A Schottky defect is a type of point defect in a crystal lattice named after Walter H. Schottky

Walter H. Schottky

Walter Hermann Schottky was a German physicist who played a major early role in developing the theory of electron and ion emission phenomena, invented the screen-grid vacuum tube in 1915 and the pentode in 1919 while working at Siemens, and later made many significant contributions in the areas of...

.

The defect forms when oppositely charged ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric

Stoichiometry

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

 units, to maintain an overall neutral charge in the ionic solid. The vacancies are then free to move about as their own entities. Normally these defects will lead to a decrease in the density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of the crystal. The following are the chemical equations in Kröger-Vink Notation

Kröger-Vink Notation

Kröger–Vink notation is set of conventions used to describe electric charge and lattice position for point defect species in crystals. It is primarily used for ionic crystals and is particularly useful for describing various defect reactions. It was proposed by F. A. Kröger and H. J. Vink.-General...

 for the formation of Schottky defects in TiO₂ and BaTiO₃.
Ø
Ø
This can be illustrated schematically with a two-dimensional diagram of a sodium chloride

Sodium chloride

Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

crystal lattice:

Definition

If in an ionic crystal of type A⁺B^- an equal number of cations and anions are missing from their lattice sites so that electrical neutrality as well as stoichiometry is maintained this is called a Schottky Defect.

It is a vacancy defect (due to missing ions) and also a stoichiometric defect, as the ratio of the number of cations and anions remains the same.

Examples

This type of defect is shown in compounds with :

• highly ionic compounds
• high co-ordination number
• small difference in sizes of cations and anions

Examples : NaCl, KCl, CsCl, KBr, AgCl. etc.

Experimental observations show that at room temperature in an NaCl crystal there is one Schottky defect per 10¹⁶ ions.

Effect on Density

The total number of ions in a crystal with this defect is less than the theoretical value of ions, thus, the density of the said crystal is less than normal.