X-Ray Topographic and Polarisation-Optical Study of the Low-Temperature Phase Transitions and Domain Structures of KLiSO4

The transformations between the three polar “low-temperature” phases of KLiSO 4 III (space group P6 3 ), IV (P31c) and V (C1c1 or P112 1 ), were investigated by low-temperature X-ray topography and polarisation optics. The disappearance of the phase-III growth twinning (twin law “m‖[001]”) and the appearance of the phase-IV transformation twins (twin law “2‖ [001]”) during the very sluggish III → IV transition (ca. 225 K, thermal hysteresis ≈ 50 K) was directly observed. In both phases the domain boundaries are parallel (0001), normal to the polar axis [001] (pronounced memory effect). The III → IV transition is strongly promoted by dislocations. The IV-V transition (190 K) shows the formation of three pseudo-hexagonal domains and the local metastable coexistence of phases III, IV and V. The ambiguities of the phase-V symmetry are discussed.