Chapter 4 Imperfections in Solids Session I

Point Defects The simplest defects are point defects. e.g. a vacancy due to a missing atom. Results from imperfect packing during crystallization or thermal vibrations at high temperatures, or 2 vacancies combine to form a divacancy or 3 to form tri-vacancy In ceramics vacancies occur in such a way that electro-neutrality must be maintained. i.e. whenever a positive ion is missing, (-ve) ions with a corresponding charge must be missing too. The absence of a positive or (-ve) ions from a crystal is called Schottky Defect. When an extra atom is lodged in the crystal in a position that does not belong to the crystal lattice, this defect is called Interstitialcy. When an ion leaves its normal place and lodges itself into interstitial site, the compound defect is known as Frenkel defect (Vacancy + interstitialcy)

Point Defects. (a) Vacancy, D. (b) Di-vacancy (two missing atoms) Point Defects. (a) Vacancy, D. (b) Di-vacancy (two missing atoms). (c) Ion-pair vacancy (Schottky defect). (d) Interstitialcy. (e) Displaced ion (Frenkel defect).