STRUCTURAL STUDY OF A COMMENSURATE TGB(A) PHASE AND OF A PRESUMED CHIRAL LINE LIQUID-PHASE

We report the first experimental observation of a commensurate twist-g rain-boundary smectic-A phase (TGBA). The number of blocks per pitch i s found to vary with temperature but is independent of the thickness o f the well aligned sample. For the first time the variations with temp erature of the distance between the screw dislocations l(d) and the gr ain boundaries l(b) have been determined in the TGB(A) phase. The rati o l(b)/l(d) is found to be very close to the one predicted by Renn and Lubensky. We also report the first structural study of the phase pres umed to be the chiral line liquid phase, the liquid-crystal analog of the vortex liquid phase in type-II superconductors.