Ratio of ionic radii

On this basis, we would expect to be able to use the ratio of ionic radii to predict possible crystal structures for a given substance. But does it work Unfortunately, only about 50% of the structures are correctly predicted. This is because the model is too... 

Investigation of what would be the crystal energy for the NaCl, or rock-salt, structure, in which the ratio of ionic radii is smaller than 0 73/1, reveals that, where six negative ions surround the positive ion, these negative ions still can make contact with the positive ion. 

The larger ions, however, will not make contact for the same ratio of ionic radii in the zinc-blende structure, with the result that the crystal energy is a minimum for this structure, which is, therefore, the stable one. 

Ratios of Ionic Radii r+,0> First Peak Positions of g (0 and and Coor-... 

Radius ratios Simple geometrical arguments based on hard-sphere ions give predictions of structure according to the ratio of ionic radii. These are qualitatively useful but not quantitatively reliable. 

TABLE 1. The different coordination numbers (CN) and coordination figures as a function of the ratio of ionic radii... 

Therefore, ratio of ionic radii = 124/181 = 0.685 (not valid for cubic structure). 

---