from use of the Dirac equation have recently been incorporated An fee structure was assumed for all the metals in both series of calculations. 

Owing to the large variety of surfactants, metal ions, and complex metal ions that have been incorporated into LB films, variations of the stoichiometry given in Eq. (2) are plentiful. Some of these are outlined in the following section. With fatty acid films, metal ions and complex metal ions containing something other than a divalent charge include Ag+, Fe3+, Ti(IV) from the transition metal series, U(III) from the actinide series, and M3+ from the lanthanide series (7). 

Atomic-like f electron states in condensed matter were first studied in rare-earth and actinide metallic or non metallic compounds. There the multiplicity of the f states and related properties like magnetic moment, Curie-Weiss susceptibilities and spectra (where the crystal field splitting is measured) indicate that for most of the rare-earth series (RE) it is a good approximation indeed to consider those f electrons as atomic-like states. Then for the calculation of properties we can treat the f electrons in those compounds within the same approximations as for the core electrons and assume that the interaction between f electrons in different sites is carried through the conduction or the valence electrons. 

The occurrence of a heavy-electron state in metals is most distinctly observed in compounds where one of the chemical constituents is an element of the rare-earth (4f) or actinide (5f) series. Within these series, it is the elements at the beginning or the end of the res-Sective row of the periodic system that are most likely involved in this effect (Ce, Yb, U, Np). 

The electrical resistivities of most of the lighter actinide metals - due to their f electron participation in bonding -differ remarkably from those of "normal" metals. The resistivities start to increase along the actinide series after Pa, and reach a maximum at Pu, before localization of 5f electrons sets in. (Figure 2). 

The physicochemical properties of actinide metals confirm the presence of band-like 5f electrons up to Pu. The participation of these 5f electrons in the metallic bond is assumed to begin with Pa. In the first half of the actinide series, 5f electrons are similar to d electrons in typical transition metals the 5f electron orbitals are more extended than 4f orbitals for the light actinides, 5f electrons are "delocalized" and hybridized in a rather large band with 6d and/or 7s electrons. Starting with Am, the 5f electrons are localized again, like 4f electrons in the lanthanides. 

The actinide element series, like the lanthanide series, is characterized by the filling of an f-electron shell. The chemical and physical properties, however, are quite different between these two series of f-electron elements, especially in the first half of the series. The differences are mainly due to the different radial extension of the 4f- and 5f-electron wavefunctions. For the rare-earth ions, even in metallic systems, the 4f electrons are spatially well localized near the ion sites. Photoemission spectra of the f electrons in lanthanide elements and compounds always show "final state multiplet" structure (3), spectra that result from partially filled shells of localized electrons. In contrast, the 5f electrons are not so well localized. They experience a smaller coulomb correlation interaction than the 4f electrons in the rare earths and stronger hybridization with the 6d- and 7s-derived conduction bands. The 5f s thus... 

Fig.7.2. Calculated and measured equilibrium atomic raddi for the light actinide metals [7.6]. The decreasing radii in the beginning of the series, Ac-Pu, are caused by increasing occupation of bonding 5f states while the calculated anomaly between Pu and Am is due to the onset of spin polarisation...

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