Antiferroelectric Smectic Ordering as a Prelude to the Ferroelectric Nematic: Introducing the Smectic Z Phase*

In 2020, the Boulder group demonstrated direct evidence of ferroelectricity in the nematic phase of the molecule RM734 [1,2]. DIO [3] is one of the few other molecules demonstrating nematic ferroelectricity, and the Boulder group has recently shown that RM734 and DIO have the same ferroelectric nematic (NF) phase [4]. As previously reported [3], there is also an intermediate phase of unknown structure in DIO, between the nematic and NF phases. Here we report that this phase is a new class of smectic liquid crystal, one having the molecular long axes on average parallel to the layers. Synchrotron-based non resonant SAXS and electro-optical microscopy experiments show that the intermediate phase is density modulated with a 90Å periodicity. The layers are parallel to director n which is the limit opposite to that of the smectic A, so we call this phase the smectic Z. The space-filling periodic structure of the SmZ phase results in textures familiar in smectics, such as the bookshelf layer organization, including the formation of chevron textures having zig-zag walls. The SmZ is antiferroelectric, with polarization along n and oppositely directed in neighboring layers.