Schottky Defect Overview
A Schottky defect occurs in ionic crystals when oppositely charged ions leave their lattice positions and become integrated, for example, at the surface, resulting in oppositely charged vacancies. To maintain an overall neutral charge in the ionic solid, these vacancies are produced in stoichiometric units. The vacancies that comprise the Schottky defects contain opposing charges, resulting in a mutually attractive Coulomb force. They may form bonded clusters at low temperatures.
What is the Schottky Defect?
The Schottky defect (small shot effect) is named after Walter H. Schottky, a well-known German physicist who was given the Royal Society's Hughes prize in 1936 for discovering this defect. According to his hypothesis, defects arise in ionic crystals when oppositely charged ions leave their lattice positions, resulting in the formation of vacancies. These voids are then filled in order to keep the crystal neutral. The model also describes how the surrounding atoms migrate to fill these voids. When the defect is observed in non-ionic crystals, it is known as a lattice vacancy defect. Schottky defect, on the other hand, differs from Frenkel defect in that atoms in Schottky defect permanently exit the crystal, whilst atoms in Frenkel defect often stay within the solid crystal.
Examples of Schottky Defect
Crystals of strongly coordinated or highly ionic substances are examples of Schottky faults. In the Schottky defect lattice, the size difference between anions and cations is negligible. The following are some of the examples of Schottky Defects-
- Chloride of sodium (NaCl)
- Chloride of Potassium (KCl)
- Bromide of potassium (KBr)
- Bromide of Silver (AgBr)
Schottky Defect Characteristics
Some of the most prevalent Schottky faults are as follows-
- The difference in size between cation and anion is insignificant.
- The cation and anion both leave the solid crystal.
- Atoms are permanently expelled from the crystal as well.
- In most cases, two positions are created.
- The density of the solid reduces significantly.
Schottky Defects: The Impact on Density
The density drops in a Schottky faulty lattice. This is due to the fact that the volume of the crystal lattice remains constant in a compound while the mass of ions drops. Both anions and cations have a tendency to depart the lattice, causing it to lose mass. Thus, density is the mass-to-volume ratio, which tends to decrease as cations and anions decrease. Furthermore, the crystal lattice density is smaller than the theoretical density of the material.
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Calculation of Schottky Defects
The above formula can be used to compute the number of flaws in an MX crystal.
nₛ≈ N exp (- ΔHₛ/2RT)
where,
nâ‚›=denotes the number of Schottky faults per unit volume.
H = defect formation enthalpy
R is the gas constant.
T stands for absolute temperature (in K).
We may use the formula to compute N;
N = density of the iconic crystal compound x NA / molar mass of the ionic crystal compound
Features of Schottky Defects
The following are some of the features of schottky defects-
- Schottky defects develop in ionic crystals where the size of the anion is almost equal to the size of the cation.
- Schottky defect occurs when one anion and one cation escape the crystal at the same time. None of them return to the interstitial webpage.
- One Schottky defect causes two vacancies to develop.
- The electrical neutrality of the whole crystal is preserved.
- As vacancies form, the density of the crystal drops for Schottky faults.
Frenkel vs Schottky Defects
When oppositely charged atoms (cation and anion) depart their lattice positions, they generate a pair of Vacancy Defects. Frenkel Defect occurs when an atom (ion, particularly cation) moves from its original lattice location to an interstitial place in the same crystal.
The table below summarises the differences between the Schottky defect and the Frenkel defect-
Schottky Defect | Frenkel Defect |
Both an anion and a cation exit the solid crystal in the Schottky defect. | Only the minor cation departs its lattice site in the Frenkel defect, but the anion stays in its lattice sites. |
Two atoms are removed from the crystal. | The number of atoms in the crystal after and before the Frenkel defect is the same. |
The atoms never return to the crystal. | The atoms move away from the original lattice position and into interstitial space. |
The Schottky defect reduces the density of the solid by causing vacancies to develop. | Because no atom exits the solid following the Frenkel defect, the density of the solid crystal remains constant. |
KBr (Potassium Bromide), NaCl (Sodium Chloride), AgBr (Silver Bromide), ThO2 (Thorium Dioxide), and CeO2 are some frequent examples (Cerium Dioxide) | ZnS (Zinc Sulfide), AgBr (Silver Bromide), and AgCl are some frequent examples (Silver Chloride). |
Important Considerations for the Schottky Defect
The following are some things to remember about Schottky's Defect-
- The Schottky defect is a point defect in which both cation and anion are absent in equal amounts from the lattice site.
- For example, NaCl, CaCl, and so forth.
- Walter H. Schottky, a German scientist, developed the Schottky Defect, or tiny shot effect.
- Schottky flaws are commonly observed when ionic compound crystals are heated.
- The Schottky and Frenkel defects are point defects that only arise in ionic substances.
- Schottky defects form when both the cation and anion depart their lattice positions, resulting in a pair of Vacancy Defects.
- Frenkel Defect occurs when an atom (particularly a cation) moves from its initial lattice location to an interstitial place on the same crystal.
- In the Schottky model, the density of the crystal lattice drops.
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Sample Questions for Practice
Q.1 Answer the following-
(i) Schottky faults decrease the density of linked solids.
(ii) Doping silicon with phosphorus boosts its conductivity.
(iii) Schottky faults decrease the density of linked solids.
(iv) Doping silicon with phosphorus boosts its conductivity.
Q.2 Answer the following questions-
(i) What form of stoichiometric defect does the crystal exhibit?
(ii) How does this flaw impact the density of the crystal?
(iii) What types of ionic compounds exhibit such flaws?
(iv) What kind of non-stoichiometric point defect is responsible for LiCl's pink color?
(v) What form of stoichiometric defect does NaCl exhibit?
(vi) Describe the type of magnetism observed when the magnetic moments are aligned in uneven quantities in parallel and anti-parallel directions.
(vii) Which stoichiometric defect reduces crystal density?
Q.3 Answer the following-
(i) What form of stoichiometric defect does the crystal exhibit?
(ii) How does this flaw impact the density of the crystal?
(iii) What types of ionic compounds exhibit such flaws?
(iv) What kind of non-stoichiometric point defect is responsible for LiCl's pink color?
(v) What form of stoichiometric defect does NaCl exhibit?
Q.4 Answer the following-
(i) Describe the type of magnetism observed when the magnetic moments are aligned in uneven quantities in parallel and anti-parallel directions.
(ii) Which stoichiometric defect reduces crystal density?
(iii) How do Schottky and Frenkel flaws impact the following crystal properties?
(iv) Why is the Frenkel defect absent in pure alkali metal halides?