  Observed under the polarizing microscope, the B7bis phase in the banana
compound D14F3 [J.P. Bedel et al, Liq. Cryst., 27, 1411 (2000)] displays two
types of textures of defects, namely (a): helical ribbons, that nucleate in
large quantities when the samples are quenched from a sufficiently high
temperature in the isotropic phase (b)- shapes with no helicity having the
structure of developable domains much akin to those observed in columnar
phases, either resulting from the annealing of the helical ribbons or
nucleating under slow cooling processes. The existence of these two kinds of
defects points toward the complex nature of the structure of the B7 phase,
which is at the same time a columnar and a smectic phase. Our observations fit
the model [M. Kleman, J. de Phys., 46, 1193 (1985)] according to which the
geometry of a helical ribbon is that one of the central region of a screw
dislocation with a giant Burgers vector, split into two helical disclination
lines of strength k = 1/2 which bound the ribbon. Textures and defects, already
partly documented, and growth features and annealing processes, not yet
reported in the literature, are analyzed. We conclude that the helical ribbons
and the developable domains with no helicity are textures of two different B7
states, namely a metastable state and the ground state respectively.
Comparative textural analysis is performed for two other banana compounds
exhibiting B2 phases.
