Lattice Defects in ionic solid compound.pptx

lattice of ionic solid

1. CRYSTAL DEFECTS Ideal crystalline solid is valid only at … Internal energy is minium Entropy contribution = 0 Influnces : Many physical properties such as optical absorption electrical conductivity mechanical strength

2. Imperfections or defects in crystalline solid can be broadly classified into four groups, namely, point defect, line defect, surface defect and volume defect . Point defect is considered as the zero dimensional (0-D) defect, as by mathematical definition, a point is unit-less dimensionless quantity. The point defect is the smallest possible defect in any material.

3. Definition of Point Defect : A point defect occurs when one or more atoms of a crystalline solid leave their original lattice site and/or foreign atoms occupy the interstitial position of the crystal. Occurence : : There exist three possibilities by which point defect may occur, as provided below : One or more original atoms of the crystal are missing from their corresponding lattice site and shifted to the interstitial position in same crystal. One or more foreign atoms occupied the interstitial position of the crystalline solid. One or more foreign atoms replaced the original atom of the crystal and subsequently occupied the interstitial position.

4. Features of Point Defect in Crystalline Solid : random or an arranged manner. Weight and density may increase, decrease or even remain constant; Point defects are inherent to the natural material, whose temperature is above absolute zero temperature (0K). However, perfect crystal can be obtained in scientific laboratories with tight control of environment. Point defect can affect the equilibrium of neighboring atoms and can put them under tension or compression. Point defects can change the physical and chemical properties of the solid, such as it can convert an insulator to electrically conductor one, it can increase the hardness of the material. Sometime point defects are intentionally created in the solid in a controlled way in order to modify the properties of the material for better functionality.

5. Causes of Point Defect in Crystalline Solid: Diffusion (solid-solid ). Increase or decrease in temperature in presence of other medium. Plastic deformation. Particle or ion bombarding, such as sputtering or ion irradiation. D eliberate alloying to enhance properties. Presence of residual stress.

6. Line defects : These defects are edge and screw dislocations. Edge dislocation:   It arises when an extra half plane of atoms “inserted” into the crystal lattice. Due to the edge dislocations metals possess high plasticity characteristics:  ductility  and malleability.

7. Edge Dislocation

8. Type of defects: 1. Stoichiometric defect In this kind of point defect, the ratio of positive and negative ions (Stoichiometric) and electrical neutrality of a solid is not disturbed. Sometimes it is also known as intrinsic or thermodynamic defects. Fundamentally , they are of two types : Vacancy defect:  When an atom is not present at their lattice sites, then that lattice site is vacant and it creates a vacancy defect. Due to this, the density of a substance decreases. Interstitial defect:  It is a defect in which an atom or molecule occupies the intermolecular spaces in crystals. In this defect, the density of the substance increases. Example : A non-ionic compound mainly shows vacancy and interstitial defects. An  ionic compound  shows the same in Frenkel and Schottky defect.

9. Frenkel Defect : In ionic solids generally, the smaller ion ( cation ) moves out of its place and occupies an intermolecular space. In this case, a vacancy defect is created on its original position and the interstitial defect is experienced at its new position. It is also known as dislocation defect. The density of a substance remains unchanged. It happens when there is a huge difference in the size of anions and cations . Example:   ZnS and  AgCl .

10. Schottky Defect This kind of vacancy defects is found in Ionic Solids. But in ionic compounds, we need to balance the electrical neutrality of the compound so an equal number of anions and cations will be missing from the compound. It reduces the density of the substance. In this, the size of cations and anions are of almost the same .

11. SCHOTTKY DEFECTS Cause: migration of a metal atom or ion to the surface in a binary ionic crystal A + B - ( Na + Cl - ) The energy absorbed during the creation is called schottky defect. Observed in the case of closed packed structure. Example: NaCl , MgO  

12. No. schottky defects depend on: The total no. of ion pair i.e. mass of the ionic crystal. The average energy required to produce schottky defects. temperature