The light lanthanides

TABLE 9 The magnetic properties of the light lanthanides.The magnetic moments are all  

The Hund s rule ground state was chosen throughout  

TABLE 10 The magnetic properties of neodymium on a variety of crystal lattices (fee, bee, simple cubic, and hexagonal lattices) calculated as described in the text. The magnetic moments are all written in units of the Bohr magneton. The second column is the spin magnetic moment of the valence s, p, d, and f-states. The third column is the orbital moment associated with the valence electrons. The final two columns are the spin and orbital magnetic moments of the Sl-coirected f-states 

It is notable that the spin and orbital contributions of the localized states are nearly the same as for the ionic case. This shows that in the light lanthanides, the f-states are as localized as for the heavy lanthanides. For the paramagnetic moments, the agreement between theory and experiment is very satisfactory (Table 9) and much better than using a band description of adding the contributions of the localized and delocalized states (Table 8). Sm and Eu are less well described by this procedure and it could be that the assumption of pure trivalent Sm and pure divalent Eu are not fully applicable and some divalent Sm and some trivalent Eu might be mixed into the groimd state. 

Now let us consider the effect of crystal environment on the magnetic moment of the lanthanides. In Table 10, we show the results of calculations of the magnetic moment of neodymium on several common crystal lattices. A trivalent Nd ion yields a spin moment of 3/lb and an orbital moment of 6/ib- In the final two columns of Table 10, we see that the SIC-LSD theory yields values slightly less than, but very close to, these numbers. This is independent of the crystal structure. The valence electron polarization varies markedly between different crystal structures from 0.34/ib on the fee structure to 0.90/Zb on the simple cubic structure. It is not at all surprising that the valence electron moments can differ so strongly between different crystal structures. The importance of symmetry in electronic structure calculations cannot be overestimated. Eor example, the hep lattice does not have a centre of inversion symmetry and this allows states with different parity to hybridize, so direct f-d hybridization is allowed. However, symmetry considerations forbid f-d hybridization in the cubic structures. Such differences in the way the valence electrons interact with the f-states will undoubtedly lead to strong variations in the valence band moments. 

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Usually lanthanides are divided into several subgroups the light lanthanides, from La to Nd, medium lanthanides, from Sm to Dy, and heavy lanthanides. Ho to Lu. Alternatively, nomenclature such as ceric RE, from La to Nd, and yttric RE, from Sm to Lu plus Y, is used. 

It is easy to reduce anhydrous rare-earth hatides to the metal by reaction of mote electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pt mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphoms, and hydrogen at elevated temperature and remove these elements from most other metals. 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

As in aqueous solution, the lanthanide contraction favors a change from nine-coordination for the light lanthanides to eight-coordination for the light lanthanides such that [Ln(DMF)8]3+ is the major species when Ln3+ = Ce3+-Nd3+, and that this becomes the only detected species when Ln3+ = Tb3+-Lu3+ in dimethylformamide perchlorate solution (11, 92, 93, 321-323). Thus, Nd3+ is characterized by AH° = -14.9 kJ mol-1, AS0 = -69.1 J K"1 mol-1, and AV° = - 9.8 cm3 mol-1 for the equilibrium shown in Eq. (25) (93). The molar volume of DMF is 72 cm3 mol- and it therefore appears that the substantially smaller magnitude of AV° is a consequence of significant... 

This well known alloy produced by fused chloride electrolysis of the light lanthanide elements constitutes over 90% of the rare earth metals (RE3 l s) consumed for steeLnaking in the western world. It is estimated that approximately 3,000 metric tons of mischmetal, worth about 35 million, are added to liquid steel every year,... 

Mischmetall is a cheap alloy of the light lanthanides and is used in large amounts for industrial applications. 

In the first approximation we have neglected the dimers in the vapour pressure analysis. On the basis of the mass spectrometric results this seems to be justified for the light lanthanide trichlorides, and good agreement is found for the enthalpies of sublimation derived from the... 

The number of vapour pressure studies of the tribromides is significantly less. Gietmann et al. (1996) systematically measured the vaporization of the lanthanide bromides using mass spectrometry, showing the importance of the dimeric molecules in the vapour. For the light lanthanide tribromides (La, Ce, Nd) the fraction of dimers is around 0.01, but for TbBr3 it is 0.03,... 

Interestingly, Peters (1988) notices that the methylene protons are enantiotopic for the light lanthanides (R = Ce-Dy) and become diastereotopic for R = Ho-Yb which implies a dynamic intramolecular interconversion between the two helical enantiomers P- R(Ll-2H)3]3 M-[7 (L1-2H)3]3 occurring at a moderate rate on the NMR time scale. The... 

By far the most complete parametrized analysis to date is that of van Pieterson which covers the light lanthanides (Ce3+, Pr3"1", Nd3+, Sm3+ and Eu3+) doped in YPO4, CaF2 and LiYF4... 

The thin films of those obtained RPO4 NPs of 2-5 ran in size could be fabricated via spin coating. Therefore, the NEXAFS studies could be carried out (Suljoti et al., 2008). The results show that the light lanthanides tend to form monoclinic monazite phase of 3-5 nm in size and the heavy lanthanides tend to form the tetragonal zircon phase of 3-5 nm in size, while the medium lanthanides tend to form a mixture of the two phases with 2 nm in sizes. 

For elements with localized 5f-electrons (Am to Cf), the symmetric dhcp metal structme resembles that of the light lanthanides. However, high pressure relieves the f-f overlap and the americium structure becomes the same as uranium. 

The enhancement of the extraction of Ln(III) ions with ttfaH in cyclohexane or benzene by the addition of Cr(acac)3 was investigated . The equilibrium analysis suggested that the effect of Cr(acac)3 could be ascribed to the formation of a binuclear 1 1 La(ttfa)3-Cr(acac)3 adduct. The formation constants of adducts along the lanthanide series decreased with the decrease of the ionic radii among the light lanthanides and were constant for their heavy counterparts. UVV, IR and H NMR spectroscopic studies were also performed to explain the molecular structural differences between the light and heavy lanthanide complexes. 

Traditionally the item most widely associated with cerium has probably been the pyrophoric iron-mischmetal alloy for lighter flints, still in use. Mischmetal is to be termed the mixture of metals of the light lanthanides La. Ce, Pt and Nd. 

Separation of the light lanthanides, after removal of the Ce, has been accomplished in many ways, based mainly on solubility differences fractional crystallisation of the double magnesium nitrates, 2Ln (N03)3.3Mg(N03)2.24 H2O, was an early method (James, 1908). Tlie heavy lanthanides from the double sulphate solution (above) and from ores such as xenotime have been separated by fractional crystallisation of the bromates (James, 1908). Prandtl (1938) used double ammonium oxalates. Hartley (1952) obtained a 85% yield of mixed anhydrous lanthanide chlorides by direct chlorination of a mixture of monazite and carbon at 900" most of the impurities are more volatile. 

At the end of this stage, the transplutonium elements are rid of most of the light lanthanides and other fission products, it may be observed that, contrary to the data in Table III, the decontamination factors of the transplutonium elements in Ce and Eu are comparable. An example of good separation of the couple Am/Cm in M.A. treatment is given in figure ( X At the end of the L.A. treatment 2 t Cm is pure as shown in Table VII. 

Namy has recently described an alternative method for effecting Sml2-catalyzed pinacol couplings (Eq. 3.31) [54]. Using mischmetall, an inexpensive alloy of the light lanthanides ( 12/kg from Fluka), acetophenone can be reductively dimerized in 70% yield in contrast to the Endo system, no Me SiCl is necessary. Carbon-carbon bond formation is presumed to involve coupling of samarium ketyls, based on identical diastereoselectivity in the presence of catalytic and stoichiometric SmU. In the absence of Sml2, there is no reaction. 

The decompositions of the light lanthanide [81] oxalates (613 to 713 K), were generally similar to that described above for the lanthanum salt. The relative stabilities of these six salts, and also of the normal and basic carbonates, is Gd > Sm > Nd > La > Pr > Ce. With few exceptions, the intermediates (identified by composition and infi-ared measurements) were ... 

In 0.2 M Na2C03, the light lanthanides (cerium group) are precipitated quantitatively, while the remaining lanthanides and scandium are only partly precipitated [18). Separation of Ce(IV) as the hydroxide (pH 1) enables the separation of Ce from other REE. Ti, Zr, or Fe(III) can be used as carriers. Ce(IV) may also be precipitated as iodate. 

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