Ferroelectricity in dipolar liquids: the role of annealed positional disorder

TL;DR

This paper demonstrates that annealed positional disorder in dipolar liquids induces hindered dipole rotation, leading to an intrinsic bulk ferroelectric phase transition, contrasting with mean-field and local-structure approaches.

Contribution

It introduces a novel mechanism where annealed positional disorder causes hindered dipole rotation, driving ferroelectricity in liquids, validated by classical density functional theory.

Findings

Annealed positional disorder shortens effective dipolar interactions.

Ferroelectric transition is intrinsic and bulk in nature.

Results are exact in infinite dimensions and valid in high finite dimensions.

Abstract

Ferroelectric order in polar liquids has been observed in numerical simulations and liquid-crystal experiments. In mean-field frameworks, this behavior is associated to sample-shape dependent, surface contribution to the free energy. This remain nonzero in the thermodynamic limit due to the long-range nature of dipolar interactions. Yet, numerical simulations performed under conducting periodic boundary conditions, where surface term vanishes, still exhibit ferroelectric order, pointing to an intrinsic bulk origin of the transition. Moving beyond the mean-field approximation, Kirkwood seminal study on the dielectric properties of polar liquids highlights the role of hindered dipole rotation in shaping the corresponding pair correlations. In that study, hindered rotation stems from the mean force between nearest-neighbor dipoles, pointing the focus on local structure. Introducing a different perspective while retaining the central role of hindered dipole rotation in the onset of ferroelectricity, the present study establishes, as an original finding, that annealed averaging over positional disorder of dipolar interaction generates hindered rotation of dipole pairs in the liquid, able to drive a ferroelectric phase transition. As a result, in contrast to approaches centered on local structure, ferroelectricity emerges not in spite of the liquid nature, but exactly because the positional degrees of freedom remain liquid. This ferroelectric phase transition is shown to be intrinsic to the bulk. Annealed positional disorder defines an effective dipolar interaction that is shorter ranged than the bare potential. Derived within classical density functional theory, these findings are exact for infinite dimensional and remain valid within the optimized cluster expansion for finite dimensions larger than two.