X-RAY-DIFFRACTION STUDY OF THE SMECTIC-(A)OVER-TILDE FLUID ANTIPHASE AND ITS TRANSITIONS TO SMECTIC-A1 AND SMECTIC-A2 PHASES

A high-resolution x-ray diffraction study has been carried out on a bi nary liquid crystal mixture of pentylphenylcyanobenzoyloxy benzoate (D B5CN) and cyanobenzoyloxypentyl stilbene (C5 stilbene) that is 49.5 mo le % C5 stilbene. This mixture exhibits on cooling the phase sequence monolayer smectic-A (SmA1)-smectic-A fluid antiphase (SmA)-bilayer sme ctic-A (SmA2). We observe the presence of a broad coexistence region b etween the SmA phase and the SmA2 phase, contrary to previous work whi ch reported a smectic- A crenelated phase. The behavior of the smectic - A fluid antiphase ordering and the evolution of the system to SmA2 o rder via a two-phase coexistence region are described in detail. Our r esults are compatible with previous high-resolution heat capacity meas urements on the same sample.