[AgG]

does a substance have to be 100% crystalline in order to perform an X-ray crystal structure determination?  What if there are two compounds in your crystal? and lastly, does anyone know of any single crystalline polymers?

[sdekivit]

2 compunds can be analyzed with X-ray diffraction, think about the interaction between an enzyme bound it's ligand. The interaction between these two molecules can be examined with this technique.

[AgG]

an interesting point sdekivit and not one that I had originally aniticipated.  You are correct, of course, but I was talking more along the lines of an impurity. So, two compounds which crystallize together to form a lattice.

[AWK]

Substance that can be examined by X-ray should be crystalline, but it does mean 100 %. There are many substances that are disordered, and its structure can be solved. At the moment I am doinh with substancet that show 25 % of disorder (measured as scattering power for X-ray) and they be solved without any trouble
(but their refinemet is horrible).
Crystalline polymers are always disordered in some extent