Restricted molecular rotation in hexatic B and crystalline B mesophases as studied by the electron paramagnetic resonance method

Molecular motions have been examined by the electron paramagnetic resonance method in orthogonal smectic phases for materials with SmA-HexB-CryB phase sequence, doped by a paramagnetic copper (II) complex. For the hexatic as well as crystalline B phases, two types of subphases were found: with free or constrained rotational molecular motion. The rotation becomes hindered either at the SmA-HexB phase transition or a few degrees below it. In both cases the order parameter describing the restricted rotation increases continuously upon decreasing temperature and does not change appreciably at the HexB-CryB phase transition point. The restricted rotation is attributed to a local herringbone order of molecules. (C) 1997 American Institute of Physics.