Itinerant 5f electrons

The contributions Ef to the bonding energy E), of the itinerant 5f electrons can be evaluated in a simple way following the lines of Friedel s model Essentially the density of state peak Nf(E) of the 5 f band is substituted by a square function (see Fig. 16 in Chap. A) ... 

The whole treatment turns around the efficiency of screening of valence electrons we shall discuss this point more in detail. In light actinides, itinerant 5f electrons may provide efficient screening. 

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... 

The lighter actinide metals, from Pa to Pu, are characterized by the presence of itinerant 5f electrons. The wider radial distribution of 5f orbitals contributes to the chemical bonding in metallic phase and alloys. On the other hand, properties of the heavier actinide metals, having localized 5f electrons, resemble those of the lanthanides, which have localized 4f electrons. The itinerant 5f electrons of lighter actinide metals hybridize with the conduction electrons and contribute to the metallic bonding. 

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. 

Figure 5 gives the variation of the atomic volume in the actinide series, for the room temperature crystal structures as well as for the ccp and bcc high temperature allotropes, which exist for a number of actinides. The graph is based on the lattice parameters of Table 1, which includes also recent results. The marked dip in the curve from Th to Am illustrates the shrinkage of interactinide distance which is linked to the itinerancy of the 5f electrons in this part of the actinide series. 

The 5f electrons are itinerant and bonding, and from narrow 5f-bands (eventually hybridized). As discussed however in chapter A, they are bound to be spin-polarized, so that ... 

The energy of the 5 f electrons consists of their band-energy Ef and of a spin polarization term Asp. Ajp has an atomic origin, and tends to localize the itinerant 5f... 

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. 

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. 

Bulk magnetic data under applied pressure (Fournier et al. 1980) gave strong evidence for itinerant magnetism in UN, in contrast to UAs (and UP), where f electron itinerancy is weak, if present at all. The xSR data for the comparable AFM structure in UAs is not fundamentally different from that of UN, which lends direct evidence to the statement made earlier that xSR is not particularly sensitive to 5f electron delocalization. 

The recent observations of Coulomb transitions in actinide intermetallic compounds (McEwen et al. 1990, Osborn et al. 1990) are of particular interest because the 5f electrons are on the boundary between localised and itinerant behaviour. The neutron results give direct evidence of the persistence of strong intra-atomic correlations in, e.g., the heavy-fermion compound UPtj. The theoretical challenge is to reconcile this with the substantial evidence that the f electrons also form a coherent Fermi liquid (Fulde et al. 1988, Zwicknagl 1988). 

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