Actinide magnetic coupling

The incorporation of radicals is a highly promising way to construct larger magnetically coupled clusters in 4f chemistry [44, 64], but this is yet to happen for the actinides [43]. A promising way forward seems to be the use of redox-active ligands [65]. 

The behaviour pertinent to the opposite limit of the well localized f states is found in most rare-earth compounds. The 4f states are situated more than 5 eV below EF in most of them. The strength of the interaction of f and conduction-band electrons is considerable (= 0.1 eV), but contributes only indirectly to the magnetic coupling of f-moments via polarization of conduction electrons (RKKY), the 4f-moment magnitude remaining preserved. For f states closer to EF, as is the case of y-Ce or some Ce compounds (a situation comparable To some actinide compounds), the interactions between the f- and conduction-band states becomes stronger. The Kondo Hamiltonian can be written as... 

As the actinides are a Second f series it is natural to expect similarities with the lanthanides in their magnetic and spectroscopic properties. However, while previous treatments of the lanthanides (p. 1242) provide a useful starting point in discussing the actinides, important differences are to be noted. Spin-orbit coupling is again strong (2000-4000 cm ) but, because of the greater exposure of the 5f... 

A larger number of examples of 5f- 3d systems are known, and have been recently reviewed [43]. We reiterate here only the potentially enormously beneficial effect of sizable 5f-3d exchange couplings exploiting the large magnetic moments of first row transition metal ions and the anisotropy of actinides. Examples are the previously discussed U-Mn wheel and chain [36, 38]. 

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. 

For solids in which IN([Xf) is very near to 1, often, although no magnetic order occurs, long-range fluctuations of coupled spins may take place, giving particular form to properties such as the (Stoner enhanced) magnetic susceptibility x, the electrical resistivity, and the specific heat of the solid. Spin fluctuations have been observed in actinides, and will be discussed in more detail in Chap. D. 

A comparable temperature dependence of resistivity was obtained for a mixed phase sample of Cel2. The formation of apparently metallic phases for only the iodides of five lanthanide and actinide elements is considered in terms of the stoichiometry, the electronic structure of the cation, the possible nature of the band, and the role of the anion. In contrast, the intermediate Lai2.1,2 phase exhibits semiconduction. Its magnetic data between 80° and 300° K. can be best accounted for if the reduced component is considered to be La ", [Xe]5d with a ground term, a spin-orbit coupling constant A — 050 cm. and only small covalency and asymmetry parameters. 

---