Cation-anion radius ratio
In Condensed matter physics the Cation-Anion Radius Ratio is the ratio of the radius of the anion to the radius of the cation in a cation-anion compound. This is simply given by
The allowed size of the cation is determined by the critical radius ratio. If the cation is too small, then it will attract the anions into each other and they will collide hence the compound would collapse. At the stability limit the cation is touching all the anions and the anions are just touching at their edges. Beyond this stability limit the compound will be stable.