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I am studying solids states. So we studied different lattices of Ionic solids by introducting smaller Cations into VOIDS. Next topic is of defects. SO it is written Frankel defect arises when a smaller cation (or anion) shifts from its lattice site to Interstitial site.

Now something is wrong here. I have seen all the threads and topics, didnt find answer to it.
If we have already discussed all kind of Empty spaces as voids (octa, tetra etc), and have already filled them with cations while making ionic crystals, Now where these Interstitial spaces rises from?

Moreover, Cations are already in interstitial site, why they are even shifting to another interstitial site (which technically is already filled with cations, because we made ionic crystals by placing cations into interstitial sites.)

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    $\begingroup$ Yes, it can be confusing. But, take something simple like NaCl. In a perfect crystal, yes, each one will sit quite nicely on their own sub-lattice. To a Cl atom, it is symmetric with respect to the Na atoms, and vice versa. (Note that I don't put the cations on an 'interstitial' site - they have their own sublattice). But, for the combination of Na and Cl, there are now a new set of you 'empty' spaces defined by the spaces between all the Na and Cl atoms - those are the interstitial sites in a NaCl crystal. $\endgroup$ – Jon Custer May 4 '16 at 13:04
  • $\begingroup$ So Is there any difference between VOIDS and Interstitial Site $\endgroup$ – aman May 4 '16 at 16:00
  • $\begingroup$ @aman Would be much clear if use the words with respect to $\endgroup$ – JM97 May 4 '16 at 17:30
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    $\begingroup$ Technically, I would consider a 'void' to be an empty volume much larger than one lattice site. But, in any crystal structure there will be interstitial sites, often several different sites with different symmetries. Furthermore, the exact configuration of an 'interstitial' is often not as well defined as you might like. For example, in GaAs, the As 'interstitial' is often configured as two As atoms sharing one As site, the so-called split-interstitial. $\endgroup$ – Jon Custer May 4 '16 at 18:19

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