A structural analysis of the so-called smectic Q mesophase was perform
ed on nine different chiral compounds. Not less than four different st
ructures were observed, and they all belong to the crystal of defects
class. The defects of the liquid crystalline organization are stacked
in three dimensional crystalline lattices of tetragonal or hexagonal s
ymmetries, and the lattice constants are in the range of 65-130 Angstr
om. In all cases but one, the constitutive defects are twist grain bou
ndaries in a smectic structure with twist angles of pi/2 or pi/3. The
fourth phase organization has to be compared to that of blue phase and
it can be considered as a stack of double twist clusters.