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 ten d to the highest possible symmetrical arrangements. These arrangements have not to correspond in every case to the energetical most favourab le structure. Structures that show this distinct misfit tend to twinni ng. The examination of pentaerythrite and 1,2,4.5-tetrabromobenzene, t wo substances usually existing as twins, leads to the finding of struc tures - by lattice-energy calculations - with a better energy balance, but are not known yet as autonomous phases. Both cases showed arrange ments with lower symmetry explaining the transition between the twin i ndividuals. Following this theory the development of crystal twins is based on a structural instability caused by the conflict between the m ore favourable energetical arrangement and the highest available symme try.