Coupling between tilt and polar orders in liquid crystal phases formed from achiral bent shaped molecules

A phenomenological description is given of the mesophases which may arise b elow a liquid formed from achiral molecules possessing a transverse dipole, as the result of a coupling between their tilt and polar orders. Six stabl e mesophases are found. Two of them have a cholesteric biaxial structure. T he others display antiferroelectric or ferrielectric, incommensurately modu lated, structures with in-plane or out-of-plane spontaneous polarization. T he in-plane polarized structures become unstable under application of a lon gitudinal electric field. Other field-induced effects are discussed.