2
$\begingroup$

What is the difference between Hexagonal Close Packing (HCP) structures and the hexagonal crystal Bravais lattice? I've looked up some sites but can't seem to understand them. It would help if you backed up the answer with your specific explanation of Bravais lattices and unit cells.

Edit: I thought the difference was that the lattice consisted of many unit cells and the unit cell packing could be like hexagonal, cubic or simple. But then I looked at the stoichiometric defects which talked about molecules being in interstitial spaces. But in the unit cell, the spaces are already filled. So how is that possible.

$\endgroup$
2
$\begingroup$

Here are some important definitions:

A space lattice provides the framework with reference to which a crystal structure can be described. A lattice is different from crystal. In fact, a lattice gives rise to crystal when lattice points are replaced by atoms, ions, or molecules.

A 2-D or 3-D lattice is a regular arrangements of points. In order to specify it completely, only a small part of the lattice is described. This figure is known as a unit cell.

In 2-D, there are 5 possible lattices namely, square, rectangle, hexagonal, parallelogram and rhombic.

In 3-D, there are 14 possible lattices, and these lattices are called Bravais lattices (after the French mathematician who first described them) like cubic primitive, hexagonal primitve, etc.

For example, In a cubic system there are 3 possible Bravais lattices possible namely, primitive, body centered and face centered.

Similarly in hexagonal crystal system there is only one Bravais lattice viz, Primitive.

Crystallographers have been able to divide 32 point groups and 14 space lattices into seven crystal systems and 14 Bravais lattices. Remember that the primitive cells of any two crystals are not the same. Primitive unit cells mean that the lattice point must be at the corners of the unit cell.

enter image description here

HCP is ABA-ABA arrangement of layers in which tetrahedral void of second layer are covered by the third layer.

$\endgroup$
  • $\begingroup$ So each lattice site doesn't represent a site that could be replaced by a unit cell but actually by the constituents(atoms/ions) of the unit cell? $\endgroup$ – LeroyJD Feb 18 '17 at 6:52
  • 1
    $\begingroup$ Yes, Unit cell contains many lattice points in/on them and the constituents are placed at those lattice points. Lattice is like a framework, a blueprint but without the constituents. A crystal is the building defined by that blueprint but with the constituents at those lattice points.. $\endgroup$ – Mitchell Feb 18 '17 at 7:16
  • $\begingroup$ Also, what do you mean by point groups? $\endgroup$ – LeroyJD Feb 18 '17 at 9:31
  • 1
    $\begingroup$ Search Google for crystallographic point group..You'll find your answer there.. $\endgroup$ – Mitchell Feb 18 '17 at 9:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.