Defect In Crystals
Defects determine the behavior of solids, but because ionic compounds contain both cations and anions, they exhibit additional types of defects that are not found in metals. Real crystals contain large numbers of defects. Defects may affect only a single point in the lattice (a point defect), a row of lattice points (a line defect), or a plane of atoms (a plane defect). A point defect can be an atom missing from a site in the crystal (a vacancy) or an impurity atom that occupies either a normal lattice site (a substitutional impurity) or a hole in the lattice between atoms (an interstitial impurity). In an edge dislocation, an extra plane of atoms is inserted into part of the crystal lattice. Multiple defects can be introduced into materials so that the presence of one defect prevents the motion of another, in a process called pinning. Because defect motion tends to stop at grain boundaries, controlling the size of the grains in a material controls its mechanical properties. In addition, a process called work hardening introduces defects to toughen metals. Schottky defects are a coupled pair of vacancies—one cation and one anion—that maintains electrical neutrality. A Frenkel defect is an ion that occupies an incorrect site in the lattice. Cations in such compounds are often able to move rapidly from one site in the crystal to another, resulting in high electrical conductivity in the solid material. Such compounds are called solid electrolytes.
Summary
Defects determine the behavior of solids, but because ionic compounds contain both cations and anions, they exhibit additional types of defects that are not found in metals. Real crystals contain large numbers of defects. Defects may affect only a single point in the lattice (a point defect), a row of lattice points (a line defect), or a plane of atoms (a plane defect). A point defect can be an atom missing from a site in the crystal (a vacancy) or an impurity atom that occupies either a normal lattice site (a substitutional impurity) or a hole in the lattice between atoms (an interstitial impurity). In an edge dislocation, an extra plane of atoms is inserted into part of the crystal lattice. Multiple defects can be introduced into materials so that the presence of one defect prevents the motion of another, in a process called pinning. Because defect motion tends to stop at grain boundaries, controlling the size of the grains in a material controls its mechanical properties. In addition, a process called work hardening introduces defects to toughen metals. Schottky defects are a coupled pair of vacancies—one cation and one anion—that maintains electrical neutrality. A Frenkel defect is an ion that occupies an incorrect site in the lattice. Cations in such compounds are often able to move rapidly from one site in the crystal to another, resulting in high electrical conductivity in the solid material. Such compounds are called solid electrolytes.
Things to Remember
The crystal lattices we have described represent an idealized, simplified system that can be used to understand many of the important principles governing the behavior of solids. In contrast, real crystals contain large numbers of defects (typically more than 104 per milligram), ranging from variable amounts of impurities to missing or misplaced atoms or ions. These defects occur for three main reasons:
- It is impossible to obtain any substance in 100% pure form. Some impurities are always present.
- Even if a substance were 100% pure, forming a perfect crystal would require cooling the liquid phase infinitely slowly to allow all atoms, ions, or molecules to find their proper positions. Cooling at more realistic rates usually results in one or more components being trapped in the “wrong” place in a lattice or in areas where two lattices that grew separately intersect.
- Applying an external stress to a crystal, such as a hammer blow, can cause microscopic regions of the lattice to move with respect to the rest, thus resulting in imperfect alignment.
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Defect In Crystals
Point Defect
At absolute zero of temperature,the vibrational energy of the ion is very low so that they cannot leave their normsl lattice site and at absolute zero of temperature,perfectly ordered structure is obtained.When temperature is increased,the thermal vibration of ions on their lattice site increases so that they may leave their normal lattice site creating a defect.Such type of defect which is localized on a particular lattice point of crystal is known as point defect.The concentration of point defect increases with the increase in temperature so,it is also known as thermodynamis defect.Schottky defect and Frenkel defect are the two types of point defect.
Schottky defect
The type of defect in which a cation and anion are lost from their lattice site and creates a pair of vacancy in a crystal system is known as Schottky defect.This type of defect is more prominent in ionic crystal where cation and anion are of almost size and have a high coordination number.For example:NaCl,KCl,KBr,etc.
The number of Schottky defect per unit volume is given by
ns=Ne^\(\frac{E_s}{2K_BT}\)
where
ns=number of Schottky defect per unit volume
N=total number of ions per unit volume
Es=average energy required to create Schottky defect
T=absolute temperature
KB=Boltzmann constant
Thus,no. of Schottky defect per unit volume depends on the no. of ions pair,average energy required to create Schottky defect and absolute temperature.
Consequences of Schottky defect
i)In Schottky defect,cation and anion pairs are removed from the normal lattice site to the surface of a crystal.Thus,volume of the defect increases when concentration of the defect increases but mass remains constant.so,the density of the defected salt decreases as compared to the undefected salt.
ii)Conductivity increases when concentration of the defect increases.This can be explained by the hole mechanism.When electricity is passed,the nearby ions moves from its normal lattice site to the hole.Thus,creating a hole in new position.When this process happens many times,the hole may be migrated across the entire crystal.This is equivalent to the moving of charge in opposite direction.
Schottky defect in a crystal
Frenkel Defect
A type of point defect in which an ion is missing from its normal lattice site and occupies the interstitial is known as frenkel defect.Generally,cations are smaller than anions.So,they are mostly found to occupy such position.Frenkel defect is found in a crystal in which cations and anions are of different size and have a low coordination number.eg:ZnS,AgCl,AgBr,etc
Frenkel defect in a crystal
Consequences of Frenkel defect
i)Conductivity of defective salt becomes high as compared to non-defective salt.
The inclusion of large number of ions in interstitial position may distort the crystal lattice and hence increase the unit cell dimension.
iii)As missing ions are present in interstitial position,so that the volume as well as mass does not change.Thus,density of the defected salt remains constant.
F-Centers
In this case,an anion is absent from its lattice site and hole created by it is occupied by an electron maintaining electrical conductivity.The vacant site occupied by electrons are called F-centers which is principally responsible for the colour by absorbing light in the visible region.This defect is found in Schottky defect forming compound.For eg:If a colourless NaCl crystal is heated in presence of sodium vapour and rapidly quenched,extra sodium atom enter the last lattice and get ionised.
Na→Na++ 1e-
Thus,formed sodium ion occupies a normal cationic vacancy of crystal and electron is trapped in anion vacancy.
Interstitial ions and electron defect
This defect arises as a cation occupies an interstitial position and for electrical neutrality,an electron is included in interstitial position.This defect is also known as metal-ion excess defect.Such type of defect is common in those crystal which form a Frenkel defect.
Line defect
The defect of crystal which extends through the line of crystal is known as line defect.It is also known as one dimensional defect.There are two types of line defect.i)Edge dislocation
ii)Screw dislocation
i)Edge dislocation:
In this type of disloaction,an extra half plane of atom is inserted into the crystal lattice.the dislocation extend along the line which bends the crystal during its growth.
ii)Screw dislocation:In this type of dislocation,the atoms in one part of the crystal are displaced relative to rest of the crystal forming a ramp around the dislocation line.There is a distortion of the latttice cell in the immediate vicinity.The displacement direction for screw dislocation is parallel to the dislocation line.
Point Defect
At absolute zero of temperature,the vibrational energy of the ion is very low so that they cannot leave their normsl lattice site and at absolute zero of temperature,perfectly ordered structure is obtained.When temperature is increased,the thermal vibration of ions on their lattice site increases so that they may leave their normal lattice site creating a defect.Such type of defect which is localized on a particular lattice point of crystal is known as point defect.The concentration of point defect increases with the increase in temperature so,it is also known as thermodynamis defect.Schottky defect and Frenkel defect are the two types of point defect.
Schottky defect
The type of defect in which a cation and anion are lost from their lattice site and creates a pair of vacancy in a crystal system is known as Schottky defect.This type of defect is more prominent in ionic crystal where cstion and anion are of almost size and have a high coordination number.For example:NaCl,KCl,KBr,etc.
The number of Schottky defect per unit volume is given by
ns=Ne-Es/2KBT
where
N=total number of ions per unit volume
T=absolute temperature
Thus,no. of Schottky defect per unit volume depends on the no. of ions pair,average energy required to create Schottky defect and absolute temperature.
Consequences of Schottky defect
i)In Schottky defect,cation and anion pairs are removed from the normal lattice site to the surface of a crystal.Thus,volume of the defect increases when concentration of the defect increases but mass remains constant.so,the density of the defected salt decreases as compared to the undefected salt.
ii)Conductivityincreases when concentration of the defect increases.This can be explained by the hole mechanism.When electricity is passed,the nearby ions moves from its normal lattice site to the hole.Thus,creating a hole in new position.When this pricess happens many times,the hole may be migrated across the entire crystal.This is equivalent to the moving of charge in opposite direction.
Schottky defect in a crystal
Frenkel Defect
A type of point defect in which an ion is missing from its normal lattice site and occupies the interstitial is known as frenkel defect.Generally,cations are smaller than anions.So,they are mostly found to occupy such position.Frenkel defect is found in a crystal in which cations and anions are of different size and have a low coordination number.eg:ZnS,AgCl,AgBr,etc
Frenkel defect in a crystal
Consequences of Frenkel defect
i)Conductivity of defective salt becomes high as compared to non-defective salt.
The inclusion of large number of ions in interstitial position may distort the crystal lattice and hence increase the unit cell dimension.
iii)As missing ions are present in interstitial position,so that the volume as well as mass does not change.Thus,density of the defected salt remains constant.
F-Centers
In this case,an anion is absent from its lattice site and hole created by it is occupied by an electron maintaining electrical conductivity.The vacant site occupied by electrons are called F-centers which is principally responsible for the colour by absorbing light in the visible region.This defect is found in Schottky defect forming compound.For eg:If a colourless NaCl crystal is heated in presence of sodium vapour and rapidly quenched,extra sodium atom enter the last lattice and get ionised.
Na→Na++ 1e-
Thus,formed sodium ion occupies a normal cationic vacancy of crystal and electron is trapped in anion vacancy.
Interstitial ions and electron defect
This defect arises as a cation occupies an interstitial position and for electrical neutrality,an electron is included in interstitial position.This defect is also known as metal-ion excess defect.Such type of defect is common in those crystal which form a Frenkel defect.
Line defect
The defect of crystal which extends through the line of crystal is known as line defect.It is also known as one dimensional defect.There are two types of line defect.
i)Edge dislocation
ii)Screw dislocation
i)Edge dislocation:
In this type of disloaction,an extra half plane of atom is inserted into the crystal lattice.the dislocation extend along the line which bends the crystal during its growth.
ii)Screw dislocation:
In this type of dislocation,the atoms in one part of the crystal are displaced relative to rest of the crystal forming a ramp around the dislocation line.There is a distortion of the lattice cell in the immediate vicinity.The displacement direction for screw dislocation is parallel to the dislocation line.
Plane defect
The type of defect which is situated at a plane of crystal is called plane defect.There are two types of plane defect.They are:
i)Grain defect
ii)Stacking defect
Grain Boundaries
In a polycrystalline material,there are number of small interlocking crystals or grains which are randomly oriented in a crystal are called grain boundaries.The angle between the orientation of adjoining crystal are large and structure of grain boundary is complex.
Stacking Fault
Let a crystal of successive layers with atomic arrangement ABC ABC ABC............ as a cubic and AB AB....... as hexagonal closed packing.Now,if the sequence is ABABABCBCBC it becomes cubic instead of hexagonal and if the sequence is ABCABCBCABC,the structure becomes hexagonal instead of cubic.Such defect is known as stacking fault.
References
Paula, P.Atkins & J.de. Elements of Physical Chemistry. New Delhi: Oxford University Press, 2009.
Lesson
Solid State Chemistry
Subject
Chemistry
Grade
Bachelor of Science
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