Lanthanides selenates

The only study of the complex formation between selenate and trivalent lanthanides found is an X-ray diffraction investigation of some lanthanide selenate solutions by Johansson, Niinisto, and Wakita [85JOH/NI1], The radial distribution function was interpreted to show that inner-sphere complexes were formed. For Er an average of 0.4 SeO per Er was estimated for a solution containing 0.78 M Er and 1.55 M SeOj. The conditional equilibrium constant for the inner-sphere complex formation ... 

The solubilities of other lanthanide selenates (penta- and octahydrates) are more in line with the result obtained by Friend, while the result by Serebrennikov et al. appears excessive. The review cannot find the reason for the discrepancy and hence no value of the solubility of Nd2(Se04)3-5H20 will be selected. 

Structural Studies.—The structures of some compounds of scandium and yttrium, in addition to those of some of the lanthanides and actinides, have been reviewed. The crystal structure of scandium selenate pentahydrate, Sc(Se04)3,5H20, has been determined by single-crystal JT-ray diffraction. The scandium atoms were shown to be octahedrally co-ordinated by the oxygen atoms of the selenate groups and water molecules. The mean Sc—O distances in the scandium co-ordination polyhedra vary from 2.08 to 2.10 A and the mean Sc—O bond lengths in the selenate groups vary from 1.61 to 1.64 A. Two formula units were found in each cell. 

Thermal studies of other lanthanide salts have been reviewed by Niinisto and Leskela (1987). For most salts the total dehydration occurs in a single-step reaction, although for a few systems (e.g. the selenate and iodate hydrates) dehydration occurs via a multi-step process. 

The lattice parameters decrease from Sm to Lu according to the lanthanide contraction. The selenates are isomorphous with the sulfates and form a continuous series of mixed crystals [6]. Interplanar spacings of the powder diagram given by [7] for M = Y, Tb to Lu correspond well with the results of [6]. 

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