In the last few years, considerable interest has centered on the ferroelectric phenomena in fluoropolymers. An extensive review is given on the recent development of structural studies connected with ferroelectric phase transitions in polyvinylidene fluoride (PVDF) and copolymers of vinylidene fluoride/trifluoroethylene (P·VDFx/TrFE100-x) . PVDF exhibits five polymorphic forms, although the form-V is still ambiguous in its structure and properties. Various types of transitions among these forms, induced mechanically, electrically and thermally, are introduced and their mechanisms are discussed. In PVDF, the ferroelectric to paraelectric transition is not yet clearly understood. On the other hand, the Curie temperature has been recently observed in P·VDFx/ TrFE100-x. There are two kinds of phase transition from the paraelectric phase, that is, to the low temperature phase or to the cooled phase, depending on the composition x. The structures of these three crystal phases and the unique properties of the transition are introduced. The mechanisms of the phase transitions are discussed in terms of statistical theory in relation to cooperative conformational changes in the molecular chains. Finally, molecular motions in the paraelectric phase, domain structure and future problems on ferroelectric polymers are described.
Fluorites have received a considerable amount of experimental and theoretical research because of one of the most important and simplest systems of superionic conductors. Diffraction studies on defect structures of fluorites by X-rays and neutrons are reviewed.
The method of HADOX (high-angle double-crystal X-ray diffractometry) has been improved for some aspects and applied to the high-resolution study of the structural phase transitions in SrTiO3 and KMnF3. The discussion is given on the basis of the determination of the temperature dependence of the lattice constant. It is also shown that the peak profile provides us with more detailed information. As an example, the case of high-quality, flux-grown SrTiO3 crystals is presented. [J. Cryst. Soc. Jpn. 26, 130 (1984) ] .