Evidence of a first-order smectic-A–smectic-$C^{/emph>\mathrm{}}$ transition and its approach to tricritical behavior

Evidence of a first-order smectic-A–smectic-$C^{\mathrm{*}}$ (SmA-Sm$C^{\mathrm{*}}$) transition is presented. Detailed x-ray studies on 4-(3-methyl-2-chloropentanoyloxy)4’-heptyloxybiphenyl show that the SmA-Sm$C^{\mathrm{*}}$ transition in this material is characterized by a discontinuous jump in the layer spacing (and hence in the tilt angle) and by a narrow two-phase region in which the density modulations due to the smectic-A and the smectic-$C^{\mathrm{*}}$ phases coexist. Addition of a second compound drives the SmA-Sm$C^{\mathrm{*}}$ transition towards second order.