X-ray and optical studies of the thickness dependence of the phase diagram of liquid-crystal films

A comprehensive study of the thickness dependence of the phase diagram of freely suspended films of the liquid crystal 4-n-heptyloxybenzylidene-4-n- heptylaniline (7O.7) between 50 and 69°C is reported. In thick films (thicker than about 300 layers and characteristic of bulk samples) there is a low-temperature crystalline-G phase followed by five crystalline-B phases with different stacking arrangements at higher temperatures. In thinner films there are two additional crystalline-B phases and two tilted hexatic phases, smectic-F and smectic-I, which do not appear in bulk samples. The in-plane and interlayer correlations in the tilted hexatic phases are anisotropic with a clear dependence on the molecular tilt direction; the in-plane correlations are more developed (longer range) perpendicular to the molecular tilt direction and the interlayer correlations are more developed parallel to the tilt direction. © 1987 The American Physical Society.