Answer to Question #120691 in Inorganic Chemistry for Yuki

Question

Metal structures commonly adopt one of three common structure types, ccp, hcp and bcc. What are the differences and similarities between these structure types?

Answer 1

The ccp structure type has a coordination number of 12 and contains 4 atoms per unit cell. The hcp structure type has a coordination number of 12 and contains 6 atoms per unit cell. The bcc structure type has a coordination number of 8 and contains 2 atoms per unit cell. The bcc structure type is less closely packed than the ccp and hcp structures types.

Although bcc materials have higher number of slip systems than ccp, the slip planes are not as closely packed as in ccp. Therefore, ccp materials tend to deform more easily and are more ductile than bcc. The bcc materials don't deform easily and are stronger than ccp materials. The main difference between hcp and ccp is that the repeating structure of hcp has 2 layers of spheres whereas the repeating structure of ccp has 3 layers of spheres.

The density of hcp and ccp is the same. The bcc structure is slightly less closely packed than ccp or hcp and is often the high temperature form of metals that are close-packed at lower temperatures. Metals which are bcc are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure.