Try Fox.
From the description:
> FOX is a program for the ab initio structure determination from powder diffraction (neutrons, X-Ray).
There are several others for xtallography processing and visualisation, that's just the first I could find.
I highly recommend that you also look at the Debian Science Blend \- lists of science and research related packages either already packaged for debian, in progress towards being packaged, and some not packaged for debian at all. The Chemistry and Nanoscale Physics sections will probably be of most interest to you, but there's also a lot of useful software in other sections.
Other distros will also have similar science package pages which will be worth looking at. Scientific Linux in particular.