Simple experimental assessment of smectic translational order parameters

A procedure to obtain the smectic translational order parameter Sigma from the temperature-dependent intensity I of the fundamental (001) smectic layer peak, observed in small-angle x-ray scattering experiments, is presented. We report results obtained with this procedure on the smectic-A or chiral smectic-A phases of six different liquid crystals, among them a de Vries-type 2-{4(')-[1('),1(')-dihydro-2(')-(2(')-perfluorobutoxyperfluoroethoxy)- perfluoroethoxy]}phenyl-5(')-octylpyrimidine(3M8422) and the antiferroelectric R-4-(1-methylheptyloxycarbonyl)phenyl-4(')-octloxybiphenyl-4-carboxylate (MHPOBC) materials. The smectic order parameters obtained are in the range between 0.5 and 0.9. For the de Vries-type 3M 8422 we found remarkably high values of Sigma in the order of similar to 0.9.