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Schottky defect is a vacancy defect shown by ionic solids in which an equal number of cations and anions are missing from their lattice sites. Only the Ionic solids containing similar-sized cations and anions show this type of defect. Examples of solids showing this defect are: Sodium Chloride (NaCl), Caesium Chloride (CsCl), and Silver Bromide (AgBr).
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Key Terms: Schottky Defect, Characteristics of Schottky Defects, Concentration of Defects, Frenkel Defect, Bound, Dilute Defects
What is Schottky Defect?
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Schottky Defect is a point defect in which both cation and anion remain missing from the lattice site in equal numbers. Due to this defect, the density of the crystal reduces. The Schottky defect is found in highly ionic compounds that have same sized cations and anions.
Schottky Defect (Cation and Anion Absent from their Lattice)
Schottky Defect model by Walter H. Schottky shows that ionic crystals are imperfect when oppositely charged particles leave their cross-section locales. This leads to formation of vacancies. These vacancies maintain a neutral charge in the crystals.
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| Related Articles | ||
|---|---|---|
| Electrical properties of solids | Conductors | Insulators |
| Classification Of Crystalline Solids | Density of Unit Cell | Metallic Bonds |
What are the Characteristics of Schottky Defect?
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Key Characteristics of Schottky Defect are as follows:
- There is a small difference between the size of the anion and cation.
- Usually, two vacancies are formed.
- Anion and Cation both leave the solid crystal at the same time.
- Atoms also move out of the crystal permanently.
- The density of the solid reduces considerably.
Schottky defects generally occur when heat is applied to crystals of ionic compounds. Heat increases the temperature, which leads to occurence of thermal oscillations inside the crystal. This creates gaps in the crystal pattern.
| Example: In an ionic compound of the formula XnYm, the "n" ions of X and the "m" ions of Y will leave the lattice to create a vacancy. A group of these vacancies can also be called a Schottky cluster. |
Schottky Defect Formula
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Schottky defects are formed by applying heat to the solid crystal. At a given temperature, following formula can be used for calculating Schottky Defect:
ns \(\approx\) Nexp \((-\frac{\Delta H_s}{2RT})\)
Where,
ns = number of Schottky defects per unit volume at temperature T (in Kelvins)
\(\Delta\)Hs = Enthalpy for creating one defect.
R = Gas constant
T = Absolute temperature (in K)
N can be computed by using the following formula:
\(N=\frac{Density of the ionic crystal compound \times NA}{Molar mass of the Ionic Crystal Compound} \)
Difference Between Frenkel Defect and Schottky Defect
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The Schottky defect appears when oppositely charged atoms (cation and anion) leave their lattice sites and create a pair of Vacancy Defects. Frenkel Defect appears where an atom (ion, especially cation) leaves its original lattice site and occupies an interstitial position on the same crystal.
Differences between the Schottky defect and the Frenkel defect are mentioned in the table below:
| Frenkel Defect | Schottky Defect |
|---|---|
| In the Frenkel defect, only the minor cation leaves its lattice site’ but, the anion remains in its lattice sites. | In the Schottky defect, both anion, and cation leave the solid crystal. |
| The number of atoms present in the crystal after and before the Frenkel defect remains equal. | Two atoms are reduced from the crystal for each Schottky defect. |
| The atoms leave the original lattice site and occupy an interstitial place. | The atoms permanently leave the crystal. |
| The density of the solid crystal after and before the Frenkel defect remains equal as no atom leaves the solid. | Due to the formation of vacancy, the Schottky defect decreases the density of the solid. |
| Some common examples are: ZnS (Zinc Sulfide), AgBr (Silver Bromide), AgCl (Silver Chloride) | Some common examples are: KBr (Potassium Bromide), NaCl (Sodium Chloride), AgBr (Silver Bromide), ThO2 (Thorium Dioxide), CeO2 (Cerium Dioxide) |
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Things to Remember
- The Schottky defect is a point defect in which both cation and anion remain missing from the lattice site in equal numbers. For Example- NaCl, CaCl, etc.
- Schottky Defect or small shot effect was discovered by German physicist Walter H. Schottky.
- Schottky defects generally occur when heat is applied to crystals of ionic compounds.
- The Schottky defect and Frenkel defect are point defects and occur only in ionic compounds. Schottky defect appears when both cation and anion leave their lattice sites and create a pair of Vacancy Defects. Frenkel Defect appears when an atom (especially cation) leaves its original lattice site and occupies an interstitial position on the same crystal.
- The density of crystal lattice decreases in the Schottky Defect.
Schottky Defect Detailed Notes PDF
Sample Questions
Ques: Account for the following:
(i) Schottky defects lower the density of related solids.
(ii) Conductivity of silicon increases on doping it with phosphorus. (All India- 2013, 2 Marks)
Answer: (i) Schottky defect occurred when an equal number of cation and anion are missing from lattice as a result of which the density of the lattice solid decreases.
(ii) The conductivity of silicon increases due to negatively charged extra electrons of doped pentavalent phosphorus.
Ques: Answer the following questions :
(i) What type of stoichiometric defect is shown by the crystal?
(ii) How is the density of the crystal affected by this defect?
(iii) What type of ionic substances show such defects? (Delhi, 2 Marks)
Answer: (i) Schottky defect
(ii) Density of the crystal decreases
(iii) NaCl (Ionic solids having approximate equal size of cations and anions)
Ques: (i) What type of non-stoichiometric point defect is responsible for the pink colour of LiCl?
(ii) What type of stoichiometric defect is shown by NaCl? (All India- 2014, 2 Marks)
Answer: (i) The pink colour of LiCl is due to a metal excess defect in LiCl due to anionic vacancies in which the anionic sites are occupied by unpaired electrons (F-centres).
(ii) The stoichiometric defect shown in NaCl is Schottky’s defect.
Ques: (i) Write the type of magnetism observed when the magnetic moments are aligned in parallel and anti-parallel directions in unequal numbers.
(ii) Which stoichiometric defect decreases the density of the crystal? (All India- 2014, 2 Marks)
Answer: (i) The magnetism observed when the magnetic moments are aligned in parallel and anti-parallel directions are Ferrimagnetism.
(ii) Schottky defect decreases the density of the crystal because the ions are removed from the lattice in this defect.
Ques: How are the following properties of crystals affected by Schottky and Frenkel defects?
(i) Density
(ii) Electrical conductivity (All India 2013, 2 Marks)
Answer: (i) In Schottky defect the density of the lattice decreases whereas the density of lattice remains the same in the Frenkel defect.
(ii) The electrical conductivity increases in both Frenkel and Schottky defects.
Ques: Define the following :
(i) Schottky defect
(ii) Frenkel defect
(iii) F-centre (Delhi- 2015, 3 Marks)
Answer: (i) Schottky defect: In the Schottky defect, an equal number of cations and anions are missing from their lattice sites so that electrical neutrality is maintained.
(ii) Frenkel defect: In Frenkel defect, ion leaves its site from its lattice site and occupies the interstitial site and maintains electrical neutrality.
(iii) F-centre: The centres which are created by the trapping of electrons in anionic vacancies and which are responsible for imparting colour to the crystals are called F-centres.
Ques: Explain:
(i) Why is Frenkel defect not found in pure alkali metal halides?
(ii) Zinc oxide is white but it turns yellow on heating.
(iii) CaCl2 will introduce Schottky defect when added to AgCl crystal. (All India 2015, 3 Marks)
Answer: (i) Frenkel defect is not found in pure alkali metal halides because the ions cannot occupy the interstitial sites as they are very large in size.
(ii) When ZnO is heated it loses oxygen because Zn2+ ions are in the interstitial sites and electrons are in the neighbouring interstitial sites to maintain electrical neutrality. This results in metal excess defect. This is why it turns yellow due to the presence of free electrons in the interstitial sites.
(iii) CaCl2 on adding to AgCl results in impurity defect. The addition of one Ca2+ ion will replace two Ag+ ions to maintain electrical conductivity. One of the positions of Ag+ will be occupied by Ca2+ ion and the other will be left as a vacancy. Thus, a vacancy is created similar to the Schottky defect.
Ques: (a) Calculate the number of unit cells in 8.1 g of aluminium if it crystallizes in an f.c.c. structure. (Atomic mass of Al = 27 g mol-1)
(b) Give reasons:
(i) In stoichiometric defects, NaCl exhibits Schottky defect and not Frenkel defect.
(ii) Silicon on doping with Phosphorus form n-type semiconductor.
(iii) Ferrimagnetic substances show better magnetism than antiferromagnetic substances. (Delhi- 2017, 5 Marks)
Answer: (a) Given that:
Mass of Al = 8.1 g
Atomic mass of Al = 27 g mol-1
No. of atoms = η × 6.022 × 1023
= 8.1 / 27 × 6.022 × 1023
= 0.3 × 6.022 × 1023
= 1.8066 × 1023
Since one f.c.c. unit cell has 4 atoms
∴ No. of unit cells = (1.8066 × 1023 ) / 4
= 4.5 × 1022 unit cells
(b) (i) Schottky defect is shown by the ionic solids having a very small difference in their cationic and anionic radius whereas Frenkel defect is shown by ionic solids having a large difference in their cationic and anionic radius.
NaCl exhibits Schottky defect because the radius of both Na+ and Cl– have very small differences.
(ii) Phosphorus is pentavalent which means it has 5 valence electrons. An extra electron results in the formation of n-type semiconductors on doping with Silicon.
(iii) In antiferromagnetic substances, the magnetic moments of domains are half aligned in one direction and the remaining half in opposite direction in the presence of a magnetic field so the magnetic moment will be zero in antiferromagnetic substances.
In ferrimagnetic substances, the magnetic moments of domains are aligned in parallel and anti-parallel directions in unequal numbers hence show some value of the magnetic moment.
Ques: (i) What type of stoichiometric defect is shown by KCl and why?
(ii) What type of semiconductor is formed when silicon is doped with As?
(iii)Which one of the following is an example of molecular solid: CO2 or SiO2
(iv) What type of substances would make better magnets, ferromagnetic or ferrimagnetic? (All India 2013, 3 Marks)
Answer: (i) KCl shows Schottky defects because K+ and Cl- have very little difference in their size.
(ii) When Silicon is doped with As, the n-type semiconductor is formed.
(iii) CO2 is an example of a molecular solid.
(iv) Ferromagnetic substances can make better magnets.
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