Twinning

Maclage (Fr). Zwillingsbildung, Verzwillingung (Ge). Geminazione (It). Maclado (Sp). Двойники (Ru).

A twin consists of two or more single crystals of the same species but in different orientation, its twin components. They are intergrown in such a way that at least some of their lattice directions are parallel. The twin law describes the geometrical relation between the twin components. It specifies a symmetry operation, the twin operation, that brings one of the twin components into parallel orientation with the other. The corresponding symmetry element is called the twin element.[1]

Simple twinned crystals may be contact twins or penetration twins:

· Contact twins share a common surface and often look like mirror images across the boundary. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other.[2]

· In penetration twins the individual crystals are passing through each other in a symmetrical manner.[3]

· If several twin crystal parts are aligned by the same twin law, they are referred to as multiple or repeated twins, where the twin law is the set of twin operations mapping two individuals of a twin.

· Individuals or domains related by operations, such as reflection, inversion or rotation, form a twin called, respectively, reflection twin, inversion twin or rotation twin.[4]

Fig. 4 Example of a Quartz twin crystal, Peru.

Obtaining of the crystal structure is not easy, as handling of the twinned crystals in X-ray crystallography is not trivial, as they produce a complex diffraction pattern, which is difficult to solve. On the other hand, the smaller the crystal is, the less chances that the crystal is formed of different domains. In that sense, electron diffraction could be a helpful tool to get the structure of those crystals which repeatedly like to grow as (multi)twin domains. For such nano crystals with a less probability of twinning, ED experiments will be the approach to take.

References:

1 E. Prince, “International tables for crystallography”, Volume C, 3rd Ed., Kluwer Academic publishers, Dordrecht/Boston/London 2004, 10-14.

2 “Crystal twinning”, accessed on September 21, 2020, https://en.wikipedia.org/wiki/Crystal_twinning

3 Ibid.

4 “Twinning”, accessed on September 21, 2020, https://dictionary.iucr.org/Twinning