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-(4 n)-decylaniline (TB10A) has been examined for a possible continuous transit ion via the intermediate smectic-L (S-L) phase. X-ray diffraction measureme nts of thick, single-domain, freely suspended films are used to classify th e phases and to determine the hexatic order parameter C-6 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 direc tion of the bond-orientational order relative to the molecular tilt. The ti lt of the molecular form factor in the hexatic phases is inconsistent with the tilt estimated from published layer spacing measurements, suggesting th at the hexatic phases of TB10A must have the molecular cores oriented at an angle relative to the tails. This result taken together with published res ults on the S-C phase suggests that the S-C-->S-I-->S-F transitions are dri ven by changes in the conformation of the hydrocarbon tails. Examination of the harmonic scaling relation between the hexatic order parameters shows m ean-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 cross over from mean field to the XY behavior seen in other hexatic systems.