Presumably, both the general increase in concentrations and the changes in relative abundances of the lanthanides are the result of planet-forming processes. Relatively little attention has been given to internal separations within the lanthanide group during condensation of the solar nebula.
Lanthanides and yttrium in common sedimentary rocks
THE GEOCHEMISTRY AND MINERALOGY OF THE RARE EARTHS 9 can be igneous (crystallized directly from a melt), older sediments or metamorphic rocks (igneous or sedimentary rocks changed in mineralogy and usually in composition by the action of heat and pressure). Along the edge of the ocean closest to the continent (the continental shelf), conditions can be quite calm tectonically, and relatively pure end-joint sediments can then accumulate (sandstones of almost pure quartz, shales of almost pure clay, and carbonates precipitated from the shallow seas).
Lanthanides in igneous rocks
The basalts of the C r e s c e n t Formation (northwestern Washington) are a good example of welding of partially ocean floor-like materials onto the continent (Glassley, 1974). Rocks considered to be early crystallized products of fractionation had lanthanide distributions similar to those in the debris.
Lanthanides in Earth's interior
GEOCHEMISTRY AND MINERALOGY OF THE RARE EARTHS 39 ultramafic r o c k s (peridotites) which might have been lifted for the s u b o c e a n i c crust. 1971); with one exception, their lanthanide distributions are c o n s i d e r a b l y enriched in light lanthanides relative to chondrites or typical o c e a n floor igneous materials (fig. 21.16). The mass properties of eclogites, like those of peridotites, are consistent with the bulk properties of the upper mantle.
Toward a quantitative understanding
D values reflect the selectivity of individual phases for different members of the lanthanide series. The values fell near the middle and upper part of the range of those obtained by the p e n o c r y s t matrix method. The mineralogy of the nodules was consistent with crystallization or recrystallization within the upper mantle.
GEOCHEMISTRY AND MINERALOGY OF RARE-EARTHS (Fig. 21.25) all show strong relative enrichment in the light lanthanides, with lower concentrations of heavy lanthanides than typical for crustal materials. Assuming that the bulk crystallization conditions can cause the concentration of lanthanides in the final liquid, a peg mat will contain a high level of lanthanide-containing minerals. The different ion radii of the lanthanides may cause different crystal habits to be preferred.
The smaller ionic radii of the heavier lanthanides and yttrium cause them to require a tetragonal structure. GEOCHEMISTRY AND MINERALOGY OF THE RARE EARTHS 73 price o v e r the last d e c a d e , p r i m a r i l y due to the rapid expansion of their market. Crystal ionic radii of the rare earth elements in the trivalent oxidation state [data from Templeton et al.
Preliminary processing
Exploiting oxidation states
From rare earth hydroxide or oxide mixtures in which cerium has been oxidized, the separation of cerium is usually achieved by selective leaching of the more soluble tervalent hydroxides with dilute acid, or by complete dissolution in a more concentrated acid followed by hydrolytic precipitation. Solvent extraction of Ce(IV) nitrate has been studied extensively as an alternative method for separating cerium from rare earth mixtures. Furthermore, due to the relatively low occurrence of all three, it is not expedient to exploit the properties of the divalent state until a moderate enrichment has been achieved in other ways.
The two most widely used methods, prior to the development of ion exchange elution, as means of separating rare earth mixtures were fractional crystallization and fractional precipitation. Such practices have been summarized by several experts in rare earth separations, e.g. Spencer (1919), Yost et al. Hydrolysis occurs less extensively with La 3÷ than with the other lanthanones; and because of the greater solubility product of La(OH)3 compared to the less tervalent rare earth cation hydroxides (ca.
MoeUer (1973) reminds us that the homogeneous formation of anions, such as carbonate (from urea) and oxalate (from dimethyl oxalate), is more efficient than adding a soluble carbonate or oxalate, and that fractional precipitation in this way in the presence of selective complexing agents could have applications in solving mixtures of rare earth metals.
Ion exchange
As a result, the separation factor or distribution coefficient ratio can be expressed by the corresponding volumes at which the maxima of the concentrations of the individual components appear on the elution curve. A further application of the pH-gradient technique using ammonium o-hydroxyisobutyrate (which is superior to glycolate or lactate) has been reported by Wolfsberg (1962), Massartl-~c,~'¢e (1963) and Foti et al. al. Their assumption was that the linkages of LnCh with -N(CH3)~ on the resin lattice are somewhat similar to the solubilities exhibited by NaLnCh salts in water as reported by Marsh (1955) and depend on cation hydration.
Dybczynski (1964) next showed that raising the temperature caused an increase in the quality of the separation. Since A was chosen such that B will displace A (the sorbed A species), B will rapidly dissociate as I] at the front of the sorbed B - C band. Care must be taken in equipment design to ensure that the length of the sorbed band e x c e e d s 6h/log o~.
From this it is clear that the formation constants for H L n ( D T P A ) from H ÷ and Ln(DTPA) = about 1028 and are almost independent of the lanthanone chelated by the ligand.
Liquid-liquid ion-exchange chromatography
Solvent extraction
One of the liquids is usually an aqueous solution, usually containing a mineral acid or an inorganic salting agent, and in some cases an organic acid or anion that acts as a chelating agent. Based on the available assessments, none of the more exotic types appear to offer any particular advantage over tributyl or thosph ate as a selective extractant for dissolving lanthanide mixtures. With TBP, from aqueous solutions greater than 8 M in HNO3, the extraction in the organic phase increases on the order of increasing atomic number, but separation of the Ln's beyond Tb is rather difficult. With TBP, lanthanum, p r a se o d y m i u m and n e o d y m i u m of fairly high purity are obtained by extraction from 13-14 M HNO3 in only 10-14 steps.
Through a few stages of extraction with HDE HP from a dilute HCl solution, it is possible to remove almost all of the lighter lanthanones, leaving a mixture containing few percent Eu, which can then be easily recovered in high purity by a reduction method. Separation factors for adjacent Ln's are not particularly attractive, but the extraction of An's is remarkable. Bril, K.J., 1964, Mass Extraction and Separation, in: Eyring, L., ed., Progress in the Science and Technology of the Rare Earths, Vol.
Powell, J.E., 1964, The Separation of Rare Earths by Ion Exchange, in: Eyring, L., ed., Progress in the Science and Technology of the Rare Earths, vol.
86o/vHlc
Electronic densities of spherical symmetry
In rare-earth chemistry, you often come across statistically disordered, non-stoichiometric compounds, even with cubic symmetry. The nature of the ligands determines the numerical values (when sufficient information is known about the central atom) of the parameters of the description in the spectrochemical series and the nephelauxetic series. It is very likely that the N value of the scandium(III) aqua ion is greater than 6, despite the existence of octahedral Sc(III) anion complexes (such as S cF 6 3).
It is now known that this extrapolation (which was not based on spectroscopic observations) is only valid for cerium [Xe]4f5d6s 2, gadolinium [Xe]4f75d6s 2 and lutetium (which is most practical to consider as the first member of the 5d A closer analysis of this problem comes down to the estimation of the absolute energy for the ionization process It is clear that the sum of the last two quantities corresponds to the Madelung constant 2 in the special case ru = rx and to an even greater Madelung constant in all other cases.
A n y h o w is not generally recognized that a corollary of the L a t i m e r hypothesis a definite value for eq.
Electronic configurations and multiplet structure
There is no striking difference between the chemistry of the four lanthanum-like ground states (La, Ce, Gd and Lu) and the other 11. This is one observation (although it may be accompanied by a few examples of comments given below), but it is absolutely true that the monatomic units of which the discrete energy levels are classified by Moore are the best available (and only) quality. solving the Schr6dinger equation would give the energy differences if it is valid. It is noted that this barycenter refers to the actual J levels belonging (in a classification way) to a given configuration, and therefore includes conceivable differences in correlation energy in the monatomic units. n l , n'l') of interelectronic repulsion is defined for the energy of the barycenter of (nl)l(n'l') 1 (or alternatively (nl) z, giving A.
Wavenumbers (cm -1) of the electron transfer bands of 4f-group complexes in solution and in reflection spectra of crystals. There are not many other clear cases of electron transfer between two oxidation states of the same lanthanide, although Allen et al. F o r the chemists, the most interesting aspect of the photo-e c t r o n spectra of lanthanides is the v a l e n c e region that contains the signals due to R4f.
Jorgensen and Judd (1964) pointed out that many "conjugated" types of ligands (/3-diketonates, carboxylates, nitrate and, curiously, gaseous RI3) develop strong pseudoquadrupolar hypersensitive transitions with selection rules such as electric quadrupole transitions (in Russell-Saunders) coupling preserves S for the ground state, whereas the typical cases decrease both L and J by two units), but with inaccessibly large numerical values of the oscillator strength.
T. CARNALL Frequency of a transition (sec -l)
- Introduction
- Theoretical treatment of 3+ lanthanide solution absorption spectra
- Fluorescence spectra in solution
Many of the latter are based primarily on experimental results obtained with single crystals (Dieke (1968)). Energy of the lowest level in the 4f~-'5d configuration relative to that of 4f N for the lanthanides. The angular parts of the interactions can be precisely calculated using Racah's tensor operator methods (Judd, 1963).
The values of the energy level parameters obtained from the absorption spectra corresponding to the lanthanide solution are given in Table 24.2 (Carnall et al., 1968a). RARE-EARTH IONS IN SOLUTION 191 where c ( a , S, L ) are the numerical coefficients resulting from the simultaneous diagonalization of the t e r m s given in Eq. Oscillator strengths ( P ' ) of the main magnetic dipole transitions in the absorption spectra of lanthanides 3+ in solution.
As in the absorption process, there is an implicit assumption that all (crystal-field) components of the initial state are equally populated.
