Concentration of Point Defects

In summary, Frenkel defects occur when a cation is displaced into an interstitial position, while Schottky defects occur when both an anion and cation are displaced. The dominant defect in a crystal structure is determined by the relative sizes of the cation and anion, with Schottky defects being more likely when the vacancy energies are similar. The article provided may also offer further insight into this topic.
  • #1
RPI_Quantum
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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!
 
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  • #2
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
Yes, that makes sense. I suspected that the relative sizes of the ionic radii would have some bearing on it. Thanks!
 

1. What are point defects in materials?

Point defects are imperfections or irregularities in the crystal lattice of a material. They can occur at a single point or a small group of points within the lattice and can have a significant impact on the material's properties and behavior.

2. How do point defects form?

Point defects can form through a variety of processes, such as thermal vibrations, impurity atoms, or irradiation. They can also be intentionally introduced through doping or other manufacturing techniques.

3. What is the concentration of point defects?

The concentration of point defects refers to the number of defects present in a material relative to the total number of atoms in the material. It is typically measured in parts per million (ppm) or parts per billion (ppb) and can vary greatly depending on the type of material and its processing history.

4. How does the concentration of point defects affect material properties?

The concentration of point defects can have a significant impact on the mechanical, electrical, and optical properties of a material. For example, an increase in the concentration of defects can lead to decreased strength and ductility, increased electrical resistivity, and changes in optical transparency.

5. How is the concentration of point defects measured?

The concentration of point defects can be measured using various techniques, such as electron microscopy, X-ray diffraction, and spectroscopic methods. These techniques can provide information on the type, size, and distribution of defects within a material.

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