Non-Stoichiometric Compound

Introduction of non-stoichiometric compound

Non-stoichiometric compounds are any solid chemical compounds in which the number of atoms of the particular elements can not be expressed in a ratio of a small whole number.

In this, the atomic ratios are represented in the form of small integers.

A conventional non-stoichiometric compound is defined as one that has a small and fixed composition and is represented in a small integer ratio among its atomic components are known as non-stoichiometric compounds and the concept of the non-stoichiometric compound is related to the Dalton law of multiple proportions.

The atoms present in these compounds are located accurately at their appropriate crystal lattice points and in these compounds, neither defects nor impurities are allowed to exit.

Non-stoichiometric compounds

In the introduction of non-stoichiometric compounds, we have already discussed some facts of non-stoichiometric compounds. The compounds in which the ratio of positive and  negative ions present in the compounds differs from that required by the actual formula of the compound are called non-stoichiometric compounds. The non-stoichiometric compounds are mainly related to the transition elements. The existence of non-stoichiometric compounds is related to the presence of impurities and defects present in the crystal lattice structure of the substances, such as the absence of ions from its appropriate sites that would be normally occupied by other ions.

All the non-stoichiometric compounds deviate from all the laws followed by that of stoichiometric compounds like the law of definite proportion, the law of constant composition and the law of conservation of mass etc.

The non-stoichiometric compounds exhibit properties like conductivity, colour, magnetism and they also have a catalytic nature which is very important for chemical reactions.

The properties of the non-stoichiometric compounds depend upon the orientation, structure and phase of the crystal. So, it proves that the unit cell parameters play an important role in non-stoichiometric compounds, In which variation occurs in a definite manner.

The ceramic materials which are newly discovered to have superconducting properties at high temperatures are non-stoichiometric compounds. for example, yttrium barium copper oxide.

The compounds in which the ratio of positive to negative ions present in the compounds differs from that required by the actual formula of the compound are called non-stoichiometric compounds.

Some of the examples of non-stoichiometric compounds 

Some of the examples of non-stoichiometric compounds are

Fluorides, hydrides, sulphides, nitrides, oxides(iron oxide, nickel oxide etc).

Types of non-stoichiometric defects 

There are mainly two types of non-stoichiometric defects present such as metal excess defects and metal deficiency defects :-

1.Metal excess defects:

a)Metal excess defect due to anionic vacancies: In this negative ions are missing from their lattice point which create holes after leaving their places in which the electrons are present to maintain the electrical neutrality of the compounds.for example; sodium chloride, lithium chloride etc.

b)Metal excess defect due to the presence of extra cation at interstitial sites:In this types of defects there are extra cation or positive ions are present at interstitial sites and electron present in another interstitial space to maintain the neutrality.This defect may be visual as the loss of non metals which leaves their electrons behind.for example;Zinc oxide.

2.Metal deficiency defect:

First case;The positive is missing from it’s appropriate position in lattice structure and the extra negative charge is balanced by near by atoms which acquires two positive charge instead of one.This type of defects are only possible in those atoms who share variable oxidation states.for example;nickel oxide, ferrous oxide etc.

Second case;In this any extra anion may occupy the interstitial position present in the compounds.The extra charge is balanced by the adjacent atoms present in the compounds.This type of effect are not common because of the size of anion, anions are usually very large and they cannot easily fit into the interstitial spaces present in the compounds.

Conclusion

Non-stoichiometric compound defects are defects that occur in the crystal structure that disturbs the stoichiometry of the crystal.

There are many types of non-stoichiometric defects present such as metal excess defects and metal deficiency defects.The Non-stoichiometric compound are very useful as it uses in oxidation catalysis,Ion conduction,in ceramic,in electrochemical system design and also most of the superconductors are made up of non-stoichiometric compounds.Some of the non-stoichiometric compounds are used in electronic devices for example in rectifiers,thermistor and magnets are very useful in high frequency circuits.