Ferroelectricity in dipolar liquids: the role of annealed positional disorder

Abstract

Ferroelectric ordering in polar liquids has been observed in numerical simulations and liquid-crystal experiments. Within mean-field framework, this behaviour remains associated with sample-shape dependent, surface contribution to the free energy, which does not vanish in the thermodynamic limit due to the long-range nature of dipolar interaction. Yet, numerical simulations performed under conducting periodic boundary conditions, for which the surface contribution vanishes, still exhibit ferroelectric order, pointing to an intrinsic bulk origin of the transition. Moving beyond the mean-field approximation, Kirkwood seminal study of the dielectric properties of polar liquids emphasized the role of hindered dipolar rotation in shaping the corresponding pair correlations. In Kirkwood analysis, hindered rotation stems from the mean force between nearest-neighbor dipoles, placing the focus on local structure. Introducing a different perspective while retaining the central role of hindered dipolar rotation in the onset of ferroelectricity, the present study establishes, as an original finding, that annealed averaging of dipolar interaction over positional disorder generates hindered dipolar rotation favoring dipole alignment, and able to drive a ferroelectric phase transition. As a result, unlike approaches centered on local structure, ferroelectricity emerges not in spite of the liquid nature, but because of it. This ferroelectric phase transition is intrinsic to the bulk. Annealed averaging over positional disorder generates an effective dipolar interaction that is shorter-ranged than the bare potential, analogous to the Keesom interaction where screening arises from annealed dipolar disorder. Derived within classical density functional theory, these findings are exact in the infinite-dimensional limit and remain valid within the optimized cluster expansion for dimensions greater than two.