1. Imperfections in Solids
Perfect crystals do not exist; even the best crystals have 1ppb defects.
Defects are imperfections in the regular repeating pattern and may be classified in terms of their dimensionality (Point vs. Extended).
Classification is given by;
1. Point Defects
2. Line Defects
Point Defects
The irregularities or deviations from ideal arrangement around a point or an
atom in a crystalline substance.
Line defects 
The irregularities or deviations from ideal arrangement in entire rows of lattice points.

2. 1. Point Defects Point defects can be classified into three types :
(i)   Stoichiometric Defects (E.g. Vacancy Defect, Interstitial Defect)
(ii)  Impurity Defects
(iii) Non-Stoichiometric Defects (E.g. Metal Excess Defect, Metal Deficiency Defect )
(i) Stoichiometric Defects
a) Vacancy Defect
given a perfect crystal (e.g. of Cu), an atom can be placed on the outside of the cell to produce a vacancy (≡ □); remember atom migration.
e.g. TiO has 1:1 stoichiometry and NaCl structure, but has 15% vacancies on the Ti sites and 15% vacancies on the O sites. Both sets of vacancies are disordered.

3. b) Interstitial Defect When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have interstitial defect. This defect increases the density of the substance.

4. *Ionic Crystals shows the following Defects
in pure metal compounds, don’t need to worry about electroneutrality.
in an ionic crystal, the interior and surface must remain ≈ neutral.
c) Schottky Defect
take anions and cations and place them on surface in equal numbers.
stoichiometric effect: equal numbers of anion and cation vacancies.
may be randomly distributed, but tend to cluster because of- oppositely charged vacancies.
most important with alkali halides.
at room temp, ~1 in 1015 pairs vacant in NaCl,
so 1mg sample has ~10,000 Schottky Defects.
This defect causes the decrease in density of-
the substance

5. Ag+ surrounded by 4Cl- stabilizes this defect.
b) Frenkel Defect
take cation out of position and cram it into an interstitial site (void between normal atomic position).
Ag+ surrounded by 4Cl- stabilizes this defect.
tendency for vacancy and interstitial to form nearby pair.
also a stoichiometric deffect (vacancies = interstitials).

6. (ii)Impurity Defect Due to Impurities and Doping
Impurities are elements present in the material that are different from those in the compound formula.
Dopants are intentionally added impurities (to make alloys or affect changes in properties). Alloy formation most likely when dopant anions and cations are close in size to original material.
Isovalent dopants: substitution species have the same charge.
NaCl:AgCl → Na1-xAgxCl (alloy on cation site)
AgBr:AgCl → AgBr1-xClx (alloy on anion site)
Aleovalent dopant: substitution species has different charge.
NaCl:SrCl2 could be either Na1-2xSrx□xCl or Na1-xSrxClClxi
either could happen; experimentally, first is found.

7. Figure shows the impurity defect in NaCl by Sr2+ ion

8. *Additional Information about The Impurity Defect
Due to Vacancy:
Missing atom from an atomic site
Atoms around the vacancy displaced
Tensile stress field produced in the vicinity
Tensile Stress Fields ?

9. Due to Interstitial Atom:
Relative size
Interstitial
Compressive Stress Fields
Impurity
Substitutional
Compressive stress fields
SUBSTITUTIONAL IMPURITY  Foreign atom replacing the parent atom in the crystal  E.g. Cu sitting in the lattice site of FCC-Ni
INTERSTITIAL IMPURITY  Foreign atom sitting in the void of a crystal  E.g. C sitting in the octahedral void in HT FCC-Fe
Tensile Stress Fields