Ferroelectric phase in Stockmayer fluids.

By using a density-functional theory we establish the existence of a ferroelectric nematic phase in fluids consisting of spherical particles which interact with Lennard-Jones and dipolar forces. Our detailed analysis of the thermodynamic limit shows that for a polarized dipolar fluid within a single domain the free energy density depends on the shape of the sample as well as on the dielectric permittivity of the surrounding. For different aspect ratios of an ellipsoidal sample we determine the phase diagrams which comprise the isotropic gas and liquid phases and the ferroelectric liquid phase. For the latter we obtain the full probability distribution for the orientation of the particles. Its behavior in the vicinity of the phase transition between the isotropic and the ferroelectric liquid is analyzed by a Landau expansion of the grand-canonical potential. The repercussions of domain formation are briefly discussed.