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.
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.