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. Schottky defect is one such point defect which 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.
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 leaves the crystal, hence reducing the overall mass at the same crystal volume.
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 = 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.
Frenkel defect is also a point crystallographic defect which is usually observed in ionic compounds. It is named after a Soviet physicist Yakov Frenkel and is different from 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 Schottky defect, where both the ions leave the crystal, it is usually the cation (due to its smaller size) which 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.
Schottky defect is formed by heating the crystal, while 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 crystal is reduced.
Some ionic compounds, such as AgBr, exhibit both Schottky and Frenkel defects. But as a general rule, Schottky defect is more likely to be seen in ionic compounds where the size of constituent ions is similar and Frenkel defect is more likely to be seen where 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. Schottky effect, also named after Walter H. Schottky, is a phenomenon in condensed matter physics which is out of purview of this article.
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