Actinides light

Thorium [7440-29-1], a naturally occurring radioactive element, atomic number 90, atomic mass 232.0381, is the second element of the actinide ( f) series (see Actinides AND transactinides Radioisotopes). Discovered in 1828 in a Norwegian mineral, thorium was first isolated in its oxide form. For the light actinide elements in the first half of the. series, there is a small energy difference between and 5/ 6d7 electronic configurations. Atomic spectra... 

A full discussion of thorium electrochemistry is available (3). Thorium is generally more acidic than the lanthanides but less acidic than other light actinides, such as U, Np, and Pu, as expected from the larger Th" " ionic radius (108 pm). 

Because the sequence of neutron captures inevitably leads to looFrn which has a fission half-life of only a few seconds, the remaining three actinides, loiMd, 102N0 and losLr, can only be prepared by bombardment of heavy nuclei with the light atoms jHe to foNe. This raises the mass number in multiple units and allows the f Fm barrier to be avoided even so, yields are minute and are measured in terms of the number of individual atoms produced. 

Figure 2. The connected schematic binary alloy phase diagrams for the light actinides. The diagrams for Ac through U and for AmCm are estimates based upon the pure elements.

Figure 3. The lattice parameter for the family of rock-salt structure actinide-antimonide compounds is shown where the line is for the corresponding lanthanide compounds. The metallic radii for the light actinide elements are plotted. The smooth line simply connects Ac to the heavy actinides. In both cases the smooth line represents the ideal tri-valent behavior.

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

Apart from d- and 4f-based magnetic systems, the physical properties of actinides can be classified to be intermediate between the lanthanides and d-electron metals. 5f-electron states form bands whose width lies in between those of d- and 4f-electron states. On the other hand, the spin-orbit interaction increases as a function of atomic number and is the largest for actinides. Therefore, one can see direct similarity between the light actinides, up to plutonium, and the transition metals on one side, and the heavy actinides and 4f elements on the other side. In general, the presence or absence of magnetic order in actinides depends on the shortest distance between 5f atoms (Hill limit). 

Concerning induced orbital moments of U-based intermetallic compounds, many PND experiments have been performed and have shown that the ratio iL/ -is can be used as a measure of the hybridisation [42-44] (in the light actinides, orbital and spin moments are oppositely directed and the neutron magnetic form factors are highly sensitive to the ratio uL/us). Indeed, this ratio is reduced as compared to the free ion expectations (Figure 4). 

Figure 4. Dependence of the ratio u u on the number of 5f electrons for light actinide compounds x free ion values, ° experimental values, form band calculations. The hybridisation between 5f and 3d electrons leads to the reduction of the 5f orbital moments (metallic covalency).

Figure 5.14. Compound formation capability in the binary alloys of Sc, Y, light trivalent lanthanides (as exemplified by La), heavy trivalent lanthanides (exemplified by Gd) and of the actinides (exemplified by Th, U and Pu). The different partners of the 3rd group metals are identified by their position in the Periodic Table. Notice that a sharper subdivision between compound-forming and not forming metals will result from a shifting of Be and Mg from their position in the 2nd group towards the 12th group (see 5.12.3). The behaviour of the divalent lanthanides Eu and Yb is shown in Fig. 5.7 where it is compared with that of the alkaline earth metals.

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

On the contrary, there is a spread of oxidation numbers for the light actinides (at least up to Cm), which, for Pu and Np, range from 3 to 7 After Cm, however, the trivalent oxidation state is always met, and this second half of the actinide series approaches more the behaviour of the lanthanides. 

Thus, the implications about the chemical behaviour derived from a Periodic Chart in which the actinides are all placed in one line may be somewhat misleading. Rather, it appears that we should distinguish two parts in the series one up to Cm ( light actinides ), another one from Cm on ( heavy actinides ). 

For actinides up to Am, the plot of atomic radii vs. Z is given in Fig. 3. In the same figure, the same plot is given for d-transition elements. The first obvious observation is that the situation does not ressemble at all the lanthanide one. The second observation is that, for the light actinides (up to Pu) the trend which is followed ressembles the (almost... 

Considerations on the crystal structures and other physical properties of the light actinides have triggered a large effort in quantum calculations for the wave functions of the outer electrons of actinides, including in atoms as well as in solids. 

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. 

Since band theory has proved an invaluable tool for the description of the light actinide solids. Sects. 1 and 2 will be devoted to the band description and to its limits. Sect. 3 analysing in more details the band one-electron Hamiltonian, and the ways it can be written in order to take into account electron-electron interaction. 

For the light actinides from a) and b) qualitative consequences may be drawn. It follows, from f-f overlapping, that the atomic 5 f wave functions will broaden into 5 f bands when building the metal. Moreover, since the 5 f, 6 d and 7 s energy eigenvalues are very close (see Sect. A.II) the 5 f band will hybridize strongly with the 5 d and 7 s bands. 

The f-f overlapping in light actinides may cause broadening of the 5 f wave functions into 5 f bands. On the other hand, from Am on, this overlapping having decreased, this effect occurs much less. It follows that physical properties which depend from 5f orbitals may be better understood, in one case, in the band Umit, in the other case, in the atomic limit. 

For the non-relativistic case (Schrbdinger equation), T = -V. For relativistic case (Dirac equation), T = c a p + 3mc where m is the rest mass of the electron, c is the velocity of light. We have preferred to write the T operator in a general form, covering both cases, given the importance of the relativistic approach in band calculations for actinide solids - see Chap. F... 

Thus, the theoretical approach to light actinide metals must be made through band... 

Whereas Johansson, following Herring s argument assumes that Uh should always be less than Ujt, Herbst and Watson from relativistic band structure calculation found that while this is true for light actinides, the situation is reversed above Pu, thus favoring the transition between Pu and Am. 

Fig. 20. Schematic representation of the s, p, d and f partial contributions to the total energy of electrons in the conduction band of a light actinide metal. The different R s denote the radial extension of the different contributing orbitals. R (f-included) and R 2n-f refer to the equilibrium volumes when the 5 f electrons are itinerant and when they are non-binding (from Ref. 77)...

The Cohesive Energy and the Bulk Modulus of Light Actinides in Friedel s 101... 

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