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 reso nance 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 we re found: with free or constrained rotational molecular motion. The ro tation becomes hindered either at the SmA-HexB phase transition or a f ew 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 poi nt. The restricted rotation is attributed to a local herringbone order of molecules. (C) 1997 American Institute of Physics.