The Nematic and Smectic-A1 Phases in DB7NO2: High Resolution X-Ray Study and Synthesis
We report a high resolution x-ray study of the fluctuations in the nematic and smectic-A1 phases in DB7NO2. As a function of decreasing temperature, the fluctuations evolve from an incommensurate region, through an intermediate region of coexisting (competing) incommensurate and smectic-C̃ fluctuations, to a region dominated by pretransitional smectic-Ã fluctuations which exhibit a phase lock-in smectic Ã1-smectic-Ã transition. This behavior directly confirms the current theoretical description of the smectic-Ã and smectic-C̃ phases as alternatives to the incommensurate phase. In DB7NO2, although the average structure of the smectic-Ã phase appears to be consistent with earlier findings, high resolution x-ray work reveals additional weaker (possibly satellite) peaks indicating a more complicated underlying structure. Finally, we outline a novel new synthetic route for preparation of high purity samples of DB7NO2 in large quantities.
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