Deuterium NMR Study of Uniaxial-Biaxial Phase Transitions in a Smectogen

Abstract

The effects of uniaxial-biaxial phase transition on the spectral densities measured from the deuteron Zeeman (T _1Z )and quadrupolar (T _1Q ) spin-lattice relaxation times are examined in two partially deuterated samples of 4-n-pentyloxybenzylidene-4‘-heptylaniline(50.7): one deuterated at the methine site (50.7-d₁) and one with the aniline ring deuterated (50.7-d₄). The deuterated sites are chosen so that their relaxation 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 biaxial. The results are discussed in terms of molecular reorientation and director fluctuations. Preliminary data for the sample director oriented at 90° with the external magnetic field are reported in the B and G phases of 50.7-d₁.