sir in types of unit cell we studied it can be of 4 types i.e primitive, face centred, body centred and end centred, but while studing 3D close packing it was told that it can be of 3 types i,e. simple(primitive) cubic unit cell, hcp and ccp. but i am not able  to understand that hcp is a type of unit cell or it is one of the 7 crystal system or somthing else?and if it is type of unit cell then why it is not taught under the topic types of unit cell. and if it is a crystal system then what about the rest 6 crystal system, then why rest 6 crystal system does not comes under 3 dimensional close packing?

Asked by rohitraman1115 | 10th Sep, 2019, 07:10: PM

Expert Answer:

hcp is not a type of unit cell. It is a closed packing arrangement in three dimensions. Crystal systems are different possible geometrical shapes of the crystal lattice. These two are different things. Hexagonal closed packing(hcp) is a pattern, not a crystal system. 

Answered by Ravi | 12th Sep, 2019, 11:56: AM