Point Defects

Solids are made up of a lot of small crystals. However, the defect in solids occurs during the crystallization process due to the fast or moderate rate of crystal formation. Defects are described as irregularities in the arrangement of constituent particles in general. The defect could be a point or a line defect based on the irregular arrangement. A deviation point defect occurs in an ideal crystal when there are anomalies in the arrangement around any point or atom in the crystal. Similarly, a line defect occurs when there is a variation in the arrangement of the entire row of lattice points in crystals.

Defects in Solid

Crystalline solids are formed by the well-ordered replication of a volumetric number of unit cells in all directions. An ionic crystal that consists of a single unit cell with identical lattice points throughout the crystal is known as an ideal crystal. Regardless of whether such ideal crystals exist only at absolute zero (0K) temperature. At any temperature above 0 K, the crystals are somewhat out of a completely arranged arrangement. I

Any deviation from the perfectly arranged arrangement of constituent particles in a crystal is called a disorder or defect. Hence, Imperfection in solids refers to any abnormality in the pattern of crystal arrangement in solids. When crystals form, faults develop. It can happen very quickly or at a slower pace. Because particles don’t have enough time to arrange themselves in a regular manner, this happens.

Additional defects may occur in the crystal due to the presence of certain impurities. The term disorder or imperfection is widely practised to refer to an intimate way of going from the perfectly arranged state of the constituents of a crystal. These imperfections will not only alter the properties of the crystal but will also give rise to new properties.

Causes of Imperfections or Defects in a Solid

Various Causes of defects in solids are,

• Different types of Impurities found in the crystal lattice or in a solid make various defects in the solid.
• Vacancies in the lattice are responsible for various defects.
• Nonstoichiometric proportions of the ions cause defects in the solid.
• Dislocation of a particle in the lattice is also responsible for defects in solids.

Types of Defects

The defects are of two types:

• Point Defects: Defects arising due to irregularities or deviations from the ideal arrangement of atoms around a point or an atom in a crystalline material are called point defects or atomic defects.
• Line Defects: The defects that roll out due to irregularities or deviations from the ideal arrangement in an entire row of lattice points are called Line defects.

These irregularities are called crystal defects or crystal imperfections.

Point Defect in Solid

When crystallisation occurs quickly, point defects arise, preventing a perfect crystal structure from forming. It should be noted, however, that even during a slow crystallisation, there exist defects. Point Defects can be divided into three categories.

It is probably noted that vacancy and interstitial defects are shown by non-ionic solids. On the other hand, ionic solids must always maintain electrical neutrality.

The point defects in ionic crystals presumably behind-the-scenes as:

• Stoichiometric Defect
  • Frenkel Defect
  • Schottky Defect
• Non Stoichiometric Defect

Stoichiometric Defects

Stoichiometric compounds are those in which the number of positive and negative ions is in the ratio indicated by their chemical formulas. For the sake of simplicity, we can consider AB-type compounds that have an equal number of A⁺ and B^– ions. Defects in these types of compounds are called stoichiometric defects. Generally, two types of defects are observed in these compounds. 

The NaCl crystal with no defect is shown in the image added below.

Following are the types of Stoichiometric Defects

• Schottky Defect
• Frenkel Defect

Schottky Defect

The Schottky defect arises if some of the atoms or ions are missing from their normal lattice sites. The lattice sites which are abortively are called lattice vacancies or holes. So far as the crystal has to be electrically neutral, an equal number of cations and anions are missing. The ideal AB crystal is shown above. Presence of two holes one due to missing cation and the other due to missing. It is parring excellence a vacancy defect in ionic solids.

The Schottky Defect in a crystal is shown in the image added below.

Conditions for Schottky Defects

This type of defect is usually seen in strong ionic compounds having

• High Coordination number
• Ions of approximately the same size.

For example, NaCl, KCI, KBr, CsCl, and AgBr ionic solids have Schottky defects. It has been ascertained that in NaCl, there are about 10⁶ Schottky pairs per cm³ at room temperature. In 1cm³ there are about 10²² ions and this means that there will be one Schottky defect per 10¹⁶ ions in NaCl. Forasmuch as the impendence of a broad number of vacancies in crystals, its density is markedly lowered.

Frenkel Defect

The Frenkel defect occurs when an ion disappears from its normal state and occupies an interstitial site between lattice points. The existence of a hole due to a missing cation from its normal position and occupying the interstitial position is shown. In this case, also, the crystal remains electrically neutral because the number of anions and cations remains the same This defect is also known as an interstitial defect. It creates vacancy defects. It has its original position and an interstitial defect in its new location.

The Frenkel Defect in a crystal is shown in the image added below.

Conditions for Frenkel Defects

This defect generally occurs in compounds in which the coordination number is low, anions are much larger than the cations.

In perfect alkali metal halides, these defects are not very common because the ions cannot move to the interstitial state due to their large size. Defects can be found in silver halides such as AgCl, AgBr, AgI, ZnS, etc. So far as of the small size of the Ag ions and ions, these ions can move into the interstitial sites.

Vacancies or holes exist in crystals that have a Schottky well as well as a Frenkel defect, but the former decreases the overall density of the substance, but the latter does not. , another hybrid type of defect can also arise from a combination of both.

Effects of Schottky and Frenkel Defects

 Defects in Schottky and Frank crystals give some interesting results. These are-

• The presence of these defects increases the electrical conductivity of crystals. When an electric field is about the experiment, an adjoining ion moves from its lattice site to occupy a hole. As a result, a new hole is created and another nearby ion moves into it, and so on. This process continues and thus he moves from one end to the other. Thus, it conducts electricity throughout the crystal.
• Due to the impendence of pores in the crystal, its density changes. It presumably noted that the density decreases only for crystals with Schottky defects. However, there is no change in denisty in case of Frenkel defect
• The presence of pores also reduces the lattice energy or stabilization of the crystal. The presence of too many pores can cause partial collapse of the lattice.
• In Frenkel defects, the dielectric constant of the crystal is increased by placing it in a pattern of equal charges.

Non-Stoichiometric Crystals Defects

Non-stoichiometric defects unsettle the stoichiometry of solids. The ionic sites occupied by unpaired electrons are called F-centers (Farbenzenter or colour centre). They impart colour to alkali metal halide crystals. The colours are produced by the excitation of these electrons when they absorb energy from visible light falling on the crystal. Excess of Na turns NaCl crystals yellow, excess of Li makes LICI pink, and excess of potassium turns KCl crystals purple.

• Metal Excess Defect
• Metal Deficiency Defect

Metal Excess Defect

If in the lattice structure, an anion gets displaced the number of metal cations gets larger than the number of anions causing the access of the metal cations and resulting in the metal access defect.

Metal excess defect due to anionic vacancies is shown by alkali halides like NaCl and KCl. In such defects, a negative ion is missing from its lattice site leaving a hole which is occupied by an electron to maintain electrical neutrality.

The excess of metal due to the presence of additional cations at the interstitial sites is shown by zinc oxide on heating. On heating, zinc oxide (white) loses its oxygen and turns yellow.

ZnO  +  Heating → Zn²⁺ + O₂ +2e^–

Metal Deficiency Defect

Metal deficiency defects arose due to two factors

• Cation Vacancies
• Extra anions occupying Interstitial sites.

In cation vacancies, the cation of the crystal lattice gets dispaced from the lattice structure which causes the meatal deficiency defects.

These defects can also arise if the interstitial space in the crystal lattice is filled by the extra anions.

Read More,

• Lattice Energy
• Crystal Lattice and Unit Cell
• Lattice Energy

FAQs on Point Defects

Q1: What is a Point Defect?

Answer:

Lattice defects of the zero dimension are called point defects. They are generally caused when the crystallization occurs at a very fast rate. They occur when the crystal deviates from the ideal arrangement of the crystal.

Q2: Which is a Line Defect?

Answer:

If an entire row of atoms dissociates in any crystal then the defect in the crystal is called the line defect. They are also called dislocation, the strength of a solid deteriorate by the line defects.,

Q3: Which Point Defect Lowers the Density of a Crystal?

Answer:

The Schottky Defect lowers the density of the crystal in Schottky Defect a cation and anion pairs leave the crystal making the crystal less heavy and reducing the density of the crystal.

Q4: What causes Defects in a Material?

Answer:

The defects in the crystal are caused by various factors that include,

• Non-Stichometric Proportion of the solid.
• Impurities in the Crystal lattice.
• Dislocation of the ions in the lattice.

Q5: Name a solid that has both Frenkel and Schottky defects.

Answer:

Silver Bromide (AgBr) is a special crystal that has both Frenkel and Schottky defects.

Q6: What is Frenkel Defect?

Answer:

Frenkel Defect is a point defect in which cations dislocate from their original site to interstitial sites of the lattice.

Q7: What is Schottky Defect?

Answer:

Schottky Defect is a point defect in a solid in which pair of cations and anions are missing from their lattice site.