Defects and occupancy factors

In previous discussions of crystal structures, each atom was considered to occupy a crystallographic position completely. For example, in the crystal structure of Cs3P7, described in Chapter 5, the Csl atoms occupied completely all of the positions with a Wyckoff symbol 4a. There are four equivalent Csl atoms in the unit cell. In such (normal) cases, the occupancy of the positions is said to be 1.0. 

Crystals and Crystal Structures. Richard J. D. Tilley 2006 John Wiley Sons, Ltd 

In many compounds that are not obviously solid solutions or alloys some sites will be occupied normally, with occupancy of 1, while other sites may accommodate a mixture of atoms. The spinel structure, described earlier, (see Chapter 7, Section 7.8), can be used to illustrate this. Normal spinels are written (A )[/t2]04 and inverse spinels are written (B)[AB O4, where () represents metal 

The atomic scattering factor applicable to such alloys or solid solutions is an average value, referred to as the site scattering factor, /site. In general, if two atoms, A and B, with atomic scattering factors fA and fB fully occupy a single site in a structure, the site scattering factor is 

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