Crystal cerium compounds

The better known methods for the removal of cerium from rare earth mixtures depend upon the ease of oxidation of cerium and the subsequent precipitation of cerium(IV) compounds by hydrolysis. Among these are the permanganate-phosphate/ the permanganate-cerium dioxide, the electrolytic, and the potassium bromate methods. The method of G. F. Smith, in which ammonium-cerium(IV) nitrate is crystallized from a nitric acid solution of the rare earths in the presence of an excess of ammonium nitrate, is particularly valuable as a commercial method for the production of large amounts of very pure cerium compounds. 

Thus, Mosander s activities led to the originally two-element division into a six-element division. The cerium compounds are yellow at the higher oxidation level and colourless at the lower oxidation level, lanthanum compounds are white, didymium compounds are red, yttrium and erbium compounds are white, terbium compounds are pink. Chemists existed, of course, who disputed the existence of these elements. Unequivocal identification of elements was, however, possible in later times only. In the period in question, the main characteristics on the basis of which a substance could be qualified as a new element were separability, colour, crystal shape and reactivity. Even atomic mass determinations were largely uncertain, particularly in the group of the rare earth elements, it will be seen in the... 

For the ethylsulphates, with only four independent terms, the equivalent crystal-field operators were derived by Elliott and Stevens (1952) and used by them to interpret the magnetic resonance results (Elliott and Stevens 1953). Only minor modifications of the numerical coefficients of the terms were needed to give a reasonable fit for each member of the series. Some further interactions are required, however, for two cerium compounds. In the hexagonal crystal field of the ethylsulphate, the energy levels of the ground manifold J = f are split into three... 

Figure 4 presents the change of the crystall( raphic unit cell volume for the (R, An)T4 Alg-type compounds for different T elements (Buschow et al. 1976, Baran et al. 1987). This change is a monotonically decreasing function of increasing atomic number for actinides (only light ones) but for landianides the known lanthanide contraction has some exceptions, which is most pronounced for cerium compounds, and is probably due to the valence of cerium being different from 3+, as has been documented by X-ray spectroscopy experiments (Shcherba et al. 1992) table 3 lists the valence values. 

It is now clear that the relativistic screening effect of the 6s electrons as well as the other s electrons tends to promote an itinerant nature of the 4f electrons in a crystal. It is difficult at present, however, to predict correctly in which cerium compounds the 4f electrons may be itinerant, because the strong correlations between the 4f electrons make it difficult to derive a quantitative criterion for the itinerancy of the 4f electrons theoretically. 

Much less efforts have been put in the study of the cerium monochalcogenides CeY (Y = S, Se and Te), since they were considered as rare examples of cerium compounds with normal behaviour. However, the first resistivity measurements on CeS (Schoenes and Hulliger 1985) displayed a temperature dependence similar to that found in CeAl2- This prompted a more systematic investigation of the electrical resistivity of CeS, CeSe and CeTe on single crystals in a large temperature region (2 to 1000 K). 

The role of the crystal-field interaction remain obscure in all these systems, both for actinides and cerium. Whereas in the other lanthanide NaCl-type compounds, LnX and LnZ, the easy directions are given by straightforward crystal-field considerations and the value of the crystal-field potential varies in a systematic way across the lanthanide series, this is not the case in the materials discussed here. The crystal-field energy levels can be measured in the cerium compounds with neutron inelastic scattering, but have not been observed in the uranium (or higher) actinides. It is assumed that this inability to observe directly the crystal-field levels is because they are strongly broadened by the interaction between the 5f and conduction electrons. This has been the subject of much work by Cooper and his collaborators. 

The fluorite stmcture, which has a large crystal lattice energy, is adopted by Ce02 preferentially stahi1i2ing this oxide of the tetravalent cation rather than Ce202. Compounds of cerium(IV) other than the oxide, ceric fluoride [10060-10-3] CeF, and related materials, although less stable can be prepared. For example ceric sulfate [13590-82-4] Ce(S0 2> certain double salts are known. 

The crystal structures of Hf 2 (OH) 2 (S0O 3 (H2O) i, (14) and Ce2(0H)2(S0i,)3 (H20)it (14) also have been determined and found to be isomorphous to the zirconium compound. The cell constants for this series of four isomorphous compounds reflect the effect of the ionic radii on the dimensions of the unit cell. The values for these cell constants are in Table II. Thus, the cell constants for the zirconium and hafnium compounds are nearly identical and smaller than the cell constants for the cerium and plutonium compounds which are also nearly identical. This trend is exactly that followed by the ionic radii of these elements. 

The agreement is also satisfactory for lithium and sodium sulfide. The oxide was used in calculating the lithium radius, 0.60 A., for in this compound it is safe to assume that the anions are not in mutual contact. It is further highly pleasing to note that even in zirconium and cerium oxide, containing quadrivalent cations, our theoretical radii are substantiated by the experimental inter-atomic distances for this makes it probable that even in these crystals the ions are not greatly deformed. 

Cerium tetrakis(acetylacetonate), a venerable compound,695 exists as a and /3 modifications. Both these contain [Ce(acac)4] monomers of D2 square antiprismatic coordination but differ in the crystal packing. In the a form,696 Ce—0 = 2.36-2.43 A with the ring angle O—Ce— 0 = 72°, while in the / form697 Ce—0 = 2.32 A and O—Ce—0 = 71.3°. The tetrakis(acetyl-acetonates) of Zr, Hf, Th, U and Pu also show dimorphism. The tetrakis(dibenzoylmethanate) [Ce(dbm)4] adopts a similar square antiprismatic structure with Ce—O = 2.299-2.363 A and O—Ce—O = 70.8° or 71,2°.698 There is some tendency for Ce3+ /J-diketonates to pass into the Ce4+ tetrakis(/3-diketonate). A series of tri- and tetra-valent cerium /3-diketonates has been examined from the point of view of the effect of additional ligands such as Ph3PO on this process, and it was found that Ce3+ /J-diketonates were stabilized by adduct formation, particularly by 1,10-phenanthroline.699... 

Samarskite occurs in the Ural Mountains, Mitchell County (North Carolina, U.S.A.), Canada, and India. The tantalum content is often small, sometimes nil, and the rare earth oxides, chiefly yttria and ceria, are usually present in considerable number and proportions. The ore is radioactive and contains helium. It forms black, orthorhombic crystals. The density varies from 4-2 to G-2.5 It has been suggested that the niobium and tantalum are disintegration products of compounds of yttrium and cerium with the two higher homologues of manganese,4 masurium, and rhenium. 

A ternary compound of cerium with copper and antimony of the stoichiometric ratio 3 3 4 was identified and studied by means of X-ray analysis by Skolozdra et al. (1993). Ce3Cu3Sb4 compound was found to have the Y3Au3Sb4 type with the lattice parameters of a = 0.9721 (X-ray powder diffraction). For experimental details, see the Y-Cu-Sb system. At variance with this data, Patil et al. (1996) reported a tetragonal distortion of the cubic crystal structure Y3Cu3Sb4 for the Ce3Cu3Sb4 alloy which was prepared by arc melting the constituent ele-... 

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