Rare earth elements behaviour

The rare earth elements (REE) form a group of elements that have coherent geochemical behaviour due to their trivalent charge and similar ionic radii. They can, however, be fractionated from one another as a result of geochemical processes operating under specific physico-chemical conditions. In order to outline general trends within and differences between the individual REE, concentrations are usually normalized to a reference system (e.g. to shale). Deviations of individual elements from the generally smooth trend are referred to as anomalies. 

For example, the fission products technetium and promethium are unique, in that they do not have any stable isotopes and do not occur in nature in measureable amounts. While promethium has a number of chemical analogues in the other rare-earth elements, this is not the case for technetium, and it is thus difficult to predict its behaviour in the geosphere. 

The position of Ba in the periodic system just before the rare earth elements indicates that the electronic charge distribution in the outermost region will be very sensitive to changes in the effective nuclear charge. Previously, in Sect. 6.1, we studied the effect of collapse of the 4f-orbital on the behaviour of 4 s and 4p holes. In this section we shall investigate the effect of collapse of the 5d-shell, as already briefly discussed in Section 3.4 (see also1073). 

The position of yttrium in rare earth chemistry has always been interesting and this is also the case with respect to complex formation. The electrostatic model suggests placement of yttrium between holmium and thulium. It has been shown that it is not the case [14]. When one considers the stability constant data of group 1 ligands, yttrium is similar to the heavy rare earths. When the second group of ligands is considered, yttrium exhibits a behaviour similar to the lighter rare earth elements. 

Many minerals containing rare earths are known [135] and since lanthanides have similar properties, then each mineral that contains one lanthanide also contains other lanthanides. Table 11.17 shows the rare earth elements to have undergone differentiation during geochemical processes although they have similar chemical behaviour. 

Rare earth elements are capable of forming various species on exposure to varied and complex environmental conditions. Studies of physicochemical behaviour of rare earth elements under various environmental conditions require a knowledge of the formation and behaviour of various lanthanide species as a function of pH, redox conditions, nature and concentration of ligands present and the solubilities of the lanthanide complexes formed. 

Ecological studies involving rare earth elements although not as extensive as biochemical studies, shows some promise in studies involving (i) sedimentation and erosion, (ii) ant behaviour, (iii) sorption by aquatic insects and (iv) root sorption. 

Hack P. J., Nielsen R. L. and Johnston A. D. (1994) Experimentally-determined rare-earth element and Y partitioning behaviour between clinopyroxene and basaltic liquids at pressures up to 20 kbar. Chem. Geol. 117, 89-105. 

Hoyle J., Elder field H., Gledhill A. and Greaves M., 1984, The behaviour of the rare-earth elements during the mixing of river and sea waters. Ceochim. Cosmochim. Acta, 48, 143-149. 

The elements scandium and yttrium, which are also considered to belong to the rare earth elements (because of their similar chemical behaviour) also have a 3+ oxidation state. The atomic stmcture of the REE is further discussed in Chap. 3 (Physical and Chemical Properties of the Rare Earths). 

Fig. 1. The relationship between valency and radius for the rare earth elements (in larger font) and a number of other geochemically important species. Note the relative isolation of the trivalent lanthanides and yttrium from most common cations, which accounts for much of their geochemical behaviour. Sc on the other hand lies close to many other common trivalent cations and enters major rock-forming mineral phases. The increase in radius of divalent Eu brings it close to Sr, whose geochemical behaviour it mimics. Note the decrease in the radius of Ce" relative to Ce which brings it close both in size and valency to U and Th. (Data are from tables 1 and 2.)...

The rare earth elements, and the lanthanides in particular, have been of decisive importance in elucidating the evolution of the moon. They provide an example par excellence of the insights which can be gained from their behaviour in a natural environment, and for this reason an extended treatment is given. 

For more complicated atoms Bohr, by ingenious methods, arrived at a scheme for the structure of atoms which corresponds with the periodic table. He assigned incomplete inner groups of electrons to the atoms of the so-called transitional elements , which explained the anomalous behaviour of the rare-earth elements. This had been foreshadowed in a periodic table proposed by Julius Thomsen, in which the transitional elements occur in the middle of long rows, and was suggested by Rudolf Ladenburg (son of the chemist Albert Ladenburg), professor of physics in Breslau. Successive electrons added to the atomic structure of such elements fill shells below the valency electrons, and hence the chemical properties remain fairly constant as the atomic numbers increase. 

There are only a few papers devoted to investigation of the thermal conductivity of metallic rare earth glasses (Guessons and Mazuer 1982). No peculiarities (compared with standard metallic glasses) have been coimected with the presence of the rare earth elements. The glass behaviour of metallic glasses must display itself in both... 

Since scandium is one of the nomnagnetic elements among the rare-earth series one might expect that the physical properties of compounds with scandium and the other nonmagnetic rare-earth elements (Y, La or Lu) are the same or at least very similar. This supposition is true in principle, however because of the much smaller atomic size of scandium compared with yttrium and also lutetium a deviating physical behaviour of the corresponding scandium compounds is frequently observable. 

If the maximum information concerning a physical property of a rare earth element or intermetallic compound is to be obtained from experimental measurements, then the specimen on which the measurement is to be undertaken should be monocrystalline. Useful information can of course be extracted from powdered or polycrystalline samples, but definitive understanding of a property inevitably requires measurements on single crystals. This is particularly true for properties which show anisotropic behaviour and this applies to all measurements involving application of a magnetic field - the magnetic anisotropy of rare earth elements... 

Thorium was recently the focus of an environmental problem on extracting rare earths from ores, such as mon-azite. Actually thorium can be utilised for nuclear fertile material, thus the electrochemical process is one of the promising techniques of separation from rare earth elements. One of the systematic studies on the chemistry of the compounds containing thorium was the development of molten salt reactors [1]. To investigate the relationship between the electrochemical behaviour and physico-chemical properties of thorium is important for process design, but structural information of the related materials is still limited [2], Thus, EXAFS analysis of molten thorium fluoride in mono- and divalent cationic fluoride mixtures was systematically carried out to elucidate the variation in local structure of thorium cation in various melts. 

Volkovich, V.A., Vasin, B.D., Griffiths, T.R. et al. (2007) Behaviour of rare earth elements in molten salts in relation to pyrochemical reprocessing of spent nuclear fuels. ECS Trans, 3(35), 493-502. 

In recent years, a number of Mg alloys have been tested under in vitro and in vivo conditions to understand their corrosion behaviour and mechanisms (Witte et al. 2006 Kannan and Raman, 2008, 2010 Kannan, 2010 Walter and Kannan, 2011). AZ series alloys show a lower corrosion current than that of pure Mg. However, aluminium-containing Mg alloys may not be the ultimate choice, simply because of the potentially toxic effects of high aluminium levels in body fluid. The influence on corrosion properties of calcium and rare-earth elements added to Mg and its alloys has been investigated, but the improvement was not significant. 

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