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Non-Stoichiometric defect is a flaw that causes the number of cations to anions ratio to diverge from the predicted way by the ideal chemical formula. The defect does not affect the crystalline substance's stoichiometry. However, due to flaws in their crystal structures, a wide variety of non-stoichiometric inorganic solids are known, that contain the component elements in non-stoichiometric ratios. Metal excess defects and metal deficiency defects are the two forms of non-stoichiometric flaws.
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Key Terms: Non-Stoichiometric Compound, Non-Stoichiometric defect, Metal excess defects, Metal deficiency defects, Stoichiometry, Cations, Anions, Energy, Thermodynamics, X-ray
Characteristics Of Non-Stoichiometric Compounds
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Here are some important characteristics of Non-Stoichiometric Compounds-
- Nonstoichiometric compounds have a homogeneous physical phase in which unit cell characteristics continuously fluctuate with composition.
- The free energy of such compounds is a function of the system's composition and temperature.
- The chemical potential of a solid is determined by its composition, and structural and thermodynamic properties alter as a result of microscopic and macroscopic features.
- Because of nonstoichiometry, several properties of the crystallographic cell, as determined by X-ray diffraction analysis, alter. The content of this parameter changes with time.
- The fundamental criterion for classifying nonstoichiometric compounds is their element makeup.
- Only the condensed state exists for these chemicals. Transition metal oxides are nonstoichiometric in general, and some nitrides and sulfides fall into this group as well [4, 5, and 6].
- The compound results in the creation of certain lattice defects due to a divergence from stoichiometric composition.
- As a result, there are anionic vacancies in some nonstoichiometric compounds and cationic vacancies in others.
- Defects in the lattice structures of crystalline solids, such as the lack of ions from locations that would typically be occupied, cause these vacancies to occur.
- The Frenkel defect and the Schottky defect are the two most common point defects in nonstoichiometric compounds.
Non-Stoichiometric Defect Diagram
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Types Of Non-Stoichiometric Defects
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Generally, there are two types of Non-Stoichiometric Defects:
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Metal Excess Defects
The metal excess defects occur due to the following reasons and circumstances:
Metal Excess Defects caused due to anionic vacancies
Here are some Metal Excess Defects due to anionic vacancies-
- Alkali halides, such as NaCl and KCl are examples that have metal excess defects due to anionic vacancies.
- The sodium atoms are deposited on the surface of NaCl crystals when they are heated in a sodium vapor environment.
- Cl–ions diffuse to the crystal's surface and mix with Na atoms to form NaCl. This occurs when sodium atoms lose one electron, resulting in the formation of Na+ ions.
- The ejected electrons spread within the crystal and take up residence in anionic sites.
- As a result, there is now an overabundance of sodium in the crystal.
- F-centers (from the German word Farbenzenter for color center) are anionic sites populated by unpaired electrons. They give the NaCl crystals a yellow color.
- Excitation of these electrons occurs when they absorb energy from visible light falling on the crystals, resulting in color.
- Similarly, too much lithium turns LiCl crystals pink, whereas too much potassium turns KCl crystals violet (or lilac).
Metal interstitial defect:
Presence of additional cations may also cause non-stoichiometric defects in the crystal-
- At normal temperature, zinc oxide is a white powder. It loses oxygen and becomes yellow when heated.
ZnO → Zn2+ + ½ O2 +2e–
- The crystal now has an overabundance of zinc, and its formula is Zn1+ x O. Excess Zn2+ ions migrate to interstitial sites, whereas electrons migrate to nearby interstitial sites.
Metal Excess Defects
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Metal Deficiency Defects
Non-Stoichiometric defects that happens due to Metal Deficiency Defects may have the following characteristics and causes-
- At some sites, higher valence metal ions are incorporated into the crystal, causing metal deficiency defects.
- Many solids are difficult to make in a stoichiometric proportion and contain less metal than the stoichiometric proportion.
- FeO, which is generally found with a composition of Fe0.95O, is a good example of this kind.
- It might be anything between Fe0.93O and Fe0.96O. Some Fe2+ cations are absent from FeO crystals, but the loss of positive charge is compensated for by the existence of the requisite amount of Fe3+ ions.
Metal Deficiency Defects
Things to Remember
- Non-Stoichiometric defect can be explained as the type of defect that shows variable oxidation states of any metal which are not predicted by any ideal chemical formulas.
- There are two types of non-stoichiometric defects- Metal Excess Defects and Metal Deficiency Defects.
- Their elements form a makeup which is the fundamental criterion for classifying non-stoichiometric compounds.
- There can be anionic vacancies in some non-stoichiometric compounds and cationic vacancies in the others.
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Sample Questions
Ques. On which defects are Frenkel and Schottky defects based? (2 Marks)
Ans. Frenkel and Schottky's defects are the two common defects that can be found in non-stoichiometric defects and these defects are mainly based on crystal defects.
Ques. What causes non-stoichiometric defects? (3 Marks)
Ans. Because of the varying oxidation numbers of the cation, the ratio of cation and anion in nonstoichiometric defects is not the same as that described by the chemical ideal formula. Transition elements have a wide range of oxidation numbers.
Ques. Which metals form a simple cube when crystallized? (3 Marks)
Ans. Polonium exists in two metallic allotropic forms: α-form and β-form. The α-Po form, out of these two, crystallizes as a simple cubic crystal. Face-centered cubic crystals crystallize copper and gold, while body-centered cubic crystals crystallize iron.
Ques. Calculate Ni(OH)2 solubility in 0.1 M NaOH. Given that Ni(OH)2 has an ionic product of 2×10−15. (5 Marks)
Ans. Let’s take α=1 for NaOH
NaOH(aq) → Na+(aq) + OH-(aq)
0.1 M 0.1M
Ni(OH)2(S) ⇋ Ni+2 (aq) + 2OH-(aq)
S (0.1+2S)
Ionic Product will be- [Ni+2] [OH-]2
2 x 10-15 = [Ni+2] [10-1]2
2×10−15 = [Ni+2]
Ques. Why are crystals not always flawless, despite long-range order in particle arrangement? (2 Marks)
Ans. Crystals contain a long-range repetitive pattern of constituent particle arrangement, although defects (differences from the ideal arrangement) can be introduced during the crystallization process, therefore crystals are rarely flawless.
Ques. Why does table salt (NaCl) seem yellow sometimes? (2 Marks)
Ans. Metal excess defects cause unpaired electrons to occupy anionic sites, giving sodium chloride its yellow color. F-centers are the names given to these locations. The activation of these electrons requires energy from the visible range, which causes the crystal to look yellow.
Ques. When white ZnO (s) is heated, why does it become yellow? (3 Marks)
Ans. When ZnO is heated, it loses oxygen due to the following reaction:
ZnO —-> Zn2+ + \(½ \)O2 +2e–
Zn2+ ions and electrons travel to interstitial sites, and F-centers are formed, giving ZnO its yellow color (s).
Ques. Explain why gallium doping increases the conductivity of germanium crystals. (5 Marks)
Ans. Some of the places of the germanium lattice are filled by galium when it is doped with gallium. There are just three valence electrons in a gallium atom. As a result, the neighboring germanium atom's fourth valency is not fulfilled. The location is still unoccupied. Because this location is devoid of electrons, it is referred to as an electron-hole or an electron vacancy. An electron from a nearby atom enters the gap and fills it, leaving a hole in its original location. Electrons migrate towards positively charged plates through these holes under the influence of an electric field and conduct electricity. The holes appear to be moving in the direction of negatively charged plates.
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