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

## TL;DR

The paper explores stacking variations in xanthine crystals using electron diffraction, revealing new structural insights and potential widespread nanoscale disorder in layered organic crystals.

## Contribution

The study identifies a new xanthine polymorph and visualizes nanoscale disorder using multi-dimensional electron diffraction.

## Key findings

- A new xanthine polymorph was identified as a polytype of Form I.
- Interfaces between twin and polytype domains were visualized.
- Planar disorder was observed through streaking in diffraction patterns.

## Abstract

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.

Layered crystal structures often exhibit twinning, planar disorder, or polytypes due to stacking variations. Here, the authors use multi-dimensional electron diffraction to uncover nanoscale disorder in xanthine, identifying a new polymorph and visualizing interfaces, suggesting such disorder may be prevalent in layered organic molecular crystals.