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Effect of Alkyl Chains Length on Ferroelectric Properties of Km/n and KLLm/n Homologous Series

In this work, a phenomenological thermodynamic model is used to describe the temperature variations of spontaneous polarization P0, tilt angle θ0, and helical pitch p of Sm C* phase near Sm A–Sm C* phase transition in two homologous series, Km/n and KLLm/n, of ferroelectric mesogens. The free-energy density of the system is written in terms of tensor orientational order parameter Qij, scalar smectic order parameter ψ, vector polarization order parameter P, and the resulting couplings between these order parameters. The tilt vector ξ is not considered an independent order parameter, as that Qij governs ξ. Adopting usual method, we evaluate the relative contribution of each individual term of the expansion series and the values of P0, θ0 and p as a function of temperature for the eleven ferroic mesogens, and found that the theoretical results agree very well with the experimental data of all the mesogens.