TWINNING OF CRYSTALS AS A RESULT OF DIFFERENCES BETWEEN SYMMETRICAL AND ENERGETICAL MOST FAVORABLE STRUCTURE ARRANGEMENTS .25.

It is thought that the building units of crystal structures always tend to the highest possible symmetrical arrangements. These arrangements have not to correspond in every case to the energetical most favourable structure. Structures that show this distinct misfit tend to twinning. The examination of pentaerythrite and 1,2,4.5-tetrabromobenzene, two substances usually existing as twins, leads to the finding of structures - by lattice-energy calculations - with a better energy balance, but are not known yet as autonomous phases. Both cases showed arrangements with lower symmetry explaining the transition between the twin individuals. Following this theory the development of crystal twins is based on a structural instability caused by the conflict between the more favourable energetical arrangement and the highest available symmetry.