# Pleaze help me

1. Sep 30, 2007

### hayat

I have a test after a few days and I face agreat deal with some concepts.

Really I dont understand the crystal structure?
how we know the Bravias lattice and how we get the primitive vectors and how we know
the cells that make the lattice Bravias if it not?

I want very clear description for everypoint otherwise I will fail on the test.

2. Sep 30, 2007

### malawi_glenn

Last edited by a moderator: May 3, 2017
3. Sep 30, 2007

### fasterthanjoao

hayat, I'm sure you know people on here are always around to help - but we need more informatioN! If it's a 'few days till test' and you don't know anything on the course, then there probably isn't much we can say in terms of tutorials. Essentially, it isn't reasonable to ask for a full explanation of an entire course just because you'll fail otherwise.

It's much better if there are clear, pinpointed, issues you want clearing up with. If you first of all make a list of things you're really having problems with, and then post that you will get a much better, focussed, response. Just want to make sure that you make the most of the forum's collective intelligence with the few days to go,

good luck.

4. Sep 30, 2007

### malawi_glenn

Yes, and we cant just write 100pages for you to read, we are students too, and we have limited time also. But we are many here, so if you ask some specific questions and what you are thinking about them and show work done, then you can be pointed in correct direction and get hints and specified answers

5. Oct 1, 2007

### hayat

i mean i want the best way to understand the crystal structure. I have about 5 books to study from and alot of notes also but i dont know why i cant understand the crystal structure?

in few words i want an idea or a general overview how to understand the crystal structure.

6. Oct 1, 2007

### malawi_glenn

Lattice = Points in 3-d space. If all points can be reached by a cerntain vector, then that vector is a primitive lattice vetor. And you have three of those. In your books, this should be covered, and also nice figures. Do the comparison with basis vectors in coordinate system. The lattice points are arranged by translation with the primitive lattice vectors. Then you also have rotation and mirroring, so that you are left with a certain number of lattices that are invariant under rotation and mirroring; and these lattices are called bravais lattices (but some of the baravais lattacies can be reduced even further, but some; for example bcc; is regarded as a bravais lattice since many real crystals have this lattice.

Then you have unit cells and primitive cells. A primitive cell contain ONE lattice point, and you can construct that in an infinte number of ways. Note; a lattice point should be regarded as a sphere, that can be divided by other primitive cells, but the sum of all this is equal to ONE in a primitive cell. See for example a simple cubic primitive cell with 8corners. Each corner is shared amog 8 other cells, so in one primitive cell you have 8*(1/8) = 1 lattive point. Then you have unit cells; for example the bcc cell; that is a cube with one lattice point in the middle ( at ½½½ coordinate).

The thing is to practice, try drawing each unit cell, and play with the vectors, and do excersice in finding basis and bravais lattices etc. Best (only) way to learn.

Then you have the basis, I leave that subject to another volunteer =)

7. Oct 2, 2007

### hayat

thanks for everyone try to help me.
Really i have learned alot and wish for me agood luck beacuse the test will be on wednesday.

Thanks again