High-temperature phase transitions and domain structures of KLiSO4: studied by polarisation-optics, X-ray topography and liquid–crystal surface decoration

Abstract

Abstract The transitions between the room temperature phase III (space group P63) and the two high-temperature phases II (Pcmn) and I (P63/mmc) of KLiSO4 and the domain structures generated by them were investigated by high-temperature polarisation optics (birefringence) and room-temperature X-ray topography, optical activity and nematic–liquid–crystal (NLC) surface decoration. The transition from the polar hexagonal phase III into the centrosymmetric orthorhombic phase II at 708 K leads, due to the loss of the trigonal axis and the radial temperature gradient of the optical heating chamber used, to a roughly hexagonal arrangement of three sets of thin orthorhombic {110} lamelleae with angles of 60° (120°) between them. The associated twin law “reflection m{110}orth” corresponds to the frequent growth twin m{101̅0}hex of phase III. The domains are easily ferroelastically switched. Upon further heating above 949 K into phase I (P63/mmc) all domains vanish. Upon cooling back into phase II the three domain states related by 60°(120°) reflections m{110}orth re-appear, however (due to the higher thermal agitation at 949 K) with a completely different domain structure consisting of many small, irregularly arranged {110}orth domains. Particular attention is paid to the domain structure of the hexagonal room temperature phase III generated during the re-transition from the orthorhombic phase II. Curiously, from the expected three twin laws inversion 1̅, rotation 2⊥[001]hex and reflection m{101̅0}hex only the latter, which corresponds to the frequent growth twinning, has been found. Finally a short treatise of the structural relations of the KLiSO4 high-temperature polymorphs is given.