Schottky Defect

In ionic solids, the Schottky defect is a form of stoichiometry defect. The defect is known as a stoichiometry effect because it does not change the solid’s stoichiometry. The Schottky defect is characterised by the absence of the same number of cations and anions from the solid. We’ll go through the Schottky Defect, its properties, and formula, as well as some key questions.

What is this Effect :

The Schottky defect (small shot effect) was named after Walter H. Schottky, a well-known German physicist who was awarded the Hughes prize by the Royal Society in 1936 for discovering it.

According to his hypothesis, vacancies arise in ionic crystals when oppositely charged ions leave their lattice positions, resulting in defects. These voids are also formed in order to keep the crystal neutrally charged. The surrounding atoms likewise move to fill these voids, according to the model. The defect is usually referred to as a lattice vacancy defect when it occurs in non-ionic crystals.

Definition :

Schottky defects have a stoichiometric ratio of vacant anion and cation sites. A Schottky defect is made up of a single anion vacancy (A) and a single cation vacancy (B) in a simple ionic crystal of type A-B+ , or VA•+ VB︲in Kröger–Vink notation. A Schottky cluster is made of x vacancies of A and y vacancies of B for a more general crystal with formula AxBy, preserving overall stoichiometry and charge neutrality. A Schottky defect is created when a crystal is extended by one unit cell, and the atoms that diffuse out of the interior replace the a priori empty sites, resulting in vacancies in the crystal.

Explanation with Diagram :

Characteristics of Schottky Defect :

When heat is applied to ionic compound crystals, Schottky defects are created. Thermal oscillations occur inside the crystal as a result of the heat increasing the temperature. The crystal pattern will be disrupted as a result of this. The availability of ions in chemical compounds leads to the formation of vapours.

The “n” ions of X and the “m” ions of Y will exit the lattice to generate a vacancy in an ionic compound with the formula XnYm, for example. A Schottky cluster refers to a collection of these job openings.

The following are some properties of the Schottky Defect:

• The size of the anion and cation differs by a little amount.
• Two openings are usually created.
• Both the anion and the cation exit the solid crystal at the same moment.
• Atoms leave the crystal permanently as well.
• The solid’s density decreases dramatically.

Formula :

ns N exp (Hs2RT)

Where;

ns = number of Schottky defects per unit volume at temperature T (in Kelvins)

N = Density of the ionic crystal compound * NAMolar mass of the ionic crystal compound

Hs= Enthalpy for creating one defect 

R = Gas constant

T = Absolute Temperature (kelvin)

Examples of Schottky Defect :

A Schottky defect is a type of crystal defect that occurs mostly in strongly ionic or highly coordinated substances. The size difference between the anions and the cations in the compound’s lattice is quite modest.

Sodium chloride (NaCl), potassium chloride (KCl), potassium bromide (KBr), caesium chloride (CsCl), and silver bromide are some examples of salts with Schottky defects (AgBr).

Conclusion :

The Schottky defect is a point defect in which both the cation and anion are missing in equal proportions from the lattice site. NaCl, CaCl, and other salts, for example. Walter H. Schottky, a German physicist, invented the Schottky Defect or small shot effect. When heat is applied to ionic compound crystals, Schottky defects are created. Schottky and Frenkel defects are point defects that only appear in ionic substances. When both cation and anion depart their lattice positions and form a pair of Vacancy Defects, the Schottky defect emerges. When an atom (particularly a cation) moves from its initial lattice site to an interstitial position on the same crystal, it is called a Frenkel Defect. In the Schottky Defect, the density of the crystal lattice falls.