High-Resolution X-Ray Scattering Study of the Nematic-to-Smectic-$C$ Transitions in $\overline{8}\mathrm{S}5-\overline{7}\mathrm{S}5$ Mixtures

Measurements of the mass-density fluctuations associated with the first-order nematic-to-smectic-$C$ transitions in $\overline{8}\mathrm{S}{5}_{1-x}-\overline{7}\mathrm{S}{5}_x$ mixtures are reported. This binary system exhibits nematic, smectic-$A$, and smectic-$C$ phases, and a multicritical point $x_{\mathrm{NAC}}$. As a function of decreasing temperature for $x>\mathrm{}{x}_{\mathrm{NAC}}$, the scattering in the nematic phase evolves from smectic-$A$ to pretransitional smectic-$C$ fluctuations described by a Lifshitz model of Chen and Lubensky. Important discrepancies remain with the predictions of the model near $x_{\mathrm{NAC}}$.