Phase transitions in LiKSO4 below room temperature

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

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.

High temperature structures of LiKSO4 crystals: normal and incommensurate phases

Three crystallographic phases have been observed for LiKSO4 crystals between room temperature and the melting point. The study of these phases has involved many researchers for more than two decades of intense experimental investigations. Many publications with controversial results are found in the literature especially concerning the aperiodic character of the intermediate phase (708 K < T < 943 K). A recent careful X-ray diffraction study of the intermediate phase reports evidences of satellite reflections characteristic of an incommensurate modulation. A compilation of the most significant results and the ultimate conclusions are presented here.