DEUTERIUM NMR-STUDY OF UNIAXIAL-BIAXIAL PHASE-TRANSITIONS IN A SMECTOGEN

The effects of uniaxial-biaxial phase transition on the spectral densi ties measured from the deuteron Zeeman (T-1Z) and quadrupolar (T-1Q) s pin-lattice relaxation times are examined in two partially deuterated samples of 4-n-pentyloxybenzylidene-4'-heptylaniline (50.7): one deute rated at the methine site (50.7-d(1)) and one with the aniline ring de uterated (50.7 d(4)). The deuterated sites are chosen so that their re laxation behaviours are not complicated by internal chain dynamics. It is found that the spectral densities show discontinuities at the S-C - S-B phase transition, but show no discontinuity at the S-A - S-C and S-B - S-G phase transitions. Both the smectic C and G phases are biax ial. The results are discussed in terms of molecular reorientation and director fluctuations. Preliminary data for the sample director orien ted at 90 degrees with the external magnetic field are reported in the B and G phases of 50.7-d(1).