[Chemistry Class Notes] on Schottky Defect Pdf for Exam

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.