12th chemistry solid state- Imperfections in Solids

Imperfections in Solids

Defects

• Irregularities or deviations from the ideal arrangement of constituent particles
• Two types:

• Point defects − Irregularities in the arrangement of constituent particles around a point or an atom in a crystalline substance.
• Line defects − Irregularities in the arrangement of constituent particles in entire rows of lattice points.

• These irregularities are called crystal defects.

Types of Point Defects

• Three types:

• Stoichiometric defects
• Impurity defect
• Non-stoichiometric defects

Stoichiometric Defects

• Do not disturb stoichiometry of the solid
• Also called intrinsic or thermodynamic defects
• Two types − (i) Vacancy defect

(ii) Interstitial defect

• Vacancy defect

• When some of the lattice sites are vacant
• Shown by non-ionic solids
• Created when a substance is heated
• Results in the decrease in density of the substance

• Interstitial defect

• Shown by non-ionic solids
• Created when some constituent particles (atoms or molecules) occupy an interstitial site of the crystal.

• Ionic solids show these two defects as Frenkel defect and Schottky defect.
• Frenkel defect

• Shown by ionic solids containing large differences in the sizes of ions
• Created when the smaller ion (usually cation) is dislocated from its normal site to an interstitial site
• Creates a vacancy defect as well as an interstitial defect
• Also known as dislocation defect
• Ionic solids such as AgCl, AgBr, AgI and ZnS show this type of defect.

• 
• Schottky defect

• Basically a vacancy defect shown by ionic solids
• An equal number of cations and anions are missing to maintain electrical neutrality
• Results in the decrease in the density of the substance
• Significant number of Schottky defect is present in ionic solids. For example, in NaCl, there are approximately 10⁶ Schottky pairs per cm³, at room temperature.
• Shown by ionic substances containing similar-sized cations and anions; for example, NaCl, KCl CsCl, AgBr

Impurity Defect

• Point defect due to the presence of foreign atoms
• For example, if molten NaCl containing a little amount of SrCl₂ is crystallised, some of the sites of Na⁺ ions are occupied by Sr²⁺ions. Each Sr²⁺ ion replaces two Na⁺ ions, occupying the site of one ion, leaving the other site vacant. The cationic vacancies thus produced are equal in number to those of Sr²⁺ ions.

• Solid solution of CdCl₂ and AgCl also shows this defect

Non-Stoichiometric Defects

• Result in non-stoichiometric ratio of the constituent elements
• Two types −

• Metal excess defect
• Metal deficiency defect

• Metal excess defect

• Metal excess defect due to anionic vacancies:

• Alkali metals like NaCl and KCl show this type of defect.

• When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl⁻ ions diffuse from the crystal to its surface and combine with Na atoms, forming NaCl. During this process, the Na atoms on the surface of the crystal lose electrons. These released electrons diffuse into the crystal and occupy the vacant anionic sites, creating F-centres.
• When the ionic sites of a crystal are occupied by unpaired electrons, the ionic sites are called F-centres.

• Metal excess defect due to the presence of extra cations at interstitial sites:

• When white zinc oxide is heated, it loses oxygen and turns yellow.

Then, zinc becomes excess in the crystal, leading the formula of the oxide to. The excess Zn²⁺ ions move to the interstitial sites, and the electrons move to the neighbouring interstitial sites.

• Metal deficiency defect

• Arises when a solid contains lesser number of cations compared to the stoichiometric proportion.
• For example, FeO is mostly found with a composition of. In crystals of FeO, some Fe²⁺ ions are missing, and the loss of positive charge is made up by the presence of the required number of Fe³⁺ ions.