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Abstract
Normally there exist two separate groups of substances which are intermediate between crystals and liquids. One group forms the so-called plastic crystals (often known as ODIC phases). The other group forms various phases of liquid crystals.
This lecture concerns two substances of the PAA homologous series, in which a mixed sequence was discovered. There are the n=4 and n=5 members of the PAA series. In both substances Sequences ate. crystal, ODIC crystal, nematic liquid, isotropic liquid. An indication of such a “mixed” behaviour has been obtained in the calorimetry: entropies of fusion are for both substances much lower than those for smaller and larger n. Moreover, the neutron quasielastic scattering experiments as well as the dielectric relaxation measurements show an existance of molecular rotations in the ODIC phases before melting.