MS213 Crystallography and Diffraction

09/11/2013

Crystal Structure: Cubic System (BCC, FCC)

Soon Ho KwonComputational Materials Science lab. KAISThonest@kaist.ac.kr

A crystal or crystalline solid is a solid material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.

Cf.) An "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms.

Nature 432 488 (2004)

What is crystal?

The crystal system is a grouping of crystal structures that are categorized according to the axial system used to describe their "lattice". A lattice is a set of points, regularly arranged in space, for which the environment of each point is identical.

http://www.materials.ac.uk/elearning/matter/Crystallography/3dCrystallography/

a

b

c

αβ

γ

7 crystal sys-tems

latticecrystal

4 Types of Unit CellP = Primitive I = Body-centeredF = Face-centeredC = Side-centered+ 7 Crystal System

→ 14 Bravais Lattice

4 Types of Unit CellP = Primitive I = Body-centeredF = Face-centeredC = Side-centered+ 7 Crystal System

→ 14 Bravais Lattice

14 Bravais lat-tice

Cubic system: a=b=c and α=β=γ=90°

Simple cubic: P, CN=6Body Centered Cubic (BCC): I, CN=8, APF = 68%Face Centered Cubic (FCC) : F, CN=12, APF = 74%

Simple cubic BCC FCC

Cubic system

http://www.youtube.com/watch?v=GEMw0CKV4qw

It can be described as two interpenetrating FCC structures, one at co-ordinates (0,0,0) and the other at (1/4,1/4,1/4).

Tetrahedral bonding !

Cf.) Zinc blend structure

It has a cubic close packed (face centred) array of A and the B sit in tetrahedral (1/2 occupied) sites in the lattice

Diamond struc-ture