STABILITY OF ANTIFERROELECTRICITY AND MOLECULAR-REORIENTATION IN THE HEXATIC SMECTIC I-A-ASTERISK PHASE AS STUDIED BY X-RAY-DIFFRACTION ANDNMR-SPECTROSCOPY

We have investigated molecular orientation in the hexatic antiferroele ctric SIA phase by X-ray diffraction and nuclear magnetic resonance ( NMR) spectroscopy. In the phase transition from SCA to SIA*, the laye r thickness increases and becomes larger than that in SA. The third-or der diffraction peak intensity becomes stronger than the second-order one at this transition and increases with decreasing temperature in SI A. According to the magic angle spinning NMR measurements, on the oth er hand, the isotropic chemical shift scarcely moves at this transitio n. These results suggest that the layer structure is reconstructed by the molecular reorientation but not by a molecular conformation change . We have also discussed the stability of antiferroelectricity and the origin of its appearance in hexatic ordering.