Definition of Schottky Defect
Ionic crystals exhibit a range of crystallographic defects where the prevailing crystal pattern gets disrupted either at a point, along a line, along a plane or in bulk. The Schottky defect is one such point defect that is observed in various crystals. Named after a German physicist, Walter H. Schottky, this defect occurs commonly in ionic crystals where the size of cation and anion is similar. Take, for example, KCl, Potassium (K) has an atomic number of 19 and Chlorine (Cl) has an atomic number of 17. Both the ions are of similar size, and hence it is a good candidate for showing Schottky defects.
Characteristics of Schottky Defects
Schottky defects usually occur when heat is applied to the ionic compound crystal. Heat raises the temperature, and hence the thermal vibration within the crystal. This creates gaps in the crystal pattern. The gaps are created in stoichiometric ratio, i.e. as per the availability of ions in chemical compounds. For example, in a generic ionic compound with the formula XnYm, ‘n’ ions of X and ‘m’ ions of Y will leave to create vacancies. A group of such vacancies can also be referred to as a Schottky cluster.
Schottky defect reduces the density of ionic compounds because a fraction of ions leave the crystal, hence reducing the overall mass at the same crystal volume.
Concentration of Defects
As explained previously, Schottky defects are formed by applying heat. At any given temperature, there is a concentration of defects (i.e. Schottky defects per unit volume) given by the following formula:
\[ns \approx N exp (- \frac{\Delta Hs}{2RT})\]
Where,
ns = number of Schottky defects per unit volume at temperature T (in Kelvins) in a crystal with N anion and N cations per unit volume, and ∆Hs is the enthalpy for creating one defect.
Schottky Defect and Frenkel Defect
Frenkel defect is also a point crystallographic defect that is usually observed in ionic compounds. It is named after Soviet physicist Yakov Frenkel and is different from the Schottky defect in terms of its occurrence and characteristics.
Frenkel defect generally occurs in ionic compounds where the ions are of different sizes. As opposed to the Schottky defect, where both the ions leave the crystal, it is usually the cation (due to its smaller size) that leaves its natural place in the crystal and moves to a nearby location. A compound like NaCl is a good candidate for observing a Frenkel defect.
The Schottky defect is formed by heating the crystal, while the Frenkel defect is formed by particle irradiation of the crystal. Moreover, the Frenkel defect doesn’t change the density of the crystal because ions are still present and have not left the crystal. This is different from Schottky defects where the density of the crystal is reduced. Some ionic compounds, such as AgBr, exhibit both Schottky and Frenkel defects. But as a general rule, the Schottky defect is more likely to be seen in ionic compounds where the size of constituent ions is similar and the Frenkel defect is more likely to be seen where the size of constituent ions is largely different.
As a word of caution, there is a similar-sounding term called ‘Schottky effect’. Please note this is not to be confused with the Schottky defect. The Schottky effect, also named after Walter H. Schottky, is a phenomenon in condensed matter physics that is out of the purview of this article.
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Bound and Dilute Defects
We know that Schottky defects are seen in ionic crystals and are density defects. Thus, it is clear that the gaps created in the crystal lattice consist of ions carrying opposite charges. These ions experience a mutually attractive Coulomb force that brings them both close together. These may then form bound clusters at low temperatures.
These bound clusters that are formed are generally less mobile than their dilute defect counterparts. Chemists and physicists have seen that multiple species need to move in a united manner for the whole cluster to be able to migrate. This has a plethora of applications in several fields and their effects can be seen in ion conductors, solid oxide fuel cells, and nuclear fuel.
FAQs on Schottky Defect
1. What is a Schottky defect? Explain with an example from the NCERT syllabus.
A Schottky defect is a type of point defect found in ionic crystals. It is created when an equal number of cations (positive ions) and anions (negative ions) are missing from their regular positions in the crystal lattice, creating a pair of vacancies. This ensures that the overall electrical neutrality of the crystal is maintained. This defect is common in compounds with a high coordination number where the ions have similar sizes. For example, alkali halides like NaCl (Sodium Chloride) and KCl (Potassium Chloride) are classic examples of crystals that exhibit the Schottky defect.
2. What is the main difference between a Schottky defect and a Frenkel defect?
The key differences between Schottky and Frenkel defects lie in the mechanism of their formation and their impact on the crystal's density.
- Formation: In a Schottky defect, an equal number of cations and anions leave the crystal lattice entirely. In a Frenkel defect, a smaller ion (usually the cation) is displaced from its lattice site to an interstitial site within the same crystal.
- Density: The Schottky defect decreases the density of the crystal because mass is lost from the lattice. The Frenkel defect does not change the density, as no ions leave the crystal.
3. How does a Schottky defect affect the physical properties of a crystal?
The presence of Schottky defects has several important consequences for a crystal's properties:
- Density: The most notable effect is a decrease in the crystal's density. This happens because the number of ions within the crystal lattice decreases while the volume remains essentially constant.
- Stability: The formation of vacancies requires energy, which means the presence of Schottky defects lowers the overall lattice energy and thus the thermodynamic stability of the crystal.
- Electrical Conductivity: The vacancies created by the defect can facilitate the movement of ions through the crystal under an electric field, which slightly increases the ionic conductivity.
4. Why is the Schottky defect more common in compounds like CsCl than in AgCl?
The type of defect a crystal exhibits depends largely on the relative sizes of its ions. The Schottky defect is favoured in compounds where the cation and anion are of similar size. In Caesium Chloride (CsCl), the Cs⁺ and Cl⁻ ions are comparable in size, making it easy to remove a pair without destabilising the lattice. In contrast, Silver Chloride (AgCl) has a significant size difference between the smaller Ag⁺ ion and the larger Cl⁻ ion. This size discrepancy makes it easier for the small Ag⁺ ion to move into an interstitial site, thus favouring the Frenkel defect over the Schottky defect.
5. How does an increase in temperature affect the number of Schottky defects in a solid?
The concentration of Schottky defects in a crystal is directly related to temperature. As the temperature of the solid increases, the thermal vibrations of the ions in the lattice become more energetic. This increased energy allows more ions to break free from their lattice sites, creating vacancies. Consequently, the number of Schottky defects increases exponentially with a rise in temperature. A theoretically perfect crystal with zero defects would only exist at absolute zero (0 Kelvin).
6. What is the importance of maintaining stoichiometry in a Schottky defect?
Maintaining stoichiometry is crucial because ionic crystals must remain electrically neutral. In a Schottky defect, if only a cation were to leave the lattice, the crystal would have a net negative charge, and if only an anion left, it would have a net positive charge. To prevent this charge imbalance, an equal number of cations and anions must be removed simultaneously. For an ionic compound with a formula like AX, one A⁺ ion and one X⁻ ion are removed. For a compound like AX₂, one A²⁺ ion and two X⁻ ions are removed to maintain neutrality.
7. Is there a difference between the 'Schottky defect' and the 'Schottky effect'?
Yes, these are two distinct concepts named after the same physicist, Walter H. Schottky, and they should not be confused. The Schottky defect is a topic in solid-state chemistry and refers to the absence of a pair of oppositely charged ions in a crystal lattice. The Schottky effect, on the other hand, is a phenomenon in condensed matter physics that describes the lowering of the work function of a metal surface when an external electric field is applied, facilitating thermionic emission.
