Hydrates metal-oxygen bond distances

Metal-oxygen bond distances in solid lanthanide hydrates. 

Table 1. Metal-oxygen bond distances (A) in hydrated ions (Burgess 1988), oxide minerals (see compilation in Casey 1991), and orthosilicate minerals (Brown 1980 Bish and Burnham 1984) Divalent cations in the minerals are hexacoordinated to oxygens except for ZnO, Zn2Si04, BeO, and Be2Si04, which have tetrahedral coordination. Zinc has a different M-0 distance because the coordination number is from six in the aquated ion and four in the minerals...

It seems that many workers would tend to equate die distanee (d) eorrespond-ing to the first RDF peak with the average distance between the center of the ion and the centers of the nearest water molecules, d=Tion + Twater Actually, Marcus presented a nice relationship between d, averaged over diffraction and simulation data, and the Pauling crystal radius in the form d=1.38 +1.102 rp. Notwithstanding this success, it is preferable to implicate not the water radius but instead the oxygen radius. This follows from the close correspondence between d and the metal-oxygen bond lengths in crystalline metal hydrates. ... 

The structure of type I hydrates is repeated in melanophlogite 61), and that of hydrates of types I and II is repeated in clathrate silicides or germa-nides of Na, K, Rb, or Cs 59, 60). The alkali metal atoms are enclosed in cages of Si or Ge atoms and are thereby protected from attack by atmospheric oxygen. The limiting composition is 8G 46Si or 24G-136Si, or the same with Si replaced by Ge however, neither of these compositions is reached since some voids do not contain an alkali metal atom. The unit cells vary as follows with the bond distances between pairs of vertices in the polyhedra. 

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