Physical Properties of Lanthanides

In analysis of the physical properties of lanthanide crystals the superposition model (Bradbury and Newman 1968, Newman 1971) is widespread. In this model a) the crystal field is formed only by the nearest neighbours (ligands) of an R ion b) the interaction of the 4f electron with a ligand is axially symmetric, so the Hamiltonian of an ion in the field of the v-th ligand is written as follows (the quantization axis is directed along the radius-vector Ry of the ligand, the origin placed on the R ion) ... 

TABLE 3.5 Selected Physical Properties of Lanthanide-based Ionic Liquids-... 

The arc and spark spectra of the individual lanthanides are exceedingly complex. Thousands of emission lines are observed. For the trivalent rare-earth ions in soUds, the absorption spectra are much better understood. However, the crystal fields of the neighboring atoms remove the degeneracy of some states and several levels exist where only one did before. Many of these crystal field levels exist very close to a base level. As the soUd is heated, a number of the lower levels become occupied. Some physical properties of rare-earth metals are thus very sensitive to temperature (7). 

Apart from d- and 4f-based magnetic systems, the physical properties of actinides can be classified to be intermediate between the lanthanides and d-electron metals. 5f-electron states form bands whose width lies in between those of d- and 4f-electron states. On the other hand, the spin-orbit interaction increases as a function of atomic number and is the largest for actinides. Therefore, one can see direct similarity between the light actinides, up to plutonium, and the transition metals on one side, and the heavy actinides and 4f elements on the other side. In general, the presence or absence of magnetic order in actinides depends on the shortest distance between 5f atoms (Hill limit). 

Porphyrazines with alkyl or aryl substituents are considerably more soluble than their unsubstituted counterparts (Section III. A). Consequently, various pz isomers with alkyl and aryl substituents, for example, symmetrical M[pz(A4)] and unsym-metrical M[pz(A3B)], have been reported. In particular, the symmetrical species M[pz( A4)] have been used both as vehicles to study the fundamental physical properties of metalated porphyrazines (52) as well as to make double decker or sandwich porphyrazines, cofacial dimers linked with lanthanide metal ions (34), while the unsymmetrical species M[pz(A3B)] have utilized the alkyl-aryl substituents as solubilizing groups and have been applied to all areas of pz chemistry. 

The 3rd group metals a summary of their atomic and physical properties 5.5.5.1 The rare earth metals. A summary of the main atomic and physical properties of the rare earth metals has been collected in Tables 5.11-5.13. To complete the information and the presentation of the entire series of lanthanides the data relevant to Eu and Yb have been included in these tables. However, the same data are reported also in Table 5.7 in comparison with those of the other typical divalent metals (the alkaline earth metals). As for the properties of liquid rare earth metals and alloys see Van Zytveld (1989). 

Lanthanum is the fourth most abundant of the rare-earths found on the Earth. Its abundance is 18 ppm of the Earth s crust, making it the 29th most abundant element on Earth. Its abundance is about equal to the abundance of zinc, lead, and nickel, so it is not really rare. Because the chemical and physical properties of the elements of the lanthanide series are so similar, they are quite difficult to separate. Therefore, some of them are often used together as an alloy or in compounds. 

Einsteinium has homologous chemical and physical properties of the rare-earth holmium (g Ho), located just above it in the lanthanide series in the periodic table. 

The compounds Ln(C5H5)2Cl also have been made only with the lanthanides above samarium (772). These compounds are stable in the absence of air and moisture, sublime near 200 °C, are insoluble in non-polar solvents, and exhibit room temperature magnetic moments near the free ion values (772, 113). The chloride ion may be replaced by a variety of anions including methoxide, phenoxide, amide and carboxylate. Some of these derivatives are considerably more air-stable than the chloride — the phenoxide is reported to be stable for days in dry air. Despite their apparent stability, little is known about the physical properties of these materials. The methyl-substituted cyclopentadiene complexes are much more soluble in non-polar solvents than the unsubstituted species. Ebulliometric measurements on the bis(methylcyclopentadienyl)lanthanide(III) chlorides indicated the complexes are dimeric in non-coordinating solvents (772). A structmre analysis of the ytterbium member of this series has been completed (714). The crystal and molecular parameters of this and related complexes are compared in Table 5. 

Experimental investigations of spectroscopic and other physical-chemical properties of actinides are severely hampered by their radioactive decay and radiation which lead to chemical modifications of the systems under study. The diversity of properties of lanthanide and actinide compounds is unique due to the multitude of their valency forms (which can vary over a wide range) and because of the particular importance of relativistic effects. They are, therefore, of great interest, both for fundamental research and for the development of new technologies and materials. The most important practical problems involve storage and processing of radioactive waste and nuclear fuel, as well as pollution of the environment by radioactive waste, where most of the decayed elements are actinides. 

Morss, L.R. 1994. Comparative thermochemical and oxidation-reduction properties of lanthanides and actinides. In Handbook on the Physics and Chemistry of Rare Earths, eds. K.A. Gschneider, J.L. Eyring, G.R. Choppin, G.H. Lander, pp. 239-257. Elsevier Science B.V., Amsterdam. 

The formation of luminescent lanthanide complexes relies on a number of factors. The choice of coordinating ligand and the method by which the antenna chromophore is attached to it, as well as the physical properties of the antenna, are important. In order to fully coordinate a lanthanide ion, either a high-level polydentate ligand such as a cryptate 1 or a number of smaller ligands (such as 1,3-diketones, 2) working in cooperation are required. Both 1 and 2 are two of the simplest coordination complexes possible for lanthanide ions. In both cases there are no antennae present. However, the number of bound solvent molecules is decreased considerably from nine (for lanthanide ions in solution) to one to two for the cryptate and three for the 1,3-diketone complexes. 

Recently, many researchers have paid attention to the optical properties of lanthanide-doped III-V and II-VI semiconductor nanocrystals prepared by ion implantation, molecular-beam-epitaxy (MBE) or wet chemical syntheses. Although some controversies still exist, many important results have been achieved, which may be beneficial to the understanding of the basic physical or chemical properties of lanthanide-doped semiconductor nanocrystals. 

The introduction of the btsa ligand into lanthanide chemistry by Bradley led to the isolation of the homoleptic compounds Ln(btsa)3 [7], interpreted as the first 3-coordinate lanthanide complexes. This result was indeed spectacular and exploration of the chemical and physical properties of this simple system is continuing [104-106]. 

As expected the physical properties of the sulfur derivatives 55 are very similar to those of the corresponding lanthanide(III) benzamidinates 20. Apparently no attempts have been made so far to prepare disubstituted lanthanide complexes containing diiminosulfinate ligands. 

BettineUi, M., A. Speghini, D. Falcomer, M. Daldosso, V. Dallacasa and L. Romano (2006). Photo-catalytic, spectroscopic and transport properties of lanthanide-doped Ti02 nanocrystals. Journal of Physics-Condensed Matter, 18(33), S2149-S2160. 

In summary, the special combination of physical properties of the lanthanides should translate into novel and potentially useful chemical behaviour. As stated in an earlier summary of organolanthanide chemistry 14), the challenge in the lanthanide area, therefore, is to place the lanthanide metals in chemical environments which allow exploitation of their chemical uniqueness. In the past 5 years, organometallic environments beyond the simple, original, tris(cyclopentadienyl) and bis(cyclo-pentadienyl) chloride and alkyl types have been explored and some remarkable chemistry has resulted. 

Lanthanide elements (referred to as Ln) have atomic numbers that range from 57 to 71. They are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). With the inclusion of scandium (Sc) and yttrium (Y), which are in the same subgroup, this total of 17 elements are referred to as the rare earth elements (RE). They are similar in some aspects but very different in many others. Based on the electronic configuration of the rare earth elements, in this chapter we will discuss the lanthanide contraction phenomenon and the consequential effects on the chemical and physical properties of these elements. The coordination chemistry of lanthanide complexes containing small inorganic ligands is also briefly introduced here [1-5]. 

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