Observation of a uniaxial ferroelectric smectic A phase

We report the observation of the smectic A_F, a liquid crystal phase of the ferroelectric nematic realm. The smectic A_F is a phase of small polar, rod-shaped molecules that form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the ∼10 Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field ∼2 × 10⁵ V/m is observed. The SmA_F phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic (N)-smectic Z_A (SmZ_A)-ferroelectric nematic (N_F)-SmA_F phase sequence, and 7N/DIO, exhibiting an N-SmZ_A-SmA_F phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers.