Synchrotron X-Ray Study of the Thickness Dependence of the Phase Diagram of Thin Liquid-Crystal Films

The phase diagram of freely suspended thin films of heptyloxybenzylidene-heptylaniline shows dramatic changes for thicknesses below 22 layers. The most surprising feature of the phase diagram is the inclusion of two phases lacking long-range crystalline order (smectic-$F$ and hexatic-$B$ phases) between two crystalline phases (crystalline smectic $B$ and smectic $G$). Neither the smectic $F$ nor the hexatic $B$ occurs in bulk samples. Between sixteen and ten layers the width, in temperature, of the hexatic-$B$ phase increases.