METALLIC CRYSTAL STRUCTURES

OBJECTIVE:

In this experiment you will build models for three common metallic lattice structures. Then you will examine the relationships between structure and properties for metals with these types of lattices.

Before moving on to the next structure, the coordination number must be determined:

Coordination number is defined as the number of nearest neighbors, (spheres in direct contact) for an atom buried within the crystal. Chose a sphere in the 2nd layer and count the number of spheres in direct contact with it in all 3 layers and record.

PROCEDURE:

1.  BODY-CENTRED CUBIC PACKING (bcc)

Use toothpicks to attach the styrofoam 5 cm spheres spheres to each other. Build the model layer by layer first. Then fasten the layers together being careful to fit the spheres of one layer into the depressions of the next layer.

1st layer 2nd layer 3rd layer

Coordination number: BODY-CENTRED CUBIC __8___

2. CLOSEST PACKING ( 2 types)

Use the 3 following pre-made layers:

1st layer 2nd layer 3rd layer

In addition to coordination number, examine the structure for layers of tetrahedral holes and octahedral holes created at the interface of two layers:

tetrahedral holes octahedral holes

and slippages planes: non-slippage planes:

a) HEXAGONAL CLOSEST PACKING (hcp)
Place the 2nd layer over the 1st such that the middle red sphere is in the center of the 3 blue spheres: then the 3rd layer is directly above the 1st layer:

Coordination number: HEXAGONAL __12___

Layers of Tetrahedral holes: HEXAGONAL __ 2___

Layers of Octahedral holes: HEXAGONAL __ 2___

Slippage planes: HEXAGONAL __ 1___

Also, one unique aspect of Hexagonal packing is that when looking down through the 3rd layer to the 1st, holes exist straight through to the bottom.

b) FACE-CENTRED CUBIC CLOSEST or CUBIC CLOSEST PACKING (fcp or ccp)

Place the 2nd layer over the 1st such that the middle red sphere is in the center of the 3 blue spheres: then the 3rd layer is such that the yellow triangle is over the middle red sphere, but the yellow triangle is in the opposite direction as the bottom blue triangle:

Coordination number: FACE __12___

Layers of Tetrahedral holes: FACE __ 1___

Layers of Octahedral holes: FACE __ 3___

Slippage planes: FACE __ 3___

Also, Face packing covers all holes when viewed from above.

Finally, rotate the face structure until you can find the “face” unit cell:

Last Revised 9/1/2010