STRUCTURAL INVESTIGATION OF SMECTIC Q-PHASE

Some mesogenic chiral compounds show a smectic Q phase just below the clearing point. The molecules are in a liquid state but they are organ ized on a tridimensional lattice. At lower temperatures the only possi ble mesophase-mesophase transformation is towards an antiferroelectric smectic. Three different tetragonal and one hexagonal lattices have b een identified. All these phases, except one, can be considered as cry stalline arrays of twist grain boundaries inside an antiferroelectric smectic. The twist grain boundaries perpendicular to the smectic plane s divide the smectic structure in narrow sheets (their width compares to the molecular length), the smectic planes are twisted by an angle o f pi/2 or of pi/3 on each side of a boundary and the successive sheets correspond to one another by a 4(1) or a 6(2) helical axis. The inter face between two molecular layers is described by a triply periodic mi nimal surface.