Can single crystal X-ray diffraction determine a structure uniquely?

TL;DR

This paper investigates whether single crystal X-ray diffraction can uniquely determine crystal structures, revealing that different structures can produce identical diffraction patterns, thus challenging a common assumption in crystallography.

Contribution

The study demonstrates that certain structures can have identical diffraction patterns, and develops a computer program to identify such cases in a large database.

Findings

Approximately 1000 structures found with sister structures sharing the same diffraction patterns.

Sister structures can differ in space group, topology, and crystal system.

Structures with matching diffraction data may still be incorrect.

Abstract

The diffraction technique is widely used in the determination of crystal structures and is one of the bases for the modern science and technology. All related structure determination methods are based on the assumption that perfect single crystal X-ray diffraction (SXRD) can determine a structure uniquely. But as the structure factor phases are lost in SXRD and even more information is lost in powder X-ray diffraction (PXRD), this assumption is still questionable. In this work, we found that structures with certain characteristic can have its sister structure with exactly the same PXRD or even SXRD pattern. A computer program is developed to search the ICSD database, and about 1000 structures were identified to have this characteristic. The original structure and its sister structures can have different space groups, topologies, crystal systems etc. and some may even have multiple sisters. Our studies indicate that special caution is needed since a structure with reasonable atomic positions and perfect match of experimental diffraction intensities could still be wrong.