Lanthanides atomic properties

In Part II. Figs. 8 and 11 showed that in Icinthanides the 4f electrons can be described as localized so to be treated, in all respects, as atomic electrons. Thus, for instance, in the lanthanide metcds, properties related to the conduction band (which has essentially a (5d, 6 s) character) may be - in first approximation - separated from the magnetic properties related to the highly localized 4f electrons. 

The properties of cyclopentadienyl lanthanide compounds are influenced markedly by the relationship between the size of the lanthanide atoms and the steric demand of the Cp group. The former varies from La to Lu according to lanthanide contraction, while the latter varies from the least bulky Cp to highly substituted Cp, which is appreciably larger. The Sc and Y complexes are very similar to those of lanthanides with proper allowance for the relative atomic sizes. 

The decrease in radius in moving from La3+ to Lu3+ is 117.2 to 100.1 pm which is less than 114-88 pm for elements Ca2+ to Zn2+. In the case of Sc3+ to Ga3+, the radius decreases from 88.5 to 76 pm. This comparison shows that the percent contraction is greater in the case of Sc3+ to Ga3+ and Ca2+ to Zn2+ series than lanthanides series. The fact is that the magnitude of the lanthanide contraction is small and the usual interpretation of magnetic and spectroscopic properties of the lanthanides are inconsistent with the idea of considerable shielding of 4/ electrons from the chemical environment of the ion by the 5s25p6 configuration. Thus the implication that the size of lanthanide atoms or ions is determined by the 4 fn subshell must be incorrect. 

Table 19-1 Some Properties of Lanthanide Atoms and Ions...

The formation and properties of the lanthanide complex species can be best understood by summarizing first some of the pertinent general characteristics of these elements. In their ground states, the lanthanide atoms have the characteristic valence-shell electronic configurations 4/"5 d 6 or 4 / + 6 where n = 0 for lanthanum and 14 for lutetium, overlying the closed-shell xenon arrangement. The atoms are large and readily oxidized. In both aqueous or nonaqueous systems and the solid state, oxidation... 

The major drawback of these lanthanide-copper alloys is that they are irreversibly poisoned by low concentrations of carbon dioxide, as already indicated above (Owen et al. 1987). The same authors also found that ZrCu2 and TiCu2 produced catalysts which were active for methanol synthesis, but difficult to activate. In a subsequent paper Owen et al. (1990) studied ternary lanthanide-Zr(or Ti)-Cu alloys with the hope that the addition of the larger lanthanide atom could increase the possibility of hydride formation and therefore the overall properties of these Zr(Ti)-Cu alloys. They prepared a series of... 

The sp hybridization of carbon atoms bonding to either five or four lanthanide atoms would provide strong covalent bonds. The bond strength of each R-C bond would be f and 1 electron per bond, respectively, and is sufficient to overcome the catenation tendency of carbon with the formation of C2 units. When carbon is present in amounts greater than the limiting value, the occupation of adjacent octahedral interstices with poor carbon-lanthanide bond properties results in the formation of R2C3 and R2C2 phases (McColm et al. 1971). 

Since some properties of each sublattice, ei ecially the anisotropy of the lanthanide sublattice, as experimentally established, govern the behavior of the whole crystal of the magnet, the magnetic properties of the lanthanide sublattice will affect the sublattice of the transition metals, i.e., interactions between sublattices exist. The 4f electrons, however, have almost no direct bonds with the 3d electrons of the transition-metal sublattice, so the anisotropy of the 4f electrons initially transfers to the outer orbitals of 6s, 5d and/or 6p electrons of the lanthanide atoms, and these in turn interact with 4s and/or 4d electrons of the transition-metal sublattice, which is composed of and/or spd bands with the transition metal s 3d electrons. The interactions between the 4f and 3d electrons, therefore, are indirect. Figure la schematically shows the band structure in the lanthanide-transition-metal compounds. 

The discussion in this subsection will be based on the band structure of the lanthanide-transition-metal compounds as shown schematically in figs. la,b. If we think about the effect of the anisotropy of the inner 4f electrons of flie lanthanides (Sm or Nd) on the Fe sublattices, the 4f electrons should initially transfer their characteristics to 5d, 6s and 6p band electrons of the lanthanide atom, which in turn will transfer the information to 4s, 4p and 3d electrons of the Fe atoms, finally affecting the magnetic properties, for example the crystalline anisotropy, of the entire crystal. 

The lattice parameters and magnetic properties of compounds with M=Zn are listed in table 5. According to landelli and Palenzona (1967) only the compoimds of the heavy lanthanides exist. Since the Zn sublattice is nonmagnetic the magnetic properties of these alloys are related exclusively to the lanthanide atoms. The lattice parameters reported by landelli and Palenzona (1967) follow the lanthanide contraction, however, the data of... 

Table 2.1-25A Lanthanides. Atomic, ionic, and molecular properties (see Table 2.1-25D for ionic radii)...

Provided the f" configuration maintains its atomic properties in the metallic state it is obvious that we can generalize eq. (6) to a metallic situation and obtain the energy difference between the trivalent and tetravalent metallic states, A iv, for the lanthanides from the following relation ... 

Another source for different experimental results may be the enormous sensitivity of the Lj]i absorption to the local chenjistry of the absorbing lanthanide atom. Crystalline defects or small amounts of other phases below the detection limit of the usual structural analysis or other measurements might explain different valence numbers from nominally identical materials. Most of the Lm measurements were performed on finely ground polycrystalline material. Even the few materials available as single crystals were powdered in order to prepare the required thin absorption foils (about 10 pm thick). Clearly this procedure may induce defects and imperfections which may alter the elastic and electronic properties. In particular powdered... 

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