X-ray study of phase transitions involving ordered biaxial phases

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