Miscellaneous examples of interstitial superstructures

This structure can be considered a superstructure of the AuCu(I) type with 1N atom inserted in an octahedral interstice. This structure, as the previously described cP5-Fe4N type, can be considered an interstitial ordered phase. 

The cell can be described as formed by two superimposed slightly distorted body-centred cubic subcells of the metal atoms. The O atom is surrounded by a slightly compressed Ta atom octahedron. 

In the preceding paragraphs examples of a number of so-called superstructures have been considered. Generally, it has been observed that a derivative structure has fewer symmetry operations than the reference structure it has either a larger cell or a lower symmetry (or both) than the reference structure. Typically the passage from the reference structure to the derivative structure (superstructure) may be related to the fact that a set of equipoints of a certain structure (the reference one) has to be subdivided into two (or more) subsets in order to obtain the description of the other structure. The structure of the Cu type (cF4 type), for instance, corresponds to 4 Cu atoms in the unit cell, placed in 0, 0, 0 14, 14, 0 14, 0, 14 0, 14, 14, whereas in the cP4-AuCu3 type structure the same atomic sites are subdivided, in another space group, into two sets with an ordered distribution of the two atomic species (1 Au atom in 0, 0, 0 and 3 Cu atoms in 14, 14, 0 14, 0,14 0,14,14). 

Apart from this type of superstructure, which could then be described as superstructure of the first kind, another type of structure derivation can be considered an example could be given by the relation between the CaCu5 and the CeCo3B2 structures. 

Both hP6-CaCu5 and hP6-CeCo3B2 structures are hexagonal, in the same space group (P6/mmm, N. 191), with the following occupation of the same sites  

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