Spinel interatomic distances

The ideal situation is almost never realised, and the u value for the vast majority of the known spinels ranges between 0.375 and 0.385. u increases because the anions in tetrahedral sites are forced to move in the [111] direction to give space to the A cations, which are almost always larger than the ideal space allowed by the close-packed oxygen, but without changing the overall 43m symmetry. Octahedra become smaller and assume 3m symmetry. In Table 2.1, interatomic distances are given as a function of the unit cell parameter a and the u parameter. 

Table 2.1. Interatomic distances and site radii in spinels AB O, as a function of unit cell edge a and deformation parameter u.

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