Two mechanisms for the formation of the ferronematic phase studied by dielectric spectroscopy

Abstract

A non-chiral ferroelectric nematic compound with a 1,3-dioxane unit in the mesogenic core called 2,3′,4′,5′-tetrafluoro-[1,1′-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) was studied by dielectric spectroscopy in the frequency range 0.1 Hz–10 MHz over a wide range of temperatures. The compound exhibits three nematic phases on cooling from the isotropic phase, i.e., the ordinary paraelectric nematic N; the intermediate nematic NX and the ferroelectric NF phase. The least frequency process is due to the dynamics of ions. The middle frequency relaxation process P1 is like as observed in other ferronematic compounds and this mode is a continuation of the molecular flip-flop motion in the isotropic phase to the collective dynamics of dipoles which are strongly coupled with the splay fluctuations in nematic phases. In addition to this process, DIO shows an additional collective relaxation process P2 at higher frequencies both in the N and the NX phases. This mode originates from the polar/chiral domains of the opposite chirality, these arise from the spontaneous symmetry breaking of achiral mesogens in the N phase. Both collective processes, P1 and P2, show soft mode-like characteristic behavior on cooling from the N to the NX-NF phase transition temperature and are shown to contribute independently to the formation of the ferronematic NF phase.