X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES

Abstract. - X-ray diffraction was used to investigate the following transitions involving ordered biaxial phases of TBBA : solid-smectic B,, smectic B,-smectic VI, smectic V1-phase VII, phase VII-solid. A dependence on the thermal history of the sample was observed at the solid-smectic B, phase transition, in agreement with recent DSc data. A phase coexistence was observed at the smectic VI-phase V11 and at the solid-smectic B, phase transitions. An hysteresis effect was observed at the smectic VI-phase V11 phase transition. 1. Introduction. - X-ray (or neutron) diffraction constitutes a valuable tool in the investigation of hase transitions of liquid crystal materials. However these techniques were applied so far mainly to study high temperature phase transitions, i.e. nematic (or cho- 1esteric)-smectic A phase transitions [l-51, nematic (or cho1esteric)-smectic C phase transitions [6] and smectic C-smectic A phase transitions [7]. This kind of phase transitions, which were largely investigated by other experimental techniques, were theoretically treated in detail [g-101. Many uncertainties however still exist concerning these transitions. On the other hand very few investi- gations by X-ray (or neutron) diffraction exist concern- ing phase transitions involving the solid or the more ordered low temperature smectic phases. One of the few studies of this kind was performed by Levelut, Doucet and Lambert 111, 121, who investi- gated by X-ray diffraction the solid-smectic B phase transition of several compounds including the typical substance