X-RAY STUDY OF ORDER - DISORDER PHASE TRANSITIONS. A. Pietraszko, Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland
The microscopic mechanism of the order-disorder phase transitions is a topic of considerable interest. Disorder occurs in the crystalline phases of many kinds of organic and inorganic compounds and may be the result of dynamic processes in the crystal structure (dynamic disorder). Static disorder is a result of two or more different orientations of molecule in a crystal with similar energies, or random distribution of atoms among equivalent positions with the occupancy <1. X-ray diffraction effects will be discussed for two cases: (i) when the positions of atoms are well resolved (e.g. in crystals RbLiSO4 and CsLiSO4 of AA'BX4 type), (ii) when and due to small displacements of atoms their positions overlap and cannot be resolved (e.g. Cd2NB2O7 crystals ). We define a model for AA'BX4 crystal structures with non-dynamic disorder of the orientational subsystems and examine the coupling the disordered tetrahedra BX4 with the distortion of the lattice parameters.
Dynamic disorder of one or more sublattices in the crystal is the distinguishing feature of the ionically conducting solids. The disorder can be a result of either a temperature increase or of an order-disorder structure phase transition. The X-ray study of proton superionic conductors of Me4LiH3(XO4)4 and (NH4)3H(SeO4)2 will be presented.
Recently many works have been devoted to X-ray investigations of the pretransitional phenomena with order-disorder phase transitions, because they can help to explain the nature of these transformations. The existence of such phenomena in the RbLiSO4 and CsLiSO4 crystals will be presented.