Question

In general, ionic crystals have negative ions that first layer and positive ions fill the interstitial sites. At this time, there are many types of interstitial sites, which are determined by the ratio of the atomic radius of anions and cations.

If so

a) What is the radius ratio of anion and cation for an interstitial site to place in a tetrahedral site?

b) What is the radius ratio of anion and cation for an interstitial site to place in an octahedral site?

Answer 1

Cations are smaller in size and anions are larger in size. As a result of which anions form the crystal lattice and cations occupy the interstitial sites formed by them.

a)

The radius ratio of a tetrahedral void is 0.225. This is the radius ratio of cation and anion at which the cation exactly fits the tetrahedral void. For the structure to be stable; the radius ratio of cation and anion for an interstitial site should be in the range

0.225 < r+ / r- < 0.414

so as to place in a tetrahedral site.

b)

The radius ratio of a octahedral void is 0.414. This is the radius ratio of cation and anion at which the cation exactly fits the octahedral void. For the structure to be stable; the radius ratio of cation and anion for an interstitial site should be in the range

0.414 < r+ / r- < 0.732

so as to place in a octahedral site.