The term "amorphous" is itself amorphous and open to argument. The same is true of the word "crystalline". A truly amorphous solid would be one that is a frozen gas: it does not exist as far as I can tell. So instead one thinks of a frozen liquid. That means that the solid consists of short-range order and no long-range order. The diffraction pattern is then very diffuse with not much features: no sharp peaks (of course we can argue about what sharp means too).

At the other end of the spectrum, there is no such thing as a purely crystalline material since this would imply 100% long-range order: this is unachievable in practice because the atoms are in thermal motion. The diffraction pattern consists of sharp peaks, low background (but not zero background because of the presence of thermal motion).  I am assuming here of course that we have eliminated all the other causes of background scattering that I mentioned earlier.


So we have in practice a spectrum from "amorphous" to  "crystalline".

Now suppose we take a material whose diffraction pattern consists of sharp peaks plus some measurable background. It could be that we are dealing with a crystalline material mixed with an "amorphous" material. It could also mean that we have a crystalline material that has significant short-range order (high thermal scattering or local breakdowns in molecular arrangements to give diffuse scattering). In the "crystallinity" literature these two cases have been described as 1 and 2 state models. 

In general the diffracted intensity consists of two terms:


                    I(obs)   = I(Bragg)  + I(diffuse).

Take another example. We see a diffraction pattern that has broad diffuse background and broad diffuse peaks. Amorphous or crystalline or something in between? How can one say how "crystalline" this is?

It is for these (and other reasons) that I am highly sceptical about the notion of crystallinity (I have seen in some cases the term "crystallinity index" used). So while one can go through the various methods to separate amorphous from crystalline in certain cases where this might be thought appropriate, using say whole pattern fitting methods, the true error in such a determination remains suspect and probably much larger than the fitting procedure would suggest. At best such an estimation can only be a rough guide.

______________________________________________
XRD mailing list
[log in to unmask]
http://www.jiscmail.ac.uk/lists/xrd.html
To leave the list use the webpage or send an email to:
[log in to unmask] with the text
SIGNOFF XRD