4f valence electrons

Weak crystalline field //cf //so, Hq. In this case, the energy levels of the free ion A are only slightly perturbed (shifted and split) by the crystalline field. The free ion wavefunctions are then used as basis functions to apply perturbation theory, //cf being the perturbation Hamiltonian over the / states (where S and L are the spin and orbital angular momenta and. 1 = L + S). This approach is generally applied to describe the energy levels of trivalent rare earth ions, since for these ions the 4f valence electrons are screened by the outer 5s 5p electrons. These electrons partially shield the crystalline field created by the B ions (see Section 6.2). 

Okada et al. (1984) assignment of the bands in the 37000-51000cmregion to 4f 5d transition. The spin-orbit splitting was computed as 2200 cm for the 4f state. The Mulliken-population analysis revealed a 4f occupation of 1.01 electron. It was noted by Kotzian et al. (1991a, b) that due to the compactness of the 4f shell, the 4f valence electron of Ce does not contribute to metal-ligand bonding. This is reminiscent of the bonding discussed before for the diatomic lanthanide... 

Fig. 30. 4f contribution to the photoelectron spectra of a series of Ce-based compounds. All these spectra were taken on the same instrument and the background of scattered electrons subtracted in the same fashion. These spectra are the difference between on-resonance and off-resonance spectra. They are ordered by increasing amount of 4f-valence electron hybridization from the bottom to the top (mote 7-Ce-like to more a-Uke). (After Allen et al 1986.)... 

The rapid rise of the hep-Sm-type phase boundary beyond Tm (fig. 123), and the anomalous high pressures of transformation in Y (hep - Sm-type, Sm-type dhep and dhep - fee) relative to lanthanide elements was cited by Gschneidner (1985b) as evidence for 4f valence electron hybridization having a significant role in determining the lanthanide crystal structure. He noted, however, that d occupation number probably is more important in determining which crystal structure would form, as had been proposed by others (e.g., see Duthie and Pettifor, 1977 Skriver, 1983). 

Cerium is especially intereshng because of its variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small amounts of energy are required to change the relahve occupancy of these electronic levels. This gives rise to dual valency states. 

The reason usually cited for the great similarity in the properties of the lanthanides is that they have similar electronic configurations in the outermost 6s and 5d orbitals. This occurs because, at this point in the periodic table, the added electrons begin to enter 4f orbitals which are fairly deep inside the atom. These orbitals are screened quite well from the outside by outer electrons, so changing the number of 4/electrons has almost no effect on the chemical properties of the atom. The added electrons do not become valence electrons in a chemical sense—neither are they readily shared nor are they readily removed. 

Trivalent lanthanide ions have an outer electronic configuration 5s 5p 4f", where n varies from 1 (Ce +) to 13 (Yb +) and indicates the number of electrons in the unfilled 4f shell. The 4f" electrons are, in fact, the valence electrons that are responsible for the optical transitions. 

The change of the X component allows for a change of the valence electron concentration and one can thus modify the separation between the Fermi energy and the 4f energy levels. The ternary compounds CesPd24X have significantly different properties than binary CePd3 (see Section 5.5). 

The Auger spectrum of YbAl3 in the 4d-4f-4f region showed no significant difference from that of Yb metal with 4f14 configuration. In this case the valence electron transfer is due to the 4d hole state and the processes involved in valence electron transfer are shown below. 

The process of transfer of the valence electron to the 4f level suffers from some deficiencies. The 5p level being one of the outermost levels with binding energy 25 eV is unlikely to stabilize the 4f level to a significant extent. Further, the time scale involved in the electron transfer from valence level to 4f level is not compatible (< 10-16 s). Thus it is hard to explain the observed trivalent and tetravalent states in CeN. In the case of Ce, the 3d hole stabilizes the 4f level of Ce significantly below the Fermi edge. This leads to the notion that the electron transfer may not be as fast as < 10 16 s. If the electron transfer from valence band to the 4f level due to the core is fast, then it is difficult to explain the experimental observations on the XPS of 3d region of La, Ce and Pr (Fig. 9.14). 

For the highly contracted f electrons of Gd(III) type ions, the magnetic interaction is mediated by the spin polarized 5d, 6s valence electrons. To a good approximation, the 4f exchange field can be viewed as a type of contact effect [6], which only exert its influence on the orbitals centered on the Gd atom. Both the valence 5d and 6s electrons can penetrate to some extent... 

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