X-Ray Scattering Study of Nematic and Smectic - a Critical Behavior in Liquid Crystals.

X-ray measurements of susceptibility and correlation length exponents have been carried out on mixtures of hexylphenyl cyanobenzoyloxy benzoate (DB6) and terephtal-bis-butyl aniline (TBBA). Pure DB6 has a nematic to smectic-Ad to smectic-A2 phase sequence. Mixtures with > 12 m% TBBA concentration have a smectic-A1 phase between nematic and smectic-A2 phases. For the N-Ad transition in pure DB6, and along the N-A1 line in mixtures, measured exponents are for smectic fluctuations in the nematic phase. Along the line of A1-A2 transitions in mixtures, the exponents are for A2 fluctuations, in the A1 phase. The N-Ad transition in pure DB6 has exponents (gamma) = 1.29 (+OR-) .05, (nu)(,(PARLL)) = .67 (+OR-) .03, and (nu)(,(PERP)) = .52 (+OR-) .03. Close to the multicritical point (12 m% TBBA) where a second order N-A1 line meets the first order part of the A1-A2 line, the N-A1 exponents are (gamma) = 1.09 (+OR-) .05, (nu)(,(PARLL)) = .57 (+OR-) .03, and (nu)(,(PERP)) = .43 (+OR-) .03. Anisotropy persists along the entire N-A1 line, with exponents decreasing as the multicritical point is approached. The A1-A2 line has both first and second order parts. Exponents are invariant along the second order part of the A1-A2 line. Correlation length exponents are isotropic. The result is (gamma) = 1.46 (+OR-) .05, (nu)(,(PARLL)) = (nu)(,(PERP)) = .74 (+OR-) .03. These numbers also hold, close to the tricritical point where the transition turns first order. Measurements have been made of the integrated scattering intensity from Bragg-like peaks in the smectic -A phase, and pretransitional smectic fluctuations in the nematic phase. This has been done on oxtyloxycyanobiphenyl (80CB), butyloxybenzylidene heptylaniline (40.7), and 4 -n-pentylphenylthiol-4'-n-nonyloxybenzoate ((')9S5). For a given material, the intensity exponent, x, is the same for both sides of the transition. The results are: x = .67 (+OR-) .05 for 80CB, .70 (+OR-) .05 for 40.7, and .63 (+OR-) .05 for (')9S5.