High-resolution volumetric study of the smectic-<i>A–</i>to<i>–</i>nematic transition in 4-(<i>n</i>-pentyl)phenylthiol-4’-(<i>n</i>-octyloxy)benzoate (<i>8¯</i>S5) and octyloxycyanobiphenyl (8OCB)

Abstract

High-resolution volumetric measurements have been made on pure 8\ifmmode\bar\else\textasciimacron\fi{}S5 and 8OCB in the vicinity of the smectic-A--to--nematic (A-N) transition. The molar-volume variations show in both compounds a continuous second-order A-N transition. For 8\ifmmode\bar\else\textasciimacron\fi{}S5 the A-N transition is well described by a critical three-dimensional XY exponent \ensuremath{\alpha}=\ensuremath{\alpha}'=-0.026, but for 8OCB by the exponent \ensuremath{\alpha}=\ensuremath{\alpha}'=0.19\ifmmode\pm\else\textpm\fi{}0.07. For 8OCB the anisotropic hyperscaling relation 2-\ensuremath{\alpha}=${\ensuremath{\nu}}_{?}$+2${\ensuremath{\nu}}_{\ensuremath{\perp}}$ is satisfied to within experimental error. The thermal-expansion coefficient ${\ensuremath{\alpha}}_{p}$ has been determined as a function of temperature.