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The wiki page on vacancy defects says;

It is also known as a Schottky defect, although in ionic crystals the concepts are not identical.

As far as I'm aware, they're the same. Furthermore, the wiki page for Schottky defects contains the term 'vacancies' while describing the phenomenon, which is linked back to the page for vacancy defects. That seems contradictory.

Are they the same? And is the same true for Frenkel and interstitial defects? Or are the named ones subsets?

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  • $\begingroup$ According to Kittel 'Introduction to Solid State Physics' a lattice vacancy defect is a called a Schottky defect and in the Frenkel defect, also a vacancy defect, an atom is transferred from a lattice site to an interstitial position. $\endgroup$
    – porphyrin
    Mar 13, 2021 at 14:01
  • $\begingroup$ I thought Frenkel defects weren't vacancy defects, but classified separately under interstitial defects. $\endgroup$
    – harry
    Mar 14, 2021 at 0:22
  • $\begingroup$ @porphyrin: aren't Frenkel defects a seperate class? $\endgroup$
    – harry
    Mar 15, 2021 at 1:41
  • $\begingroup$ Could you provide a link to the wiki page/s you are referring to? $\endgroup$
    – ananta
    May 6, 2023 at 23:55

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Well, you can say that they are kind of similar...the only major difference is that Schottky defects are only for ionic compounds, and the number of missing cations and anions must be equal. Vacancy and interstitial defects can be shown by non-ionic solids. Ionic solids must always maintain electrical neutrality. Rather than simple vacancy or interstitial defects, they show these defects as Frenkel and Schottky defects.

Vacancy Defect: When some of the lattice sites are vacant, the crystal is said to have a vacancy defect. This results in a decrease in the density of the substance. This defect can also develop when a substance is heated.

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Schottky Defect: It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal. Like a simple vacancy defect, the Schottky defect also decreases the density of the substance. The number of such defects in ionic solids is quite significant. For example, in $\ce{NaCl}$, there are approximately $10^6$ Schottky pairs per $\pu{{cm}3}$ at room temperature. In $\pu{1 {cm}3}$ there are about $10^{22}$ ions. Thus, there is one Schottky defect per $10^{16}$ ions. The Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, $\ce{NaCl, KCl, CsCl}$ and $\ce{AgBr}$. It may be noted that $\ce{AgBr}$ shows both, Frenkel as well as Schottky defects.

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Also, with respect to your comment: "I thought Frenkel defects weren't vacancy defects but classified separately under interstitial defects."

Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substances in which there is a large difference in the size of ions, for example, $\ce{ZnS, AgCl, AgBr}$ and $\ce{AgI}$ due to the small size of $\ce{Zn^2+}$ and $\ce{Ag+}$ ions.

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Interstitial Defect: When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have an interstitial defect. This defect increases the density of the substance.

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Also, here's why $\ce{AgBr}$ shows both Schottky and Frenkel defects, even though Frenkel defects are shown by ionic substances in which there is a large difference in the size of ions, and Schottky defects are shown by ionic substances in which the cation and anion are of almost similar sizes.

Schottky defect in $\ce{AgBr}$ is exhibited due to precipitation of both cations and anions. In $\ce{AgBr, Ag+}$ ion is small in size and when removed from lattice point they can occupy interstitial sites and hence show both Frenkel and Schottky defects.

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  • $\begingroup$ All four defects mentioned are types of stoichiometric point defects....other types of defects include impurity point defects, non-stoichiometric point defects (Metal Excess Defect and Metal Deficiency Defect) and line defects $\endgroup$ Apr 5, 2021 at 20:19
  • $\begingroup$ By "cm3" do you mean cubic centimeter? Because, if so, your calculation about the number of ions is out by a factor exceeding 10^20. If that isn't what you meant you need to define your terms carefully to avoid confusing people. $\endgroup$
    – matt_black
    May 6, 2021 at 11:29
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    $\begingroup$ I am sorry, the text didn't get edited properly, I'll edit my answer rn....i meant 10^22 and 10^16 respectively in one cubic centimeter $\endgroup$ May 7, 2021 at 16:41

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