Twinned Crystals

Single crystal structure analysis is a method that involves irradiating a single crystal sample with X-rays or electron beams and analyzing the resulting diffraction data to determine the three-dimensional arrangement of atoms and molecules within the single crystal, as well as the intermolecular interactions. To use this method, it is first necessary to produce a single crystal of ideal quality.

However, during the process of growing a crystal, the crystal may grow not as a single crystal but as a "twin crystal," where two crystals of the same type are joined together. If this fact goes unnoticed and the measurement is performed, the need arises to analyze the data obtained from a twin crystal. 

Twins are classified by how domains join. True twins—where crystals are joined according to specific crystallographic orientations—are handled differently from mere polycrystals; i.e., agglomerations without a specific relationship

True twins are further divided into: merohedral twins, where reciprocal-lattice points from different domains coincide entirely, and non-merohedral twins, where there is only partial overlap of reciprocal-lattice points. 

When it was determined that a sample had grown as a twin, researchers sometimes tried different crystallization conditions to obtain high-quality single crystals. However, crystallization takes considerable time, creating a significant barrier in traditional single crystal structure analysis methods, and in certain cases, obtaining an untwinned crystal is not possible.

Traditionally, handling twins required the use of advanced techniques by experts. Recently, however, advances in software have made it possible to analyze data obtained from twin crystals in the same way as data from single crystals, allowing for highly accurate results. This technological innovation has made it possible to shorten the time required for research and development and remove the technical barriers previously necessary for the analysis.