Actinide sublattice

In actinide compounds (see table 9), ThFe4Alg is considered as the nonmagnetic standard for the actinide sublattice because the Th sublattice is nonmagnetic. The magnetic transition temperature reported by Buschow and van der Kraan (1978) was also confirmed by neutron diffraction (temperature dependence of the [110]-peak intensity) (Ptasiewicz-Bak et al. 1988). However, other neutron diffraction examinations along witii magnetic ac and dc susceptibility and Mossbauer effect investigations (Gal et al. 

Spin freezing conditions exist for the 2a site (if occupied by magnetic lanthanide or actinide ions) as well. It is believed (Gal et al. 1990) that this behavior of the lanthanide or actinide sublattice is an outcome of conflicting RKKY interactions between the antiferromagnetically coupled (Fe) moments on the 8f site and the ferromagnetic (An) moments on the 2a sites. 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

The Stoner product of UN (see Chaps. A and D) is greater than one, in agreement with the antiferromagnetic behaviour of this solid. The antiferromagnetism was attributed to itinerant band magnetism (as in some d-metals and compounds but unlike light actinide metals). In fact, cohesive properties of this solid have been well explained in a pure spin-polarised picture and Fournier et al. have shown that the magnetic uranium sublattice moment and the Neel temperature have a similar pressure dependence. Discrepancies existed, however, between calculations and experiments ... 

For one-sublattice magnets, such as Fe and Co, the Akulov or Callen and Callen theory [81] relates the temperature dependence of the anisotropy to the spontaneous magnetization and yields hi and Af° power laws for uniaxial and cubic magnets, respectively. This theory has become popular far beyond its range of applicability [82] but is unable to describe structures such as rare-earth transition-metal magnets [16, 60], actinide magnets [83], and L10 type compounds [44, 84]. 

Among the actinides only the UFexAli2-x system has been investigated over a broad composition range. In fig. 20 we present the saturation magnetization, magnetic moment of the iron sublattice and Neel/Curie points of these alloys for Fe concentration 3 6... 

The phase and structural relationships of the lanthanide- and actinide-hydride systems are outlined here with relevant corollary evidence to provide a suitable background and preparation for some of the more esoteric discussions to follow. Historically, the first realizations of latent structural complexities appeared with the advent of more precise neutron-diffraction measurements in conjunction with new low-temperature capabilities. X-ray studies have been of limited utility because only metal atom positions are defined and only the near-surface regions of opaque heavy-metal hydrides are sampled. However, the influence of the H sublattice ordering upon the metal lattice can be deduced. 

In structures where n = 7 or above, overlapping clusters are not observed, and maximum separation of the clusters occurs consistent with the composition, dimensions and symmetries of the unit cell. Furthermore, relaxation of the cation sublattice is observed to be maximized. In particular, although in the = 7 structures double vacancies do occur along <11 1) across metal atoms and where no metal atoms separate them, the latter are not found in any other known structures, even for = 9. The only intermediate compound reported for the actinide oxides is the iota phase, An70j2 (see sections 2 and 3). However, efforts to prepare other phases have not been extensive. They might occur in the heavier oxides of the series from Pu to Es since these may possess both the 3 -F and 4+ oxidation states. The structural principles appropri-... 

Clearly, much remains to be done in the UPd3 and mixed systems. One complication is the presence of two uranium sublattices. The appearance of quadrupolar distortions (McEwen et al. 1993, Zochowski and McEwen 1994) emphasizes their importance in actinide systems similar effects occur in the oxides. 

On the other hand. Gal et al. (1990) have shown that the actinide-containing compounds AnFc4Al8 (An = Th, U, Np) are all spin glasses. The so-called spin-glass temperatures, T, where hysteretic and irreversible effects in the magnetization occur, are all about 120 K for these materials. Neutron diffraction (and Mossbauer also, in the case of the Np compound) experiments show that a moment exists on the U and Np sites, but it is not clear from these studies on polycrystalline samples whether the moment directions are random or uniaxial. The dominant spin-glass behavior is, of course, provided by the Fe sublattice. This is not only shown by the various measurements, but also by the fact that Tsg is much the same for the Th compound (no 5f moments exist on the Th) as for the other actinide compounds. 

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