# Bravais lattice

by aaaa202
Tags: bravais, lattice
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 P: 1,005 My book has an awful lot of text about these special lattices where everything looks the same from every lattice point. But why are these lattices so important? I mean, surely in some crystals the atomic arrangement must be such that the crystal lattice is not a bravais lattice? Edit: maybe you could also point me to a derivation of the 14 different bravais lattices as I cant really see intuitively why these exhaust all space symmetry possibilities.
 P: 662 The only crystals that do not form in one of the 14 Bravais lattices are Quasicrystals, which are non-periodic. All other crystals, from simple rock salt and silicon to protein crystals with huge unit cells for in Bravais lattices with repeating unit cells. Real crystals are of course finite and have imperfections, but the important properties are derived from the periodicity and the unit cell - band structure (conductivity, semi-conducting behaviour, etc), magnetism, birefringence, ... everything. That's why it is a big deal. Ashcroft and Mermin has a pretty good discussion of crystal lattices.
 P: 1,005 But I mean, can't you draw a periodic lattice without it being a bravais lattice?
 P: 662 Bravais lattice No, you cannot. That is the whole point.
 P: 1,005 What about the honeycomb lattice? It is clearly periodic in some way, but it is not a bravais lattice. What exactly do you mean by periodic?
 P: 662 The honeycomb lattice is a triangular Bravais lattice with a two-point basis. http://www-personal.umich.edu/~sunka...honeycomb.html
 P: 1,005 So by two-point basis you mean it is a bravais lattice where we don't put an atom in the middel of the hexagons.
Sci Advisor
P: 3,593
 Quote by aaaa202 So by two-point basis you mean it is a bravais lattice where we don't put an atom in the middel of the hexagons.
Exactly, a crystal consists of a basis and a lattice. The lattice is defined only by all the vectors which correspond to translations of the crystal which leave the crystal invariant.

I think there are many books on group theory who show how and why there can only be 14 lattices.
 P: 316 Can you give one of them. For me is very interesting to see point groups. There is 32 point groups. When I see rotation of 1,2,3,4,6 order. Is this 5/32 point groups? And what is liquid crystal? Do they form some crystal lattice?
 P: 662 Liquid crystals are not crystals in the sense that they do not have a regular periodic lattice. In most cases they consist of long molecules that have orientational order (i.e. they point into a defined direction), but no translational order. http://en.wikipedia.org/wiki/Liquid_crystal The five groups you mention are all crystallographic point groups. Trivially, you also have to consider the group that contains only the identity. Additional symmetries that can be added to form the remaining groups are space inversion and 2-fold axes perpendicular to the "main" axis. For the cubic point groups, you combine 3-fold axis about the body diagonals of the cube with 2- or 4-fold axes about the faces, and sometimes 2-fold axes about the face diagonal and/or space inversion. http://en.wikipedia.org/wiki/Crystal...ic_point_group
 P: 316 Thanks. One more question. What is reflection group?
 P: 662 Sorry, no clue. The Wiki page is not all that helpful...

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