First-Order Transitions and Hexatic Ordering in the Aerogel-Confined Liquid Crystal 650BC

In the liquid-crystal material 650BC confined in a silica aerogel, synchrotron x-ray diffraction shows two distinct kinds of first-order phase change behavior, depending on $r=\lambda \left(T\right)/〈p〉$, the ratio of the two-phase interface thickness at the transition, $\lambda \left(T\right)$, to the pore size $〈p〉$. For $r<\mathrm{}1\mathrm{}$, intrapore phase coexistence is observed, whereas for $r>\mathrm{}1\mathrm{}$, order in the aerogel nucleates on a scale larger than the mean pore size and grows simultaneously in pores of all sizes. We also find that in all phases translational in-plane order is short ranged and limited in the aerogel host to dimension much smaller than the pore (smectic domain) size and bulk values.