# Concentration of Point Defects

1. Mar 27, 2005

### RPI_Quantum

A question for anyone that has the ability to answer:

I understand the difference between Frenkel and Schottky defects, and I also know that in different crystal structures, which one is predominant is different. I was wondering, what dictates which defect is the dominant one?

Thanks for any responses!

2. Mar 27, 2005

### Staff: Mentor

Frenkel defect occurs when a cation is displaced into an interstitial position, whereas a Schottky defect occurs when an anion (- ion) and a cation are displaced.

I have an example in one book that shows Frenkel defect in AgCl and Schottky defect in NaCl.

Na (A=23) and Cl (A=35) are close in mass, where as Ag (A=108) is heavier. The closer in mass the cation and anion, it would seem that it is more likely both would be equally displaced from the proper lattice position. Also, ionic radius probably plays a role.

More generally, if the anion and cation vacancy energies are similar, then it would stand to reason that Schottky defects are more likely than Frenkel defects.

I don't know if this is any use, but - http://www.iop.org/EJ/article/0953-8984/11/22/102/cm11022l2.html (registration maybe required).

3. Mar 27, 2005

### RPI_Quantum

Yes, that makes sense. I suspected that the relative sizes of the ionic radii would have some bearing on it. Thanks!
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