THE MOST IMPORTANTCRYSTAL STRUCTURES

•■Sodium Chloride Structure Na+Cl-

•■Cesium Chloride Structure Cs+Cl-

•■ Hexagonal Closed-Packed Structure•■ Diamond Structure

•■ Zinc Blende

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1 – Sodium Chloride Structure

•■ Sodium chloride alsocrystallizes in a cubic lattice,but with a different unit cell.

•■ Sodium chloride structureconsists of equal numbers ofsodium and chlorine ionsplaced at alternate points of asimple cubic lattice.

•■ Each ion has six of the otherkind of ions as its nearestneighbours.

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Sodium Chloride Structure

•If we take the NaCl unit cell and remove all the red Clions, we are left with only the blue Na. If we comparethis with the fcc / ccp unit cell, it is clear that they areidentical. Thus, the Na is in a fcc sublattice.

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Sodium Chloride Structure

•■ This structure can be considered as a face-centered-cubic Bravais lattice with a basis consistingof a sodium ion at 0 and a chlorine ion at the centerof the conventional cell,

•■ LiF,NaBr,KCl,LiI,etc•■ The lattice constants are in the order of 4-7

angstroms.

)(2/

zyxa

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2-Cesium Chloride Structure Cs+Cl-

•■ Cesium chloride crystallizes in acubic lattice. The unit cell may bedepicted as shown. (Cs+ is teal,Cl- is gold).

•■Cesium chloride consists of equalnumbers of cesium and chlorineions, placed at the points of abody-centered cubic lattice so thateach ion has eight of the other kindas its nearest neighbors.

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Cesium Chloride Structure Cs+Cl-

•The translational symmetry of thisstructure is that of the simple cubicBravais lattice, and is described as asimple cubic lattice with a basisconsisting of a cesium ion at the origin 0and a chlorine ion at the cube center

•CsBr,CsI crystallize in this structure.Thelattice constants are in the order of 4angstroms.

)(2/

zyxa

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8 cell

Cesium Chloride Cs+Cl-

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3–Hexagonal Close-PackedStracture

•■ This is another structure thatis common, particularly inmetals. In addition to the twolayers of atoms which form thebase and the upper face of thehexagonal, there is also anintervening layer of atomsarranged such that each of theseatoms rest over a depressionbetween three atoms in thebase.

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Bravais Lattice : Hexagonal LatticeHe, Be, Mg, Hf, Re (Group II elements)ABABAB Type of Stacking

Hexagonal Close-packed Structure

a=b B زاویة =120, c=1.633a,basis : (0,0,0) (2/3a ,1/3a,1/2c)

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A A

AA

AA

A

AAA

AA

AAA

AAA

B B

B

B

B B

B

B

B

BB

C C C

CC

C

C

C C C

Sequence ABABAB..-hexagonal close pack

Sequence ABCABCAB..-face centered cubic close pack

Close pack

BAA

AA

A

A

A

A A

B

B B

Sequence AAAA…- simple cubic

Sequence ABAB…- body centered cubic

Packing

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Crystal Structure 12101

4 - Diamond Structure

•■ The diamond lattice is consist of two interpenetrating facecentered bravais lattices.

•■ There are eight atom in the structure of diamond.•■ Each atom bonds covalently to 4 others equally spread

about atom in 3d.

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4 - Diamond Structure

•■ The coordination number of diamondstructure is 4.

•■ The diamond lattice is not a Bravaislattice.

•■ Si, Ge and C crystallizes in diamondstructure.

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Diamond

5- Zinc Blende

•■Zincblende has equal numbers of zincand sulfur ions distributed on a diamondlattice so that each has four of theopposite kind as nearest neighbors. Thisstructure is an example of a lattice with abasis, which must so described bothbecause of the geometrical position of theions and because two types of ions occur.

•■ AgI,GaAs,GaSb,InAs,

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5- Zinc Blende

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5- Zinc Blende

Zinc Blende is the name given to the mineral ZnS. It has a cubicclose packed (face centred) array of S and the Zn(II) sit intetrahedral (1/2 occupied) sites in the lattice.

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