Polytypes and planar defects revealed in the purine base xanthine using multi-dimensional electron diffraction

Layered crystal structures are commonly found across organic and inorganic material systems. When in-plane atomic arrangement remains (nearly) identical, a stacking variation of these layers may result in twinning, planar disorder, or polytypes, a form of polymorphism derived from altering stacking sequences. In this work, we use multi-dimensional electron diffraction (ED) modalities to explore the microstructure of xanthine, an archetypal purine base with a layered crystal structure. Firstly, we identify and characterise the twin operator relating domains of Form I xanthine. We then solve the structure of a new xanthine polymorph, revealing that it is a polytype of Form I. Finally, interfaces between twin and polytype domains are visualised, whilst streaking in the diffraction patterns reveals the presence of planar disorder. Given these observations in the xanthine system, this work suggests that disorder on the nanoscale may be a commonly occurring phenomenon in layered organic molecular crystals.