X. Shen et Ry. Dong, DEUTERIUM NMR-STUDY OF UNIAXIAL-BIAXIAL PHASE-TRANSITIONS IN A SMECTOGEN, Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals, 262, 1995, pp. 293-303
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).