Question

The intrinsic or thermodynamic defect is (a) Schottky defect (b) Metal excess defect due to excess cation (c) Metal excess defect due to anion vacancy (d) Metal-deficient defect

Short answer

The intrinsic or thermodynamic defect is (a) Schottky defect.

Step-by-step solution

Identifying the Type of Defect

Intrinsic or thermodynamic defects are those which occur in stoichiometric compounds due to thermal agitation at high temperatures. These are not due to chemical impurities or external conditions but due to the intrinsic thermal energy within the crystal. There are two main types: Schottky defect and Frenkel defect. A Schottky defect involves a pair of vacancies, one from the cation and one from the anion sublattice, without changing the overall stoichiometry of the crystal. On the other hand, a Frenkel defect is a combination of a vacancy and an interstitial defect, which often happens in the cation sublattice.

Eliminating the Incorrect Options

For the given options, we rule out the metal excess defects (due to excess cation or anion vacancy) as these are non-stoichiometric defects caused by deviations from the ideal stoichiometry. Similarly, metal-deficient defects are also non-stoichiometric and result from a deficiency of the metal component. Thus, these options do not describe intrinsic or thermodynamic defects.

Choosing the Correct Answer

With intrinsic or thermodynamic defects, the crystal remains overall stoichiometric. Therefore, the only option that refers to a stoichiometric defect due to intrinsic thermal energy is the Schottky defect. Hence, option (a) Schottky defect is the correct answer.

Key concepts

Schottky Defect

A Schottky defect is a fundamental type of point defect found in crystal lattices. It arises when an equal number of cations and anions are missing from their lattice sites, creating vacancies. This type of defect is significant because it maintains the overall stoichiometry of the crystal despite the missing ions. Because of this, the physical properties of the material, such as density, can be affected, as there are fewer atoms than in a perfect crystal.
The formation of Schottky defects is influenced by the size of the ions and the lattice energy of the crystal. At high temperatures, when thermal vibrations increase, atoms or ions gain sufficient energy to leave their lattice sites, resulting in vacancies. Interestingly, these defects do not introduce new chemical species or change the composition of the material; they are purely geometrical in nature.

Relevance to Material Properties

In ceramics and some ionic compounds, Schottky defects can significantly impact the electrical conductivity, as the movement of vacancies under an applied field can carry current. They are also influential in processes like diffusion, where the movement of atoms or ions through the lattice is essential for various applications like sintering and alloying.

Intrinsic Defects in Crystals

Intrinsic defects are imperfections that occur within the crystal lattice of a material on its own, without any external influence. These defects are a result of the innate properties of the material, such as its thermodynamic stability, and the natural conditions it experiences, such as temperature. Intrinsic defects arise because of thermal energy that causes sporadic movements in the lattice atoms or ions.
Types of intrinsic defects include Schottky and Frenkel defects, as mentioned previously. It is crucial to understand that intrinsic defects are not errors or impurities, but rather natural variations that arise in all crystals at finite temperatures. These defects can affect a variety of properties ranging from electrical conductivity to mechanical strength.

Schottky vs. Frenkel Defects

While Schottky defects involve the formation of vacancies, Frenkel defects are a combination of a vacancy and an interstitial defect, where an atom or ion is displaced to a site between the regular lattice positions. Unlike Schottky, Frenkel defects do not change the overall density of the crystal significantly because the displaced ions remain within the crystal.

Stoichiometry in Crystal Defects

Stoichiometry in the context of crystal defects is concerned with the maintaining of the elemental composition and ratio as per the chemical formula of the crystal, despite the presence of defects. A stoichiometric defect like the Schottky defect does not alter the chemical composition of the crystal since there is an equal number of missing cations and anions, maintaining the balance. This is in contrast to non-stoichiometric defects, which disturb the stoichiometry, causing an excess or deficiency of some ions.

Impact of Stoichiometry

The kind of defects present in a crystal structure deeply influences the physical and chemical behavior of the material. Stoichiometric defects preserve the idealized ratio of elements in a compound, which is crucial for many applications that rely on consistent material properties. For instance, the semiconductor industry depends on precise control over stoichiometry for the fabrication of devices with predictable electronic properties. Non-stoichiometric defects, such as metal excess or deficiency, may induce changes in electronic structure and are utilized in applications where variable conductivity is required.