  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.
