Actinide atom

Magnetic measurements of PuFi, between 4.2 and 300 K are consistent at high temperatures with older measurements (10-12). The large temperature dependent diamagnetism observed earlier was not found. Up to 100 K the susceptibility is nearly temperature independent with a value of X ip 2940 x 10-6 emu. The Curie-Weiss behavior near room temperature indicates population of a higher first excited state. The structure of PuFi, is isomorphic with that of UFi, (13), where two different sets of actinide atoms are 8-fold coordinated by a distorted antiprism. 

Table 3 The Energy Needed to Reduce the Occupation Number of the 7s Orbital from Two to One in the Actinide Atoms Ac-U (in eV)a.

SYMBOL Ac PERIOD 7 SERIES NAME Actinide ATOMIC NO 89... 

SYMBOL Cm PERIOD 7 SERIES NAME Actinides ATOMIC NO 96 ATOMIC MASS 247 amu VALENCE 3 and 4 OXIDATION STATE +3 and +4 NATURAL STATE Solid... 

SYMBOL Cf PERIOD 7 SERIES NAME Actinides ATOMIC NO 98... 

Actinide atoms, in their ground configuration, comprise the closed shell electronic structure of the noble gas radon... 

The results of atomic spectroscopy as well as atomic quantum calculations have made it possible to determine the ground state of the free actinide atoms. These results (see Table 1 ) (that will be reviewed in the next section of this Chapter) confirm the progressive filling of the 5f shell. From the point of view of the electronic structure of the free atom, therefore, question ii. is solved in the sense of actinides being a series in which the unsaturated 5 f shell is progressively filled (only one or two electrons being accomodated in the 6d shell). 

The concept of valence developed in the preceding section is the basis of the first correlations aiming at a global theory of the actinide metallic bond. These correlations were established between the atomic volumes of actinide elemental metals, and the electronic configuration of the actinide atoms Their aim was to provide a general theory of actinides (i.e. to give an answer to the questions i. and ii. of Sect. A.I.l.) within the framework of a simple model of the metallic bond. 

The general shape of these curves at least for pnictides up to AnAs indicates clearly that the metallie bond is dominated by the electronic structure of the actinide atom. In the further discussion of this chapter (and in Chaps. C-F) it will be made clear that the shape is explained, contrarily to Zachariasen s hypothesis, by means of the f participation in the bond. 

The rest of the chapter is structured in such a way as to give the reader the possibility to get convinced of this description. The description originates from the peculiar properties of the 5 f electron states. Therefore, in Part II, these states will be examined in free actinide atoms. In Part III, the meaning of the atomic information for solids will be discussed the puzzling problem of atomic-like localized states, which, however, spread enough to form narrow bands and allow a delocalized description. In this part, the conceptual tools to understand the puzzle are provided. 

Most results on the free actinide atom came from atomic spectroscopy and from atomic quantum calculations of wave functions and eigenvalues pf their outer electrons. This section cannot be an exhaustive review devoted to the theory and interpretation of the very complex spectra of the actinide atoms and ions. We shall recall briefly the theoretical approach used in atomic calculations and then give some of the numerous useful informations that derived from atomic studies for solid state physicists and chemists. 

A better description of the energy level structure of the actinide atoms is obtained by adding to (3) terms of relativistic origin, which, in fact, represent the magnetic interaction of orbital and spin momenta of the electrons. (They are of particular importance for actinides since they depend on the fourth power of the atomic number Z) ... 

Magnetic and spectroscopic properties of free atoms depend on the interplay of the interactions Hi and H2, since they determine the magnetic moment and the energy spectrum of the atom. Models of this interplay (coupling models) are assumed for lanthanide and d-transition elements. We shall examine in a simple way possible couplings, and point out the difficult case of actinide atoms. 

Having examined the principal concepts of the physics of actinide atoms, we shall now relate results obtained from atomic quantum calculations, with special emphasis on 5f states. 

Fig. 7. The expectation value (r) for 5 f, 6 p, 6 d and 7 s orbitals in actinide atoms (within the ground state configurations indicated on the top of the figure) (The atomic calculations have been performed by Desclaux, J. P.)...

This competition, manifested in the structure of the actinide atomic spectra, also appears as a competition between configurations there is an accumulation of many configurations, both even and odd, at about the same height, leading to a high density of energy levels (in fact, we had arrived at the same conclusion by inspecting Fig. 1 in Sect. A.I.2). 

The energy level values of the lowest spectroscopic term of the electronic configuration of lanthanide as well as actinide atoms, were tabulated by Brewer. Such tables are very useful for phenomenological correlations concerning actinide metals (see Chap. C). From these tables one can obtain Table 1 giving the ground state and the first excited level of the actinide atoms as well as of the lanthanide atoms for comparison ... 

The problem of bonding in actinide solids has been approached through the establishment of semi-theoretical correlations between thermodynamic and structural properties and the degree of participation to the bond of the different electronic orbitals of the actinide atom. 

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