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.
So, this was a quick review of the Schottky defect. We hope you enjoyed understanding it. If you want to read more articles and improve your concepts by solving questions/numerical, please log on to the Vedantu website or download the Vedantu learning app for classes 6-10, IIT JEE and NEET.
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 defect in a crystal lattice?
A defect in a crystal lattice can be defined as an imperfection in the usual geometrical arrangement of the atoms or ions present in a solid substance. These imperfections can form due to a variety of reasons. Some of these reasons can include sudden deformation, thermal changes, or exposure to harmful radiation. X-rays or neutrons striking the solid can drastically alter its structure. These defects can be of several types. They can be found at single points, long lines, or on the entire surface of the solid. Linear and point defects both have the capacity to alter the electrical, optical, and mechanical properties of the solid.
2. What are the types of point defects?
Point defects include Frenkel defects, Schottky defects, and impurity defects. The Frenkel defect consists of the displacement of a single ion. This ion, due to external factors, is displaced from its original position in the crystal lattice and shifts to a vacant space between atoms in the lattice. In contrast to this, two ions are involved in the Schottky defect. Here, two ions of opposite charges simultaneously leave their positions in the crystal lattice. This directly also affects the density of the solid. Impurity defects are seen when foreign atoms replace some of the existing atoms making up the solid. Impurity defects are used in doping. The process of doping comes in handy in the formation of semiconductors.
3. What are the types of line defects?
Now that we have discussed point defects, let us see what linear or line defects are. Line defects are seen when atoms are displaced in a linear fashion from their existing geometric arrangement. These dislocations result in the formation of straight lines along which rows of atoms or ions present in a solid are arranged randomly. The resulting irregularity that is formed is most prominently seen along a line called the line of dislocation. Line defects can alter a solid’s strength either by weakening it or strengthening it.
4. How do surface defects affect solids?
Surface defects can arise in a crystal lattice due to many reasons. They can be present at the boundary between two atoms of a large molecule. Surface defects are quite significant and lead to a mismatch across the molecule’s external boundary. The visible external surface of a crystal can also be termed a surface defect. This is mainly because the atoms on the surface are not in their original positions. They change and adjust to make up for the absence of neighbouring atoms outside the surface.
5. How can students study crystalline solids and their imperfections?
Solid State Chemistry is quite important from the examination point of view. Solid State is a separate chapter taught in Class 12 from which several questions are asked. Properties of solids are highlighted and discussed. However, any defects in a crystal lattice can alter these properties significantly. That is why one must know all about the Imperfections in Crystal. Refer to the notes that have been prepared for you by the subject experts of Vedantu. They cover all the main topics that can possibly be asked and teach you everything about crystalline solids and their imperfections.
