X-ray study of phase transition involving ordered biaxial phases

X-ray diffraction was used to investigate the following transitions involving ordered biaxial phases of TBBA: solid-smectic Bsub(c), smectic Bsub(c)-smectic VI, smectic VI-phase VII, phase VII-solid. A dependence on the thermal history of the sample was observed at the solid-smectic Bsub(c) phase transition, in agreement with recent DSC data. A phase coexistence was observed at the smectic VI-phase VII and at the solid-smectic Bsub(c) phase transitions. An hysteresis effect was observed at the smectic VI-phase VII phase transition