Rare-earth cations, coordination geometries

Table 18.3.1. Coordination number and geometry of rare-earth cations...

Structure factor for small single crystals of C-type rare earth oxides of Y2O3, DyaOs, and H02O3 was investigated from the synchrotron X-radiation point of view [30]. Approximate symmetry in the deformation electron density (Ap) around a rare earth atom with pseudo-octahedral oxygen coordination is similar to the cation geometry. Interactions appeared between heavy rare earth atoms show a pronounced effect on the Ap map. The electron-density symmetry around second rare earth atom is also influenced appreciably by cation-anion interactions and the oxides magnetic properties also reflect this complexity. 

The isotropic shifts in the proton resonances observed in the presence of a paramagnetic lanthanide ion could arise both from a Fermi contact term and the geometric dipolar term (as an example see Quereshi and Walker (1974)). In rare earth complexes, particularly in aqueous solution where there are uncertainties regarding the geometry and composition of the coordination sphere and where there may be substantial interactions between the cation and solvent molecules, the division of the observed shift into its components is not an easy problem. Treatment of data in terms of the psuedo-contact shift only must be made with some caution even in what would appear to be the most favorable cases (Moeller, 1972). (A complete discussion of problems of these sorts will be found in chs. 18, 38, 39 and in Fischer (1973)). 

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