5f electrons, localized

The itinerant paramagnetism of UCo and probably of PuNi is a consequence of an extended 5f-5f overlap (low An-An spacings). Ferromagnetic USi and U2Ga3, on the other hand, represent a step towards the 5f electron localization with respect to UT compounds, the enhanced magnetic moment being a natural consequence. 

Pseudo-binary compounds An(X1, X2)3 X1,2 = non-transition metals Several quasi-binary An(X1, X2)3 systems (preserving the AuCu3 structure of the parent phases) were studied with the aim to follow the development of electronic properties across the onset of magnetic ordering, the latter pointing always to a certain degree of 5f-electron localization. 

In this paper, we report MCP of Use and UTe which have been carried out at AR-NE1 station of KEK, Japan, and try to separate the spin and orbital contributions of magnetic moments by combining MCP with the magnetization measurement. Furthermore, we discuss the degree of localization of 5f electrons of these samples by decomposing the MCP into localized component and itinerant component. 

Whereas there are experimental evidences for the band behaviour of 5 f electrons up to plutonium (Z = 94) (see Table 3), the same criteria show that suddenly americium (Z = 95) behaves like a normal lanthanide having well localized 5f electrons ( and Chap. C) ... 

It can be stated that up to plutonium, 5 f electrons are in the conduction band and have no magnetic moment from americium on, 5f electrons are localized and carry a magnetic moment. 

Fradin has considered the question recently and emphasized the relationship between particular measurements and the way they project out the localized and the band nature of the 5f electron states. 

We have calculated the Radon core diamagnetism for actinide ions up to americium the results are reported in Table 6. These values are smaller than usually assumed in the literature. Diamagnetic contribution for localized 5f electrons are also given - From Table 6, we see that the core diamagnetism is large and has to be taken into account when a detailed analysis of the susceptibility is made. In the case of Th metal, for example, it amounts to 40% of the experimental susceptibility. 

As an example NpOs2 and even NpAl2 display a proportional decrease of the hyper-fine field and isomer shift, indicative of increasing 5 f delocalization while no change is detected in NpCo2Si2 where 5f electrons are expected to be well localized ... 

This chapter is devoted to photoemission spectroscopy and the related inverse photo-emission spectroscopy, which are well developed experimental tools to study occupied and empty electronic levels, respectively. Special emphasis is given to the 5f electrons and their localized or delocalized character. 

Within the actinide series Pu is the most intriguing element. On the basis of the Hubbard model, and taking into account an unhybridized bandwidth Wf (due only to f-f overlapping), the Un/Wf ratio is 0.7 for U and 3 for Pu in fact, one would have expected already for Pu a 5 f electron localization, since Uh > Wf. However, a hybridization of 5 f with (6d7s) states broadens the 5f bandwidth and delays the Mott-like transition (see Chap. A) from Pu to Am This influences many properties of Pu metaf . ... 

Since the asymmetry parameter is dependent on the conduction band characteristics, i.e., specifically, on the density of states around Ep, it can be expected that the broadening effect is stronger for those actinides which still have some weak itinerant 5 f character, i.e., 5f electrons at Ep even in the nearly localized situation. 

A relativistic Hartree-Fock-Wigner-Seitz band calculation has been performed for Bk metal in order to estimate the Coulomb term U (the energy required for a 5f electron to hop from one atomic site to an adjacent one) and the 5f-electron excitation energies (143). The results for berkelium in comparison to those for the lighter actinides show increasing localization of the 5f states, i.e., the magnitude of the Coulomb term U increases through the first half of the actinide series with a concomitant decrease in the width of the 5f level. 

For elements with localized 5f-electrons (Am to Cf), the symmetric dhcp metal structme resembles that of the light lanthanides. However, high pressure relieves the f-f overlap and the americium structure becomes the same as uranium. 

The consequent increase in the nuclear charge and reduction of the shielding of the 6d- and 7s-electrons lead to a contraction of the atomic radius, similar to that previously discussed for the ionic radius. In Am and Cm, the 5f-electrons are localized in the core, which causes an expansion of the atomic radius. The differences in localization of f-electrons between light and heavy actinides are also illustrated by their different superconductive and magnetic behavior. The Th, Pa, and Am metals are superconductors Tc of 1.37, 0.42, and 0.79 K, respectively), whereas the heavier actinide metals are not superconductors but have larger magnetic moments at low temperatures. 

Additionally and equally significant, the spectral features assigned to the antibonding state of Hf 5f electrons display seven features indicating a completely removal of the Hf 4fs/2 and Hf 4f7/2 degeneracies of three and four, respectively. This is consistent with the local field induced symmetries of Hf 4f orbitals that are mixed with O 2p, and possibly O 2s states as well. This is the same mechanism that activated the Ti 3p and O 2s virtual bound state resonance absorptions in Fig. 12. The spectral widths of the Hf 5d" features (4 states) and Hf 4f features (7 states)... 

The polymorphism of the lighter actinides reflects the existence of numerous bonding (including 5f) electron states of almost identical energies. The observation of dhcp structures for the transplutonium metals indicates only a slight participation of the predominantly localized 5f electrons in the bonding. 

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... 

It is generally accepted nowadays that the sequentially increasing occupation of 5f states dominates the electronic properties in the series of actinide elements (see table 2.1). The analogy with lanthanides, in which the 4f states are gradually filled, is not complete. The 4f electronic states are confined deeply in the core of the lanthanide ion and can be treated in most cases as localized. On the other hand, a non-negligi-ble overlap of the more extended 5f wave functions belonging to neighbouring actinide atoms in a solid leads to the delocalization of the 5f states which resembles the formation of the d band in transition metals. The question about the localized versus itinerant 5f electron behaviour has become one of the most central ones within electronic structure considerations. This controversial behaviour is quite well... 

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