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

High-resolution volumetric measurements have been made on pure 8{}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{}S5 the A-N transition is well described by a critical three-dimensional XY exponent {α}={α}'=-0.026, but for 8OCB by the exponent {α}={α}'=0.19±{}0.07. For 8OCB the anisotropic hyperscaling relation 2-{α}=ν?+2ν_⊥ is satisfied to within experimental error. The thermal-expansion coefficient αₚ has been determined as a function of temperature.