Two kinds of two-dimensional order : the SmF and SmI phases

2014 A X-ray study of TBDA, a homologous compound of TBBA, shows that the smectic F and I phases exhibit a quasi-two-dimensional order (2D). A quantitative analysis of the powder diagrams yields short range order in the SmF phase and long range order in the SmI phase though at higher temperatures. This phase could be an example of a 2D solid. J. Physique LETTRES 42 (1981) L-67 L-70 ler FÉVRIER 1981, Classification Physics Abstracts 61.30 In recent years, two-dimensional (2D) systems have been the subject of a growing interest. New « liquid crystal » phases, recently discovered, provide good examples of such systems. The synthesis of TBBA homologous compounds with the general formula first allowed us to establish the existence of the smectic F phase (SmF) in the Cs compound [1] then the existence of the smectic I phase (SmI) in the C9 and C10 compounds [2]. We have synthesized new homologous of the series according to the process used for TBBA [3]. These last compounds show the following polymorphism : (*) We follow the advice of D. Demus et al. [13] concerning the labelling of the different smectic phases (SmG =5mBc). The SmI phase appears between the SmF phase and the SmA or SmC phase, which consists of stacked layers of « liquid type ». (It should be noted that the C 15 compound does not have a SmF phase.) The purpose of this letter is to study the change in order at the SmF H SmI transition in the C 10 compound (TBDA). An accurate quantitative analysis of the order cannot be done on single domains, because such samples often exhibit a large and unknown Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:0198100420306700 L-68 JOURNAL DE PHYSIQUE LETTRES mosaic spread this has been avoided by recording Debye-Scherrer diagrams on a high resolution Guinier camera (Cokal, ~, = 1.788 9 A). Moreover, we must point out that the differential thermal analysis has shown a great enthalpy variation at the SmI H SmC transition and a weak variation at the SmF H SmI transition, which means that the main change of order takes place at the transition : SmI +--+ SmC. 1. Structures of the SmF and SmI phases. a) THE SmF PHASE. X-ray patterns previously recorded [4, 5, 6] enabled us to assert the following : i) the layered structure of this phase has a tilt angle of the molecules with respect to the layer normal; ii) there is a very weak coupling between adjacent layers (as a matter of fact there do not exist hkl spots with I =F 0 : so, the structure is two-dimensional kind); iii) locally there is pseudo-hexagonal packing of the molecules within the layers. (Indeed the hkO Bragg peaks which are on a nearly regular hexagon (distorded by one percent) in the SmG phase [7] become diffuse spots in the SmF phase, centred of the same points of the reciprocal space.) Though the in-plane order does not extend to a long distance, and though the layers are very weakly coupled, one defines a local cell which is monoclinic with centred C-faces and its corresponding reciprocal lattice (Fig. Ib). The cell parameters of TBDA (C 10) at T 120 °C