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Schottky defect

Definition

Two types of defects occur in metals – point defect and line defect. In addition, most of the solids have impurity defects, and some have stoichiometric defects. Stoichiometric defects can be classified into two types – vacancy defect and interstitial defect. On the other hand, non-stoichiometric defects can be classified as metal excess and metal deficient defects. The metal excess defect can occur due to anionic vacancy or due to interstitial cation.

The Schottky defect is also a type of mental defect. To maintain electrical neutrality, equal numbers of anion and cation are missing from the metal lattice. This defect can be commonly seen in compounds with high coordination numbers.

Compounds having Schottky defect have a lower density, as some ions are missing in the lattice. Therefore, the volume of the lattice remains the same. But due to the missing ions, the mass decreases. Thus, the overall density of the lattice decreases.

Schottky defects examples include – potassium chloride and sodium chloride.

The above diagram shows the missing two anions and two cations from the crystal lattice. The Schottky defect is a type of stoichiometric defect as its stoichiometry remain undisturbed. Stoichiometry means the ratio of the number of cations to anions.

Schottky defect is different from Frenkel defect. In the Schottky defect, the ions leave the crystal lattice, whereas, in the Frenkel defect, they remain in the lattice.

Characteristics of Schottky Defect

Some of the commonly seen characteristics of Schottky defects are-

  • There is a minimal difference in size between cation and anion.
  • Cation and anion both leave the solid crystal.
  • Atoms also move out of the crystal permanently.
  • Generally, two vacancies are formed.
  • As for the density of the solid, it decreases considerably.

Examples
Some examples of Schottky defects include crystals of highly coordinated compounds or highly ionic compounds. The difference between the sizes of anions and cations is minimal in the Schottky defect lattice. Some compounds that face Schottky defects are – potassium chloride, sodium chloride, potassium bromide, silver bromide and caesium chloride.

Effect on density

As mentioned above, the density in a Schottky defective lattice decreases. This is because the volume of the crystal lattice remains the same in a compound, but the mass of ions decreases. Both anion and cation tend to leave the lattice, making it reduce its mass. Thus, density is the ratio of mass by volume, which tends to decrease due to the decrease in cations and anions. Moreover, the density of the crystal lattice is less than the theoretical density of the material.

Calculation

The number of defects for an MX crystal can be calculated by the given formula.
ns ≈ N exp (− Hs2RT )
where,
ns = number of Schottky defects per unit volume
ΔH = enthalpy of defect formation
R = gas constant,
T = absolute temperature (in K)
We can calculate N by using the formula;
N = density of the iconic crystal compund x NAmolar mass of the ionicrystal compound

FAQ on Schottky defect

Ques.1: Schottky defects lower the density of related solids. Explain.

Answer: Schottky defect occurs when an equal number of cation and anion are missing from the lattice. As a result, the density of the solid lattice decreases.

Ques.2: What type of stoichiometric defect is shown by the crystal when the ions tend to leave the crystal?

Answer: Schottky defect is a type of stoichiometric defect where cations and anions leave the crystal.

Ques.3: What type of stoichiometric defect is shown by NaCl?

Answer: The stoichiometric defect shown in NaCl is Schottky’s defect.

Ques.4: How are the following properties of crystals affected by Schottky and Frenkel defects?

(i) Density
(ii) Electrical conductivity

Answer: (i) In the Schottky defect, the density of the lattice decreases, whereas the density of lattice remains the same in the Frenkel defect.
(ii) Both Frenkel and Schottky defects experience an increase in electrical conductivity.

Ques.5: Define Schottky defect and Frenkel defect.

Answer: In the Schottky defect, an equal number of cations and anions are missing from their lattice sites to maintain electrical neutrality.

In Frenkel defect, ion leaves its lattice site, occupies the interstitial site, and maintains electrical neutrality.

Ques.6: Give reasons: stoichiometric defects, NaCl exhibits Schottky defect and not Frenkel defect.

Answer: Schottky defect is shown by the ionic solids having a minimal difference in their cationic and anionic radius. At the same time, the Frenkel defect is shown by ionic solids having a significant difference in their cationic and anionic radius. NaCl exhibits Schottky defect because the radius of both Na+ and Cl– have minimal differences.

 

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