TWINNING OF CRYSTALS DUE TO DIFFERENCES BETWEEN THE MOST FAVORABLE SYMMETRICAL AND ENERGETIC STRUCTURAL ARRANGEMENTS .2.

Twins and oriented intergrowths are often caused by structural instabi lities. A conflict arises between the highest symmetric and the most f avourable energetic structural arrangement. A necessary condition for twin growth is the presence of lattice pseudosymmetries. This was show n with two additional examples, maleic acid and 3,5-dimethylbenzoic ac id. Both substances are known practically only as twins. Using lattice -energy calculations, energetic arrangements that are more favourable were found in both cases. These structures explain the transition that can exist between the twin crystal parts.