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The transitions between the room temperature phase III (space group  P 6 3 ) and the two high-temperature phases II ( Pcmn ) and I ( P 6 3 / mmc ) of KLiSO 4  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 ( P 6 3 / 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 KLiSO 4  high-temperature polymorphs is given.

zeitschrift für kristallographie - crystalline materials

High-temperature phase transitions and domain structures of KLiSO 4 : studied by polarisation-optics, X-ray topography and liquid–crystal surface decoration