ABX4, A2BX6, etc. with rutile-like structures

Homologous series of oxides structurally related to the rutile structure. Vanadium and titanium form series of oxides M 02n i which have structures related to the rutile structure in the following way. Slabs of rutile-like structure, n octahedra thick but extending indefinitely in two dimensions, are connected across 

The positions of the Ca ions in the fluorite structure are those of cubic closest packing and the positions of the F ions correspond to all the tetrahedral holes. However, the ions which are actually in contact are the F ions (F—F, 2-70 A, twice the radius of F ) whereas the shortest distance between Ca ions is 3 8 A, which may be compared with the radius of Ca (1-0 A). The structure is therefore 

A considerable number of fluorides oxides, and oxyfluorides have structures which are related more or less closely to the fluorite structure. The relation is closest for compounds with cation anion ratio equal to 2 1 with random arrangement of cations or anions, these structures having the normal cubic unit cell examples include the high-temperature forms of NaYp4 and K2UF6 and the oxyfluorides AcOF and HoOF. As in the case of the NaCl structure there are tetragonal and rhombohedral superstructures, which are described in Chapter 10. These are the structures of a number of oxyfluorides  

The fluorite superstructure of 7-Na2UF6 is illustrated in Fig. 28.3 (p. 995). There is slightly distorted cubic coordination of M in Na3UF8 (and the isostructural Na3PuF8). Owing to the arrangement of cations in this structure (Fig. 6.10) the unit cell is doubled in one direction and is a b.c. tetragonal cell. 

An interesting distortion of the fluorite structure to give weak metal-metal interactions is reminiscent of the distorted rutile structures of certain dioxides. 

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