Types of Non-Stoichiometric Defects

Non-stoichiometric compounds are chemical compounds, usually typically solid inorganic compounds, whose elemental composition cannot be represented by a ratio of small natural numbers; most frequently, such materials lack a small percentage of atoms or have an abnormally dense lattice work.

Additionally, non-stoichiometric molecules are referred to as berthollides (as opposed to the stoichiometric compounds or daltonides). The names are derived from Claude Louis Berthollet and John Dalton, two nineteenth-century advocates of diametrically opposed theories of substance composition. While Dalton “won” the majority of the time, it was eventually found that the law of definite proportions contained significant exceptions.

Non-stoichiometric substances have an elemental composition that cannot be described numerically by proportions. A small fraction of atoms are missing or the lattice is packed excessively densely.

History

Berthollet’s challenge to Proust’s law was demonstrated to have merit for a large number of solid compounds primarily through the work of Nikolai Semenovich Kurnakov and his students. Kurnakov classified nonstoichiometric compounds as berthollides or daltonides based on whether or not their properties were monotonic with respect to composition. The word berthollide was recognised by the International Union of Pure and Applied Chemistry in 1960. The names are derived from Claude Louis Berthollet and John Dalton, two nineteenth-century advocates of diametrically opposed theories of substance composition. 

Because of defects in their crystal structures, a large number of non-stoichiometric inorganic solids are known, which contain the constituent elements in non-stoichiometric ratios as a result of these flaws.

These defects can be divided like:

• Metal excess defect

• Metal deficiency defect.

Excess Metal Defect

1. Excess metal defect caused by anionic vacancies:

This type of defect occurs in alkali halides such as NaCl and KCl. When sodium chloride crystals are heated in an environment of sodium vapour, sodium atoms are deposited on the crystal surface. Cl^– ions diffuse to the crystal’s surface and react with Na atoms to form NaCl. This occurs as a result of sodium atoms losing an electron to generate Na⁺ ions. The freed electrons disperse within the crystal and take up residence in anionic sites (Fig.).

The F-center refers to the anionic sites filled by unpaired electrons. This F-center is responsible for the crystal’s hue. As an example, NaCl has a yellow color, LiCl has a pink tint, and KCl has a violet color.

It is a component of ionic compounds.

It is responsible for the crystal’s coloration via F-centers.

For example, NaCl, KCl, and LiCl.

2. Due to the existence of additional cations at interstitial locations, a metal surplus defect occurs:

At normal temperature, zinc oxide is a white powder. It loses oxygen while heating and turns yellow.

ZnO –> Zn2²⁺ + 1/2 O₂ + 2e^–

Now that the crystal contains an excess of zinc, its formula becomes Zn_1+xO. Excess Zn²⁺ ions gravitate toward interstitial sites, while electrons gravitate toward neighboring interstitial sites.

Defect Due to a Metal Deficiency-

Numerous solids are difficult to manufacture in stoichiometric proportions and contain less metal than the stoichiometric proportion.

A classic example of this type is FeO, which is typically found as Fe_0.95O. It is possible that it varies between Fe_0.93O and Fe_0.96O.

Some Fe²⁺ cations are absent from FeO crystals, and the loss of positive charge is compensated for by the existence of the requisite amount of Fe³⁺ ions.

Applications

1. Oxidation catalysis- Numerous valuable chemicals are formed when hydrocarbons react with oxygen, a reaction facilitated by metal oxides. The method operates by transferring oxygen in a “lattice” to the hydrocarbon substrate, a step that results in the temporary generation of a vacancy (or defect). Following that, O₂ is used to restore the depleted oxygen. These catalysts rely on the metal oxide’s ability to generate non-stoichiometric phases. Other types of atom-transfer processes, such as hydrogenation and hydrodesulfurization catalyzed by solid catalysts, follow a similar series of events. These considerations further emphasize the fact that stoichiometry is dictated by the core of crystals: crystal surfaces frequently deviate from the bulk stoichiometry. The term “surface reconstruction” refers to the complex structures found on surfaces.

2. Conduction of ions- Atomic migration inside a solid is greatly impacted by defects caused by lack of stoichiometry. These defect sites create channels for atoms and ions to migrate across the crystals’ otherwise dense ensemble of atoms. Two applications that rely on oxide vacancies are oxygen sensors and solid state batteries. A sensor made of CeO₂ is one example. It is used in vehicle exhaust systems. When O₂ partial pressures are low, the sensor enables the entry of more air to achieve more complete combustion.

3. Superconductivity- Numerous superconductors exhibit non-stoichiometry. For example, yttrium barium copper oxide is a non-stoichiometric solid with the formula YBa₂Cu₃O₇ possibly the most well-known high-temperature superconductor. The superconductor’s critical temperature is dependent on the precise value of x. x = 0 in the stoichiometric species, although this value can be as high as 1.

Conclusion 

Nonstoichiometric compounds are chemical compounds that break the law of definite proportions because their elemental composition cannot be represented by a ratio of well-defined natural numbers.

Non-stoichiometric defects are those that affect the stoichiometry of the compounds. These problems are caused by either an excess of or a lack of metal ions.