Lanthanides 452 Subject

Previous studies of the hydrothermal hydrolysis of tetravalent Th, U and Np (1-4) have shown a remarkable similarity in the behavior of these elements. In each case compounds of stoichiometry M(0H)2S0i, represent the major product. In order to extend our knowledge of the hydrolytic behavior of the actinides and to elucidate similarities and differences among this group of elements, we have investigated the behavior of tetravalent plutonium under similar conditions. The relationships between the major product of the hydrothermal hydrolysis of Pu(IV), Pu2(OH)2(SO.,)3 (H20) t, (I)> and other tetravalent actinide, lanthanide and Group IVB hydroxysulfates are the subject of this re-... 

Hg is much more dense than Cd, because the decrease in atomic radius that occurs between Z = 58 and Z = 71 (the lanthanide contraction) causes the atoms following the rare earths to he smaller than might have been expected for their atomic masses and atomic numbers. Zn and Cd have densities that are not too dissimilar because the radius of Cd is subject only to a smaller d-block contraction. 

The application of these methods is described in some detail for recovery of base metals and platinum group metals in Sections 9.17.5-9.17.6 focusing mainly on solution-based hydrometal-lurgical operations, largely those involving solvent extraction, because the nature of the metal complexes formed is usually best understood in such systems. NB. Extraction of lanthanides and actinides is not included as this subject is treated separately in Chapters 3.2 and 3.3. 

At the beginning of this century slow magnetic relaxation in the paramagnetic phase of a ID system was reported for a Co2+ nitronyl-nitroxide chain [58]. The dynamics was under many respects very similar to that of SMMs and these ID systems were later named SCM to underline the analogies [9]. Since then an intense research activity, though not as spread out as for SMMs, has been devoted to SCMs. The interested reader can find extensive literature on the subject, including exhaustive reviews and a book chapter [59-63]. The aim here is to provide some basic concepts of the phenomenon and to introduce some selected examples of SCMs based on lanthanide ions. 

The situation for the triiodides is only slightly better than for the tribromides. There is only one set of values, obtained many years ago at 20°C (292). These are listed, and compared with the analogous set of solution enthalpies for the trichlorides obtained under analogous conditions by the same workers, in Table XI (and see Fig. 1). As for the chlorides, the solution enthalpies for the iodides must be subject to some small degree of uncertainty in view of the possible presence of impurities (oxoiodides, lower iodides, etc.) in the samples used. It is harder to obtain really pure anhydrous iodides than chlorides in the lanthanide series (cf. Section II). As for the chlorides, solution enthalpies for the iodides become more negative going across the lanthanide... 

Once assignments of polysaccharide signals are known, they may be used as a basis in determination of the position of substitution by such groups as acetate, malonate, phosphate, and sulfate, whose a- and /3-shifts may be estimated by referral to suitable monosaccharide models. For phosphate, the phosphated carbon atoms and adjacent resonances may be identified, as they give coupled signals and are subject to lanthanide-induced shifts. These data are described in Section VI, 8. 

Lanthanides properties and general references. For a systematic treatment and general references of the physical and chemical properties of the rare earths and their compounds and alloys mention can be made to a periodical publication in which several contributions to these subjects are being collected. See for instance Gschneidner and Eyring (1978) and Gschneidner etal. (2005). We would also like to quote a sentence, included in the prefaces of all these books, which hints at the complexity and richness of the rare earth behaviour and the ever-increasing interest in their properties and applications. The mentioned sentence is as follows ... 

Preparation of base metals by coupled reduction with platinum group metals. Very pure metals of the alkaline- earth, lanthanide and actinide series can be prepared from their oxides (or fluorides) through coupled reduction by pure hydrogen in presence of platinum group metals. According to a precursory paper on this subject (Berndt et al. 1974), the preparation scheme of Li, Ca, Sr, Ba, Am and Cf was described. As an example, Ca can be obtained by synthesis of a Pt compound, followed by its vacuum decomposition and recovery by distillation of the more volatile base metal ... 

Californium is a transuranic element of the actinide series that is homologous with dysprosium (gjDy), just above it in the rare-earth lanthanide series. Cf-245 was the first isotope of californium that was artificially produced. It has a half-life of just 44 minutes. Isotopes of californium are made by subjecting berkelium to high-energy neutrons within nuclear reactors, as follows + (neutrons and A, gamma rays) — °Bk — °Cf + (3- (beta particle... 

With the present knowledge we now turn to our proper subject, viz. the influence of these c.t. and 4f 5d states on the luminescence properties of lanthanides. 

Meerwein-Pondorf-Verley reduction, discovered in the 1920s, is the transfer hydrogenation of carbonyl compounds by alcohols, catalyzed by basic metal compounds (e.g., alkoxides) [56-58]. The same reaction viewed as oxidation of alcohols [59] is called Oppenauer oxidation. Suitable catalysts include homogeneous as well as heterogeneous systems, containing a wide variety of metals like Li, Mg, Ca, Al, Ti, 2r and lanthanides. The subject has been reviewed recently [22]. In this review we will concentrate on homogeneous catalysis by aluminium. Most aluminium alkoxides will catalyze MPV reduction. 

There are a number of discussions of the photophysical properties of the lanthanide elements and previous reviews of this area have covered this subject (3,5). The features essential to the subject of this review are given below. 

For any particular lanthanide, upon subjection to full collapse, the marked drop ( 2 e V) that is incurred in the position of the bottom of the t2g band is coupled intimately with the strong contraction in ag. Firstly, the tZ-function, given its newfound occupancy, becomes more tightly bound, just as for VO2 vs TiO2,... 

Complexation between lanthanide ions and edta or related ligands was invesigated in a number of early studies, and has been the subject of a short review405 which is still useful... 

Excitation of the Eu3+ or Tb3+ ions has traditionally been indirect, by broad-band UV excitation of a conjugated organic ligand which is followed by intramolecular energy transfer to the lanthanide ion / system, followed in turn by /- / emission.614 However, more recently, following the advent of tunable dye lasers, direct excitation of an excited / level is in many cases preferable. By scanning this frequency, an excitation spectrum can be obtained whose energy values are independent of the resolution of a monochromator and not subject to spectral interferences. 

In spite of considerable similarities between the chemical properties of lanthanides and actinides, the trivalent oxidation state is not stable for the early members of the actinide series. Due to larger ionic radii and the presence of shielding electrons, the 5f electrons of actinides are subjected to a weaker attraction from the nuclear charge than the corresponding 4f electrons of lanthanides. The greater stability of tetrapositive ions of actinides such as Th and Pu is attributed to the smaller values of fourth ionization potential for 5f electrons compared to 4f electrons of lanthanides, an effect that has been observed in aqueous solution of Th and Ce (2). Thus, thorium... 

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