Cation-Anion Radius Ratio
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In Condensed matter physics the Cation-Anion Radius Ratio is the ratio of the radius of the cation to the radius of the anion in a cation-anion compound. This is simply given by rC / rA
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, this case occurs when the radius ratio drops below 0.155. At the stability limit the cation is touching all the anions and the anions are just touching at their edges (radius ratio = 0.155). Beyond this (radius ratio > 0.155) stability limit the compound will be stable.
