X‐ray studies of the layer thickness in smectic phases

X‐ray investigations of nine smectogenic substances exhibiting the smectic Ad, A1 and crystalline E phases were performed at various temperatures. X‐ray patterns yielded the layer thickness d (Ad, A1 phases) and orthorhombic unit cell parameters (E phase). The layer thickness of the Ad phase in 4′‐n‐alkyl‐4‐cyanobiphenyls (nCBs) has different temperature coefficients for shorter (n = 8–10) and longer (n = 12–14) members, which is explained as resulting from two competing effects: a weakening with temperature of the intermolecular association energy that favours an increase in d, and the increasing number of conformers which reduces the molecular length. A small anisotropy of the thermal expansivity in the smectic phases was found by comparing the linear quantity d(T) with the linearized bulk characteristic of the system, V −3(T), where V = 1/ρ is the specific volume, ρ is the density. Differences between the slopes of the two quantities are less in the case of the A1 phase of two nDBTs (5‐n‐alkyl‐2‐(4′‐isothiocyanatophenyl)‐1,3‐dioxanes). The present X‐ray data and recent results of studies of the low frequency relaxation process in these compounds (under atmospheric as well as elevated pressures) give a consistent picture of molecular reorientations around the short axes in the smectic phases.