TWIN DOMAINS AND PHASE TRANSITIONS OF KliSO4. H. Klapper, Th. Hahn, Ch. Scherf, H.M. Park, G. Heger, Institut fuer Kristallographie, Rheinisch-Westfaelische Technische Hochschule, D-52056 Aachen, Germany.
We have investigated phase boundaries, twin formation and domain shapes during four phase transitions of KLiSO4 in the temperature range 150K to 980K. Single-crystal plates were examined in-situ by polarisation-microscopy, synchrotron-radiation Laue topography and Lang X-ray topography.
The tetrahedral framework structure of KLiSO4 exhibits a rich temperature polymorphism. In the room-temperature phase III (space group P63) four types of grown-in merohedral twins are distinguished. Heating above 708K from phase III and cooling below 949K from the high-temperature phase I leads to the intermediate orthorhombic phase II. Both transitions produce a dense twinning with three ferroelastic domain variants rotated by 120deg. around [001] (pseudo-hexagonal twins). In the "parent" phase I no twins are found, which confirms its hexagonal symmetry P63/mmc or P63/mc. After the transition cycle III'II'III phase III shows an irregular domain arrangement with the same twin laws as the growth twins.
During cooling from phase III the sluggish transition into phase IV (P31c) starts at about 208K. The growth twins (m // [001]) of the hexagonal phase vanish and thin (0001) lamellae with the twin law 2 // [001] appear. Upon further cooling below 190K the pseudo-hexagonally twinned monoclinic phase V is formed. After re-transition into phase III the original grown-in twin pattern appears again (memory effect).
Crystal-structure analyses of phases I, II and V are in progress, based upon the group-subgroup relations of the phase transitions.