5f electrons, delocalization

The heavier actinides with localized 5f-electrons adopt the dhcp structure (similar to some lanthanides) at ambient pressure and temperature. At high pressures they undergo structural transitions to lower symmetry structures, marking the pressures at which 5f-electron delocalization takes place 23 GPa for Am (Benedict et al. 1985a), 43 GPa for Cm (Benedict et al. 1985b), 32 GPa for Bk (Benedict et al. 1984) and 41 GPa for Cf (Benedict et al. 1984). 

Discussion Since dv v exceeds the Hill limit in these materials the hybridization with ligand states dominates the mechanisms of the 5f electron delocalization. The d-states are situated well below EF in AnUX3 compounds and therefore they do not hybridize with the 5f states of the actinide atoms. If d-states with relatively high N(E) are present in the valence band (which is undoubtedly the case in UT3 compounds) the strength of the 5f-d hybridization becomes a very critical parameter. It depends mostly on details of the mutual position (overlap) of the d-states and the 5f-states which remain pinned at (or near) EF. 

The NpT2X2 compounds are usually treated as materials with localized 5f states, and crystalline electric-field effects are held responsible for the depression of the magnetic moments (2.4/tB is expected for the free-ion moment of Np). However, the heavy-fermion behaviour (found in NpCu2Si2) is able to introduce an instability of the localized 5f states. In this sense the reduction in hyperfine field at the Np nucleus could be understood as being due to a partial loss of the orbital moments resulting from 5f-electron delocalization. 

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. 

Intermetallic compounds, various alloys, and additional semimetallic compounds of berkelium should be prepared and characterized to extend the knowledge of the physicochemical behavior of berkelium in these kinds of solids. Studies of such materials under pressure would be of interest in determining the effects of the non-berkelium component on physical properties such as bulk modulus (compressibility), pressure for the onset of 5f-electron delocalization, and possible volume collapse associated with a change in the metallic valence of Bk from 3 to 4. 

Unlike 1,3-dithiepin anion 144a, the evidence for the instability of 145a and for the lack of aromaticity associated with lOn-electron delocalization through the sulfur atom has been reported 91,92). The reaction of the disodium salt of c/s-dimercaptoethylene (155) with either l,2-dibromo-3-propanol or l,3-dibromo-2-propanol yielded 6,7-dihydro-5f/-l,4-dithiepin-6-ol (156). Treatment of the methoxy derivative 157 derived from 156 with two equivalents of lithium dicyclohexylamide resulted in an effective elimination of methanol to give 5//-l,4-dithiepin (145) as a colorless liquid. Lithiation of 145 with n-butyllithium in tetrahydrofuran at —70 °C... 

In a) and P) the non bonding-hypothesis for 5 f electrons is retained, differences in cohesive energy being only due to promotion of outer electrons from one to another orbital state and ionization energies (or electron affinities) due to the different valence states attained. Therefore, any further discrepancy found with experimental values, is indicative of the metallic bonding introduced by delocalization of the 5f electrons (point y). 

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. 

In most instances, the magnetic structure of a compound can be understood to be based on interacting localized spin centers, such as classical 3d/4d/5d transition metal ions and 4f lanthanide or 5f actinide cations with unpaired electrons. Note that while the assumption of localized moments is valid for many compounds comprising such spin centers, even partial electron delocalization in mixed-valence coordination compounds renders many localized spin models inapplicable. 

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. 

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... 

NpAli (Cl5 Laves phase) orders ferromagnetically at Tq = 56K with a moment Wnp = 1.5 Ub. That value is considerably below the free-ion value (of either Np" + or Np " ") and has been taken in connection with high-pressure Mossbauer data as evidence for Np moment delocalization because of 5f-5f electron overlap. A summary of bulk properties is given by Foiunier and Troc (1985). The Mossbauer results are discussed in detail by Potzel et al. (1993). 

The key feature is likely the dual nature of the U 5f-states, i.e., the presence of both localized and delocalized U 5f-electrons. The theoretical investigation proceeds in three steps. Firstly, band-structure calculations have been performed starting from the self-consistent LDA potentials but excluding the U 5f j = 5/2, = 5/2 and jz = 1/2 states from forming... 

At present we do not know why UCr4Alg does not follow the Curie-Weiss law (Baran et al. 1987). Magnetic order as the cause of this behavior is excluded by a neutron diffraction study (Bouree-Vigneron et al. 1990). Therefore, the cause could be a partial delocalization of the uranium 5f electrons. 

Valence-fluctuation (VF) and heavy-fermion (HF) systems have been widely studied in the past two decades (for review articles, see Wohlleben and Coles 1973, Wohlleben 1976, Jefferson and Stevens 1978, Robinson 1979, Grewe et al. 1980, Lawrence et al. 1981, Stewart 1984, Varma 1985, Steglich 1985, Lee et al. 1986, Moshchalkov and Brandt 1986, Newns and Read 1987, Ott 1987, Bickers 1987, Fulde et al. 1988, Schlottmann 1989, Grewe and Steglich 1991). These systems contain either lanthanide (R) elements such as Ce, Sm, Eu, Tm, or Yb or actinides such as U or Np. They can be diluted or can form concentrated alloys or compounds with other systems. In both VF and HF systems, the 4f or 5f electrons partly delocalize due to the mixing with the conduction or valence electrons of the outer shells or of the host. In this way they become itinerant in a similar way as the d electrons in transition metals. 

In Mossbauer spectroscopy, isomer shift, which is one of the important parameters to discuss the hybridization between 5f eiectrons at uranium atoms and electrons at the other atoms, is difficult to observe as mentioned above. However, hyperfine coupling constant at nuclei is a complementary parameter to discuss the hybridization at the uranium site. Typical coupling constants in uranium-based intermetallics are about I50T//xb, to our best knowledge. When coupling constants smaller than 150 TZ/ b te obtained in some compounds, it can be concluded that the nature of the 5f electrons in them is more delocalized than that in typical uranium-based intermetallics because of the expansion of the wave functions of 5f electrons. 

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