solid state structure of nacl structure of cscl defects in solids
TRANSCRIPT
DEFECTSIN
SOLIDS
STRUCTUREOF
CsCl
STRUCTUREOF
NaCl
SOLIDSTATE
UNIT CELL OF NaCl OR SPACE
LATTICE OF CsCl
HORIZONTAL VIEW
VERTICAL VIEW
FRONT VIEW OF UNIT CELL OF NaCl
3 – D VIEW OF UNIT CELL OF NaCl
CS+
Cl-
_
chloride
cesium
Unit cell of CsCl Space lattice of CsCl
IMPERFECTION IN SOLIDS
POINT DEFECTS ELECTRONIC DEFECTS
IMPURITYNON STOCHIOMETRICSTOCHIOMETRIC
FRENKELSCHOTTKY
METAL DEFICIENTMETAL EXCESS
EXTRA CATIONANION VACANCY
IONIC SOLIDSCOVALENT SOLIDS
ANY DEPARTURE FROM PERFECTLY ORDERED ARRANGEMENT OF ATOMS IN CRYSTAL IS CALLED IMPERFECTION OR DEFECT.
A+B_
A+A+
A+
A+A+
A+A+
B_
B_
B_
B_
B_
B_
B_
A+B_
A+A+
A+
A+A+
A+A+
B_
B_
B_
B_
B_
B_
B_
X
PERFECT CRYSTAL IMPERFECT CRYSTAL
IMPURITY DEFECT
NON-STOICHIOMETRICSTOICHIOMETRIC
IF IMPERFECTIONS IN THE CRYSTAL ARE SUCH THAT THE RATIO BETWEEN THE CATIONS AND ANIONS REMAINS THE SAME AS REPRESENTED BY THE MOLECULAR FORMULA,THE DEFECTS ARE CALLED STOICHIOMETRIC DEFECTS.
SCHOTTKY DEFECTS
FRENKEL DEFECTS
A+B
_
A+A+
A+
A+A+
A+A+
B_
B_
B_
B_
B_
B_
B_
A+B
_
A+A+
A+
A+A+
A+A+
B_
B_
B_
B_
B_
B_
B_
SCHOTTKY DEFECT FRENKEL DEFECT
IT IS DUE TO EQUAL NUMBER OF CATIONS AND ANIONS MISSING FROM THE LATTICE SITE.
IT IS DUE TO MISSING OF IONS(CATIONS) FROM LATTTICE SITES AND THESE OCCUPY INTERSTITIAL SITES.
DECREASE IN DENSITY OF THE CRYSTAL.
NO EFFECT ON THE DENSITY OF THE CRYSTAL.
FOUND IN HIGHLY IONIC COMPOUNDS WITH HIGH CO-ORDINATION NUMBER AND HAVING CATIONS AND ANIONS OF SIMILAR SIZES.
e.g.-NaCl, KCl, CsCl
FOUND IN CRYSTALS WITH LOW COORDINATION NO. AND WHERE THE DIFFERENCE IN THE SIZE OF CATIONS AND ANIONS IS VERY LARGE.
e.g.-ZnS, AgCl, AgI .
IF AS A RESULT OF THE IMPERFECTIONS IN THE CRYSTAL THE RATIO OF THE CATIONS TO THE ANION BECOMES DIFFERENT FROM THAT INDICATED BY THE IDEAL CHEMICAL FORMULA,THE DEFECTS ARE CALLED NON-STOICIOMETRIC DEFECTS.
(A)METAL EXCESS (A)METAL EXCESS
(B)METAL DEFICIENCY(B)METAL DEFICIENCY
BY ANION VACANCY
A+B
_
A+A+
A+
A+A+
A+A+
B_
B_
B_
B_
B_
B_
B_
A A+ +
Na Na+ + e_
e_
F-CENTRE
A+B
_
A+A+
A+
A+A+
A+A+
B_
B_
B_
B_
B_
B_
B_
DUE TO EXTRA CATION IN THE INTERSTISTIAL SPACE
ZnO Zn++ + ½ O2 + 2e_
METAL EXCESS MAY ALSO BE CAUSED BY AN EXTRA CATION OCCUPYING THE INTERSTITIAL SITE.THE ELECTRICAL ELECTRON PRESENT IN ANOTHER INTERSTITIAL SITE.
BY ANION VACANCIES BY PRESENCE OF EXTRA CATIONS IN THE INTERSTITIAL SITES
A NEGATIVE ION MAY BE MISSING FROM ITS LATTICE SITE,LEAVING A HOLE WHICH IS OCCUPUIED BY AN ELECTRON THEREBY MAINTAINING ELECTRICAL BALANCE.
METAL EXCESS MAY ALSO BE CAUSED BY AN EXTRA CATION OCCUPYING THE INTERSTITIAL SITE.THE ELECTRICAL NEUTRALITY IS MAITAINED BY AN ELECTRON PRESENT IN ANOTHER INTERSTITIAL SITE.
A++A+
A+B
_
A+A+
A+
A+
A+A+
B_
B_
B_
B_
B_
B_
B_
CATION VACANCY
METAL ACQUIRING HIGHER CHARGE
DUE TO CATION VACANCY
Fe+++
Fe+++
Fe++ S_ _
Fe++Fe++
Fe++ Fe++
Fe++
Fe++ S_ _
S_ _
S_ _
S_ _
S_ _
S_ _
S_ _
Fe++
METAL ACQUIRING HIGHER CHARGE
CATION VACANCY
DUE TO CATION VACANCY
A++A+
A+B
_
A+A+
A+
A+
A+A+
B_
B_
B_
B_
B_
B_
B_
B_
METAL ACQUIRING HIGHER CHARGE
ANION OCCUPYIG INTERSTITIAL SITE
DUE TO PRESENCE OF EXTRA ANION
THESE DEFECTS ARISE WHEN FOREIGN ATOMS ARE PRESENT AT THE LATTICE SITE(IN PLACE OF HOST ATOMS) OR AT THE VACANT INTERSTITIAL SITES.
(A) INTRODUCING IMPURITY DEFECT IN COVALENT SOLIDS
(B) INTRODUCING IMPURITY DEFECT IN IONIC SOLIDS
These defects arise when foreign atoms are present at the lattice site (in place of host atoms, forming substitutional solid solids) or at the vacant interstitial sites (forming interstitial solid solutions).This is used in increasing the conductivity of semiconductors.
The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors. Pentavalent impuritiesImpurity atomw with 5 valence electrons produce n-type semiconductors by contributing extra electrons.
The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).
The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes". It is typical to use B2H6 diborane gas to diffuse boron into the silicon material.
These defect in ionic crystal arise due to the presence of some impurity ions at the lattice sites (in place of host ions) or at the vacant interstitial sites. If molten NaCl containing a little amount of SrCl2 is allowed to crystallise,some of the sites of Na+ ions are occupied by Sr++ ions
If NaCl is doped with 10 -3 mol % of SrCl2 , what is the concentration of cation vacancy ?
Sr++ Na+
Na+Na+
Na+
Na+Na+
Cl_
Cl_
Cl_
Cl_
Cl_
Cl_
Cl_Cl
_
CATION VACANCY