Mechanism to stabilise the ferrielectric phase in ferroelectric smectics

A model with a freedom of molecular sense has been proposed recently for the purpose to certify the assumptions in the ANNNI model with the third nearest neighbour interaction for successive phase transitions in ferroelectric smectics. The phase diagram of that model at the absolute zero temperature and orderings of the sense of molecular long axis are examined to clarify the mechanism to stabilise the ferroelectric phase (SmC*gamma) and the intermediate antiferroelectric phase (AF) appearing in such phase transitions. These phases are shown to be stable in certain area on a parameter space, which coincides qualitatively with the results based on the ANNNI model and consequently justifies the minimal theory carried out previously in which negative definiteness of the second nearest neighbour interaction and the existence of the third nearest neighbour one (or, positiveness) are certified.