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The elements X to be considered here are S, Se, Te, Cr, Mo, W and U. In almost all cases the anion has the composition XO4, but this can arise in one of two ways. The commonest is for an isolated tetrahedral anion XO4 to be formed; alternatively, octahedra XO6 share two-thirds of their oxygen atoms to give the stoichiometry XO4. There are two further situations which will be discussed, though these are definitely known only in synthetic substances. One involves the existence of materials containing an X2O7 group. This may be a genuine pyro-anion, as in K2Cr2O7, or it may be an assembly of octahedra (perhaps similar to that in pyrochlore), as in some diuranates. The other occurs where the formula of a material may be written as that of a salt hydrate, whereas the material is really derived from a hydroxy-acid. An example is Na2TeO4, 2H2O which is really the disodium salt of Te(OH)6, i.e. Na2H4TeO6.

We may therefore divide the materials to be studied into five classes, according to the structure of the anion involved:

Class 1. Isolated tetrahedral anions XO42-, formed always by S, Se and in many cases byCr, Mo, W.

Class V. Compounds derived from hydroxy-acids. All the Te compounds discussed are of this type.

As in the previous chapter, the cations are grouped so as to emphasize the structural similarities; a few odd cations are placed arbitrarily in groups so as to avoid having too many groups. The groups are:

Group A.

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