Can anyone tell me formula for lattice $a$, $b$ and $c$ in a hexagonal structure?

Can anyone tell me formula for lattice constants $a$, $b$ and $c$ in a hexagonal structure? $a$ , $b$ and $c$ are units cell of structure. As we see in cubic structure we have a formula to calculate side $a$, $b$ and $c$ method known as braggs law. So what should be the method for calculating it in a hexagonal structure. I want to define the XRD (X-Ray diffraction) structure of my crystal.

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Hello! Please clarify your question . – ABC Mar 29 '13 at 15:00
more answer needed – ghanendra Mar 30 '13 at 7:18

I still don't understand what your question is about, but e.g. this paper contains everything about the real and reciprocal lattice of a hexagonal structure (on the first 1.5 pages).

"Unit cell of structure" is not a common term; I think you are referring to the lattice constants, of which graphene has only one, not three.

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thanks for your help. – ghanendra Mar 29 '13 at 15:08
If you think your question is answered, you should click the "accept" symbol to the left of this answer. – Rafael Reiter Mar 29 '13 at 15:09
no i have not got the answer. i will do it answered when i get a conclusion. – ghanendra Mar 29 '13 at 15:11