Application of thermodynamic model to study the ferroelectric properties of nAL and nHL homologous series

Abstract

Using a thermodynamic model developed by us in an earlier paper [A. Singh and S. Singh, Thermodynamic model for the description of smectic C* and smectic A–smectic C* phase transition properties, Phase Trans. 83 (2010), pp. 205–222] for the description of SmC* and SmA–SmC* phase transition properties, we study, in detail, the temperature variation of spontaneous polarization P ₀, tilt angle θ₀ and helical pitch p of two homologous series nAL(n = 9, 10, 12) and nHL(n = 9, 10, 12, 14, 16) of ferroelectric mesogens. In this model, the free-energy density of a system is expanded in terms of three degrees of freedom – tensor orientational order , scalar smectic order ψ and polarization vector P and their coupling terms. The tilt vector ξ is not considered as an independent order parameter on the plea that governs ξ . The coupled equations for these order parameters are obtained, as usual, by the minimization of the free-energy density with respect to respective order parameters together with the criterion for the thermodynamic stability. Adopting usual method, we evaluate the relative contribution of each individual term of the expansion series and the values of P ₀, θ₀, and p as a function of temperature for the above-mentioned eight ferroic mesogens. We found that the theoretical results agree very well with the experimental data of all the mesogens.

Keywords:

• thermodynamic model
• ferroelectric mesogens
• phase transition properties
• free-energy density

Acknowledgements

We are grateful to the University Grants Commission (UGC), New Delhi and the Department of Science and Technology (DST), New Delhi for the financial support.