Non-Stoichiometric Defects

In solids, the crystal arrangement is in a symmetrical pattern, and it’s called imperfection or defect solids. For the formation of crystal processes, defects take place. Defects happen in a very fast way, because particles are closely packed in a solid and they do not rearrange themselves. Small crystals are arranged together closely and made solid. Defects of solid are mainly classified into 2 types-point defect or line defect. 

Crystal defect:

Absolute crystals are formed only at 0K temperature, and other crystals are not absolute. Crystal defect can be defined as the withdrawal of complex order and particles’ periodic arrangement in the crystal lattice.

Cause of crystal defect:

crystal defect takes place in following factors-

• Lattice vacancies.
• In a lattice, disturbance of particles.
• Non-stoichiometric amount of ions.
• Lattice impurities.

Types of defects:

There are 2 main types of defects in solids:

• Line defect.
• Point defect.

Line defect:

In line defect, solid expands above the microscopic region in a row of the crystal lattice.

Point defect:

In lattice, disturbance of particles as they move from one place to another causes the imperfection or defect in the crystal lattice. There are three types of point defects:

• Stoichiometric defect.
• Non-stoichiometric defect.
• Impurity defect.

Non-stoichiometric defect: 

Non-stoichiometric compounds are those compounds in which proportion of cation and anion is not similar and defects of these compounds are known as non-stoichiometric

defects. In this defect a large number of positive and negative charges are present. As we know, crystals remain neutral so, if a spare amount of negative charge is present then it will be maintained by extra positive charge presence. Because of this positive and negative charge, solid crystal structure shapes will become improper, and solid become imperfect. 

 This defect can be happened due to 2 reasons:

• In a lattice, substances have a cation whose proportion is more than in an anion. Hence, known as a metal excess defect. 
• In a lattice, substances have a cation whose proportion is lesser than in an anion. And hence known as the metal deficiency defect.

Metal excess defect:

It happens when in a lattice, cation has more ratio than in anion. As the name suggests, there is an excess amount of metals and positive ions. This type of defect is the same as Schottky and Frenkel’s defect, but there is the absence of holes and interstitial electrons. Free electrons present in this defect can generate electricity to a level. These metal excess defects are coloured due to free electrons. There are further 2 types:

• Anionic vacancy: In this defect, to maintain electrical neutrality, electrons bind to that space where an anion is released. An anion trapped electron to catch them is referred to as f-centre. The cavities present are coloured centres. Mainly, they take place in ionic compounds. 

Example: NaCl and LiCl.

• Extra cations: In the lattice, extra cation binds to interstitial sites. Hence, an extra positive charge is created which is further maintained by electron numbers.

Example: Zinc oxide.

Metal deficiency defect:

Solids can not be prepared in the composition of stoichiometric and hence vary lower amounts of metals, (for example-FeO). It happens when in a lattice, cation has less ratio than in anion. Different oxidation states are varied by compound cations. Negative ions sizes are comparatively large and hence it is expected that negative ions would not be fixed into interstitial positions. In this defect, positive ions are missing, and they can act as a semiconductor.

Examples: Iron compound crystal and copper compound crystal. 

Conclusion

• The microscopy technique was used to study defects in solids. In some defects the density of solid is increased, in some cases, it decreases and in some solids, it does not affect. 
• Some defects help to maintain the charge of electrons and some variation in cations and anions. In the above article, we learned about non-stoichiometric defects and their types. 
• Mainly defects of solid are of 2 types: Line defect and point defect and the non-stoichiometric defect is a type of point defect. 
• These various types explain features of defects and the reason behind the imperfection of solid. The non-stoichiometric defect is the type of point defect, and hence in this defect anion and cations are not equal in proportion, and large numbers of negative and positive charges are present. 
• The non-stoichiometric defect is further divided into two classes that are metal excess defect and metal deficiency defect.
•  As their name suggests, in metal excess defect there is an excess amount of positive ions present and in metal deficiency defect positive ions are absent, hence this is the main difference between metal excess defects and metal deficiency defects.