Surface structures and reentrant transitions in tilted smectic phases of chiral liquid crystals

Four experimental projects, using a variety of optical and x-ray techniques, constitute this thesis. First, a series of surface transitions were observed in a thin film of one compound. Smectic layers near the surfaces of the film contain tilted molecules, while the bulk of the film is in the untilted Sm A phase. Simulations show that the surface transitions correspond to different tilt structures within the surfaces. The five observed structures make up a reentrant ferroelectric-antiferroelectric-ferroelectric transitions and a simultaneous double reentrant synclinic-anticlinic-synclinic-anticlinic transition.;One previous work by another research group proposed a reentrant antiferroelectric-ferroelectric-antiferroelectric transition in bulk samples of another compound. Using several complementary methods we have determined the phase sequence for the compound, conclusively showing that no reentrant transition exists. In addition, we have measured the helical pitch across the Sm C*a -SmC* transition. The transition is shown to be first order with unusual pretransitional effects. Our resonant x-ray diffraction results suggest that two different helical pitch values coexist in the Sm C*a phase at temperatures just above the Sm C*a -SmC* transition.;Using a series of binary mixtures, we have determined the dependence of the phase sequence upon doping for two compounds with similar chemical structures but inverted phase sequences in one of the pure compounds. The Sm C*FI2 phase is favored over the SmC* phase on both sides of the mixing phase diagram. Also, several Sm C*FI2 states with different polarization directions were observed. The different polarization directions result from different surface structures and different arrangements of the unit cell structure within the bulk.;In two of these binary mixtures, we observed a reentrant Sm C*FI2 -SmC*- C*FI2 transition. Resonant x-ray diffraction identifies the Sm C*FI2 phase below SmC* and null transmission ellipsometry shows that the phases on either side of SmC* have the same structure. The reentrant transition only occurs in thin films.