Orientational Order Measurements near a Possible Nematic—Smectic-ATricritical Point

We have measured a proton NMR dipolar splitting, $\Delta H$, proportional to the "nematic order parameter," in the nematic and smectic-$A$ phases of $p$-cyanobenzylidene-$p$-nonylaniline, at pressures up to 3.5 kbar. The discontinuity in $\Delta H$ at the nematic-smectic-$A$ phase transition decreases from a relatively large value at 1 atm to an unobservably small value at higher pressures. We tentatively identify a tricritical point for this transition at $t_t=140\mathrm{}\pm 4°$C and $P_t=2.89\mathrm{}\pm 0.23$ kbar.