Scandium ionic crystal radius

Goldschmidt has classed also with the ionic crystals the C-modification of the sesqui-oxides, cubic crystals with 16 M2O3 in the unit of structure. The inter-atomic distances reported by him are 2.16-2.20 A. for scandium oxide and 2.34-2.38 A. for yttrium oxide, in good agreement with the radius sums 2.21 A. for Sc+3-0= and 2.33 A. for Y+3-0". 

Most trace elements have values of D< C 1, simply because they differ substantially either in ionic radius or ionic charge, or both, from the atoms of the major elements they replace in the crystal lattice. Because of this, they are called incompatible. Exceptions are trace elements such as strontium in plagioclase, ytterbium, lutetium, and scandium in garnet, nickel in olivine, and scandium in clinopyroxene. These latter elements acmally fit into their host crystal structures slightly better than the major elements they replace, and they are therefore called compatible. Thus, most chemical elements of the periodic table are trace elements, and most of them are incompatible only a handful are compatible. 

The smaller ionic radius also leads to extensive hydrolysis, which helps to explain why scandium compounds are more difficult to crystallize than those of the lanthanides and why the number of basic compounds is large. In aqueous solutions, depending on the pH, the Sc ion can be presented as [Sc(H20)g] +, [Sc(H20)50H] + and [Sc(H20)5(0H)]2 (Komissarova, 1980) there is also evidence for the formation of other polymeric species (Biedermann et al., 1956). The main product of polynuclear hydrolysis for Sc +, + as well as for In appears to... 

On the basis of its ionic radius and known structural chemistry with oxygen donor ligands (Valkonen, 1979), scandium would be expected to have the hexahydrated ion in some of its compounds, for instance in the perchlorates and bromates. No structural data appear to be available, however, obviously because of the tendency of scandium compounds to undergo hydrolysis and the difficulty of forming good single crystals in aqueous solutions. Nevertheless, the hexahydrated R ion has been discovered recently in the solid hydrates of R perchlorates (R = La, Tb, Er) (Glaser and Johansson, 1981). For details of the structure, see table 35 and section 6. 

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