Chiroclinic effect and the phase diagram of achiral polar molecules in the antiferroelectric smectic B2 phase

Abstract

The paper presents a detailed analysis of the effects of chiral doping on antiferroelectric B 2 subphases in bent core systems. The studied system exhibits the phase sequence SmA ↔ SmAP A ↔ SmC S P A . The chiral dopant induces the tilt in the SmAP A phase or the tilt modulation in the SmC S P A phase, which is known as the chiroclinic effect, but it also influences the stability ranges and the order of the transition between the phases. The order of the phase transition changes from a continuous for weak effects of chiral doping to discontinuous for strong effects. Competition of interactions results in changes of the phase sequences, which are resumed in the phase diagram. The method that leads to these findings, which could be used for the analysis of other similar systems, is briefly presented as well.

Temperature ranges of the chiral polar smectic phases

“…We propose that the temperature T AC at which the frequency of the soft mode is the lowest is used to distinguish between the induced and the genuine phase. With higher chiral fields, the minimum of the soft mode occurs at higher temperatures, indicating an increase of temperature range of the genuine phase, consistent with [9].…”

“…Theoretical investigations of effects of chiral dopants are rare [8,9]. However, already the first analysis [8] allowed for a consistent explanation of the experimental observation and predicted a new phenomenon called the chiroclinic effect, which refers to the phenomenon when the non-tilted phase becomes tilted if doped with chiral dopant.…”

“…The observation of the phenomena would be most evident in the materials with a continuous SmAP A ↔ SmC A P A transition. Unfortunately, the materials with the continuous transition from polarly ordered smectic A phase to the homochiral tilted polar smectic C phase were not available, and quite a long time was needed to understand the preliminary experiments [10] on doped samples with a transition to the antichiral phase [9]. The analysis offered answers to several questions related to the behavior of the system under chiral doping -why the homochiral phase with a single domain expels the antichiral domain and why, eventually the homochiral domain becomes stable through the whole range of temperatures.…”

“…The system of bent-shape mesogens, with the antiferroelectric phase sequence SmA * ↔ SmAP * A ↔ SmC S P * A , where asterisk indicates that the system is doped with chiral dopant, was already considered in [9]. In that paper we studied a system in which the non-tilted antiferroelectically ordered SmAP A phase developed into the sinclinically tilted antiferroelectrically ordered SmC S P A in the system without doping.…”