Imperfection in Solids

Imperfection in Solids

Stochiometric Defects

Non- Stoichiometric

Schotkky Defects In in ionic crystal of the type A+ B-, equal number of anions and cations are missing from the lattice sites so that electrical neutrality is maintained it is called schottky defect. Types of compounds exhibiting Schottky Defect  This type of defect is shown by highly ionic compounds which have  High coordination number  Cations and anions of similar sizes

Consequences of Schottky Defect As the number of ions decrease, volume is the same, so density decreases.  Crystals with Schottky defect conduct electricity to a small extent.  Due to the presence of holes the stability of the crystal decreases.

Frenkel Defect If an ion is missing from its lattice site (causing a vacancy or a hole there) and it occupies the interstitial site so that electrical neutrality as well as stoichiometry of the compound are maintained. This type of defect is called Frenkel defect.  

Types of compounds exhibiting Frenkel defect This type of defect is present in those compounds which have  Low co-ordination number.  Large difference in size of anion and cation.  Examples: AgCl, AgBr, AgI and ZnS. 

Consequences Solids with this defect conduct electricity to a small extent  The dielectric constant of the crystal increases  The density of the solid is unchanged  Due to the presence of holes, the stability of the crystal decreases

Interstitial Defects Atoms or ions, which occupy normally vacant interstitial positions in a crystal, are called interstitials. The important factor determining the formation of interstitials is the size of the atom/ion, because they are accomodated in the voids. 

Non-Stoichiometric Defects If an imperfection causes the ratio of cations to anions to become different from that indicated by the ideal chemical formula, the defect is called non-stoichiometric.

Metal Deficiency due to anion vacancies A negative ion may be missing from its lattice site, leaving a hole, which is occupied by an electron thereby maintaining an electrical balance. The trapped electrons are called F-centers or color centers because they are responsible for imparting color to the crystal. 

By presence of extra cations in the interstitial sites Extra cations occupying interstitial sites with electrons present in another interstitial site to maintain electrical neutrality can cause metal excess. This defect is similar to Frenkel defect and is formed in crystals having Frenkel defects. Example: If ZnO is heated, it loses oxygen and turns yellow. 

Metal Deficiency due to cation vacancies In case of ionic solids, the impurities are introduced by adding impurity of ions. If the impurity ions are in a different oxidation state from that of the host ions, vacancies are created.  For e.g., If molten NaCl, containing a little of SrCl2 as impurity is allowed to cool, in the crystals of NaCl formed, at some lattice sites Na+ ions are substituted by Sr2+ ion. For every Sr2+ thus introduced two Na+ ions are removed to maintain electrical neutrality.