Anisotropy of local field and polarizability density of molecules in ferroelectric nematic FNLC919

Abstract
Understanding the molecular nature of ferroelectric nematics N_F can be aided by the following information: the anisotropy of the pair correlation function g₂(r) of intermolecular distances r, averaged over the orientations of molecules; the degree of orientation ordering of molecules S with respect to director n; the molecular polarizability components γ_j averaged over the molecular ensemble in the director's coordinate system with axes parallel (j = ||) and perpendicular (j = ⊥) to the n-axis. For a uniaxial liquid crystal (LC), the key to this information is provided by the experimental values of the Lorentz tensor components L_j. The components f_j(λ) of the local-field tensor of light wave with the wavelength λ depend on the components L_j. In this work, the experimental values of the components L_j and the components P_j = γ_j/v (where, v is the volume per molecule in the medium) of the polarizability density of molecules were determined for a ferroelectric nematic mixture FNLC919 with known refractive indices n_j(λ) in the transparency range λ = 0,4 ÷ 0,75 μm at room temperature. Taking into account the anisotropy of the components L_j, the anisotropy of spheroidal surface F(r) was found, which meets the condition g₂(r) = const and characterizes the local anisotropy of the LC. Similar and distinguishing features of the components L_j, f_j(λ) for the FNLC919 mixture in comparison with those for nonpolar nematics and other uniaxial media were noted. A relationship between the anisotropy of the L_j components and the temperature dependencies of n_j(T) in the N_F phase was established. This relationship correlates with the change of n_j(T) in the N_F-phase of the other known mixture M5. The constraints on the mean value and the anisotropy of the components P_j(λ) have been set. These constraints are due to the anisotropy of the components f_j(λ). The spectral features of the P_j(λ) functions and their implications were elucidated.

Keywords: ferroelectric nematics, mixture FNLC919, mixture M5, anisotropy of Lorentz tensor, short-range anisotropy, local field anisotropy, polarizability density