Crystal Defects : Point Defects

“Crystals are like people, it is the defects in them which tend to make them interesting!”

-Colin Humphreys

When we talk about solid crystals, we say they have a definite shape. They possess a definite structure as molecules or atoms are compactly packed in them. They have a definite arrangement in them. Thus, they possess a repeated arrangement of a unit cell which forms a space lattice structure of that solid crystal. All solid crystals are made of repeated unit cells and maintain their structure for the long run. But these crystals do not possess all perfect unit cells. Some unit cells have some imperfections. The irregularities or interruptions in regular patterns of crystalline solids are called crystallographic defects. Point defects, line defects, planar defects, bulk defects etc. are various types of crystal defects. In this article we will discuss point defects of crystalline solids in detail. 

What are Point Defects? 

Those defects in the crystals which occur around an atom or particle are called point defects. These defects occur only at or around a single lattice point. They do not extend in space in any dimension. That’s why they are also called zero dimensional (0-D) defects. These are the smallest possible defects in any crystalline solid material. Point defects occur when –

  • One or more atoms of the crystal are missing from their corresponding lattice site.

  • Atom/s is shifted from its corresponding lattice site to interstitial position in the crystal.

  • Foreign atom/s occupy the interstitial position in the crystal lattice.

  • Original atom of the crystal is replaced by foreign atom. 

Types of Point Defects 

Point defects can be further divided into following types – ( image will be updated soon)

Stoichiometric Defects – The compounds which obey the law of definite proportions, the law of constant composition and the law of conservation of mass are called stoichiometric compounds. The defects in crystals which do not disturb the stoichiometry of the compound or crystal are called stoichiometric defects. 

Stoichiometric Defects can be Divided into Following Types –

  • Vacancy Defect – The point defect which is produced when an atom goes missing from its original lattice site is called vacancy defect. It creates vacancy in the lattice site as shown in the diagram below – (image will be updated soon)

It results in a decrease in density of the substance. Number of vacancy defects depends on the temperature of the crystal. It occurs due to imperfect packing during crystallization. 

  • Interstitial Defect – The point defect which occurs when an atom takes the interstitial position of the lattice structure is called interstitial defect. The atom can be of the same crystal or foreign crystal/material. If the atom is of the same crystal, then the defect is called self-interstitial defect. It is shown below through the diagram – (images will be updated soon)

This defect increases the density of the crystal. It causes atomic distortion. 

  • Schottky Defect - 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. 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 point defect which occurs when cation and anion leave their corresponding lattice sites and create a pair of vacancy defects is called Schottky defect. In KCl crystals Schottky defect is found. It shown below – (images will be updated soon)

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.

  • Frenkel 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. 

The point defect which occurs when cation displaces to interstitial voids is called Frenkel defect. In this defect cation gets displaced as it is smaller in size than anion. AgI, CaF2, NaCl exhibit the Frenkel defect. Frenkel defect in NaCl is shown below – (image will be updated soon)

This defect does not change the density of the crystal.

Non- Stoichiometric Defects – The defects in crystals which disturb the stoichiometry of the compound or crystal are called stoichiometric defects. Non – Stoichiometric defects can be divided into following two types –

  • Metal Excess Defects – As the name suggests in this defect metal ions occur in excess in the lattice of the crystal. It can take place by following two ways –

  • Anionic Vacancy – Anion goes missing from its corresponding lattice site and creates a vacancy. This vacancy is occupied by an electron to maintain the overall electric charge zero or neutral. It is called F - center. 

Actually, this F-center electron gives colour to the compound. 

  • Extra Cations – Sometimes in some crystals extra cation fit into the interstitial site on heating the crystal. Equal number of electrons do the same to maintain electrical neutrality of the crystal. 

  • Metal Deficiency Defects – In some compounds there is a deficiency of metal than their ideal stoichiometric proportions. It is normally found in transition elements as they possess multiple valencies.

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