In ideal crystalline solids the arrangement of constituent particles is perfectly regular. That is, the constituent particles are present at fixed points in the crystal lattice. Ideal crystalline solids have zero entropy at zero absolute temperature. But it is not possible to get such an ideal crystal. Usually a solid is a group of a large number of small crystals. These tiny crystals have defects. Due to these defects, imperfection comes in solids. Defects in crystals arise due to the dislocation of constituent particles or to leave their fixed positions or due to the presence of impurities. The defects which arise due to irregularities or deviations in the ideal arrangement of constituent particles are called Point Defects. The defects which arise due to irregularities or deviations in the ideal arrangement of complete lines of lattice points are called line defects. Here we will study only point defects.
This defect in a crystal occurs due to the disappearance of a constituent particle (cation or anion or atom) from its regular position or leaving its fixed position and moving to another place in the crystal lattice. This is called a point defect. Point defects are of following types :-
Non Stoichio Metric Defect
The defects due to which there is no change in the stoichiometry of crystal are called stoichiometric defects. Basically these defects are of two types. (a) Vacancy Defects (b) Interstitial Defects
When the lattice points or lattice sites are empty in a lattice, that is, the constituent particles move out of designated place, then the defect produced is called Vacancy Defects. This type of defect is shown in the figure. This defect reduces the density of substance. These types of defects arise when substances are heated.
When some constituent particles (atoms or molecules) come into the interstitial places in a crystalline structure, the defect produced is called Interstitial Defect. As shown in Fig. Due to this defect the density of substance increases.
In general, Vacancy Defects and Interstitial Defects as explained above are possible in nonionic solids. Such defects are also found in ionic solids, but in ionic solids it is necessary to maintain electrical neutrality, so these defects are classified as Frenkel and Schottky defects.
This is basically a vacancy defect of ionic solids. In 1930, the scientist Schottky reported that during crystal formation, some ions leave their fixed positions and move out of crystal lattice, leaving a vacancy in the lattice, which is called a hole. This is called Schottky Defect. The number of cations and anions leaving the crystal lattice remains the same, so the electrical neutrality of crystal remains.
An ionic compound M+X– is shown in the figure. One M+ ion and one X– ion have left their respective positions. This pair of cation and anion is called Schottky pair. This defect is found in crystals of ionic compounds that have a high coordination number of ions. And the ions have almost the same size. E.g. NaCl, KCI, CSCI, KBr etc. At room temperature, NaCl crystals have about 10° Schottky pairs per cm³. There are about 10²² ions in one cm. Thus a Schottky pair defect occurs in the 10¹⁶ lattice locations. Due to this defect, the density of crystal decreases and with more holes, the stability and lattice energy of crystal decreases. Due to this defect, the electrical conductivity of crystal increases because on passing an electric current, the ion moves from its place into the hole and a new hole is formed which keeps on moving from front to back. With increase in temperature, Schottky defect increases.
In 1926, the scientist Frenkel said that in an ionic crystal, an ion leaves its fixed position and moves to the interstitial space. Due to which the space of that ion becomes vacant, which is called hole. This is called Frenkel Defect. Basically this defect is Interstitial Defect. Generally this defect is caused by cation because its size is smaller than that of anion. In this defect, the number of cations and anions in the crystal and their charge remain the same, so the crystal is neutral.
This defect is found in crystals of ionic compounds which have less coordination number of anions and the size of anions is much larger than that of cations. E.g. AgCI, AgBr, ZnS etc. Due to this defect a hole is created. Hence, a hole can move in a complete crystal. For this reason crystals can exhibit electrical conductivity. As the number of holes increases, the stability of crystal decreases but the density remains the same. Because the number of ions per unit volume does not decrease. In this error, as similarly charged ions come closer together, the Dielectric Constant of compound increases.
Note- (i) In some crystals both Schottky and Frenkel defects are found. For example, in AgBr.
(ii)Compared to a Frenkel defect, much less energy is required to form a Schottky defect than a Frenkel defect.
Difference Between Frenkel and Schottky Defects
|S.R||Frenkel Defects||Schottky Defects|
|1.||It is Interstitial Defects.||It is Vacancy Defects.|
|2.||In compounds composed of smaller cations and larger anions.||In compounds composed of equal sizes cations and anions of approximately.|
|3.||In this defect, ions move into the space.||In this defect, the ions leave the crystal lattice.|
|4.||The density of crystal remains unchanged.||The value of Dielectric Constant increases.|
|5||The value of Dielectric Constant increases.||The value of Dielectric Constant does not change.|
|6.||This defect is found in crystals with low coordination numbers.||This defect is found in crystals with high coordination numbers.|
Impurity defects can generally occur in ionic compounds. By adding impurities to an ionic solid, the electrical conductivity of some ionic solids can be increased. This process is called doping. This process is produced by mixing another metal ion into an ionic solid. For example, if NaCl is mixed with a small amount of SrCl2 and then crystallizes, then somewhere in the NaCl crystal, instead of Na ions, Sr+2 ions take place. Since Sr+2 has a +2 charge, for one Sr+2 ion, two Na+ ions leave their positions. Out of which one position is taken up by Sr+2 ion and one position is left vacant. These voids increase the conductivity of solid because other nearby ions can move towards this void. Other examples are the impurity of CaCl2 in NaCl, solids obtained from the impurity of CdCl2 in AgCl.
Compounds which do not obey the law of constant proportion. That is, the ratio of cations and anions is not equal to the ratio shown by the molecular formula of that compound, they are called nonstoichiometric compounds. The chemical organization ofse compounds is uncertain or variable. These are also called Bartholide compounds. The number of cations in these can be more or less than a certain ratio. These are called cation excess or deficiency defects. But the crystal is electrically neutral, which is balanced by the presence of an extra electron or an additional positive charge. This leads to defects in the structure of crystal. In general, transition metal compounds are non-stoichiometric. Because they show variable oxidation states.
Examples are NiO, FeO, Cu2S, FeS, CuO etc. The chemical composition of compounds of some transition metals is as follows – Fe0.880-1.0S, Fe0.84-0.94O, TiO0.69-1.33 etc.
Interstitial compounds of transition metals with H, C, N etc. are also non-stoichiometric. Example- ZrH1.92, VH0.56 etc. There are two types of non-stoichiometric defects.
Metal Excess Defects
This defect occurs due to excess of metal ion i.e. cation in crystal lattice. This defect can arise in the following two ways –
1. due to the presence of electrons in place of anion (due to negative vacancy)
In this some anion leaves the crystal lattice and comes out and takes their place electrons. Thus the concentration of metal ion in the crystal lattice increases. But the indifference of crystal remains.
Example – When NaCl is reacted with Na vapor, yellow NaCl is obtained, which is non-stoichiometric. Its yellow color is due to the presence of electrons in the crystal lattice instead of CI. Similarly on reacting with potassium vapour, KCl; It turns purple or lilac. When LiCl is reacted with Li vapor, it turns pinkHere the center of electron presence is called F-centre (German word Ferbe = colour) or color centre. The color becomes darker as their number increases. Due to unpaired electron, they are paramagnetic and electrically conductive. This defect is similar to Schottky defect and is found in crystals with Schottky defect.
2. Due To Presence Of Extra Cation In The Space
In this type of defect, some extra cations take up space in the interspace of crystal lattice and to maintain the neutrality of crystal, electrons also take up space in other space. For example, on heating ZnO, this defect develops in it, due to which its color becomes yellow.
ZnO = Zn+ 2 + 1/2O2 + 2e–
These defects are similar to Frankl defects. Crystals with metal ion excess defect are colourful, paramagnetic and conductive due to free electrons. They act as semiconductors (n-type semiconductors).
Metal Dificiency Defect
In this defect, the number of cations or metal ions in the crystal is less than that of anions. This defect can arise in the following two ways-
1. Cation Gap In Lattice
In this type of defect, some of cations are expelled from the crystal lattice, causing holes to form at those places, but to maintain the neutrality of crystal, additional positive charge is placed on other nearby metal ions. This defect is found in compounds of transition metals such as FeS, NiO, FeO, etc., showing variable oxidation states.
2. Presence Of Extra Anion In The Space
In this type of defect some additional anion takes up space in the inner space of crystal lattice and other nearby metal ions change to higher oxidation state to maintain the electrical neutrality of crystal. Generally, crystals with such defects are not found because due to the relatively large size of anion, it cannot easily occupy space in the space.
Crystals with metal ion deprivation defects also act as semiconductors (P-type semiconductors). Their conductivity is due to the acquisition of electrons from a neighboring ion by a metal ion with a higher oxidation state. This process goes on and on in the crystal.