Structural study of the smectic-I to smectic-F transition in freely suspended films

The smectic-I (S(I)) to smectic-F (S(F)) phase transition in terephthal-bis-(4n)-decylaniline (TB10A) has been examined for a possible continuous transition via the intermediate smectic-L (S(L)) phase. X-ray diffraction measurements of thick, single-domain, freely suspended films are used to classify the phases and to determine the hexatic order parameter C6 and its harmonics. Instead of the continuous transition suggested in the literature we find a first-order S(I)-->S(F) transition with a discontinuous change in the direction of the bond-orientational order relative to the molecular tilt. The tilt of the molecular form factor in the hexatic phases is inconsistent with the tilt estimated from published layer spacing measurements, suggesting that the hexatic phases of TB10A must have the molecular cores oriented at an angle relative to the tails. This result taken together with published results on the S(C) phase suggests that the S(C)-->S(I)-->S(F) transitions are driven by changes in the conformation of the hydrocarbon tails. Examination of the harmonic scaling relation between the hexatic order parameters shows mean-field behavior in the S(I) phase. This result will make binary mixtures of TB10A with other materials a practical system for the study of the crossover from mean field to the XY behavior seen in other hexatic systems.