  Certain nematic liquid crystals, e.g., those formed by banana-shaped
molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon
addition of chiral dopants, a chiral version of the twist-bend phase (N*tb) can
be observed below the conventional chiral nematic (N*) phase, while under
electric field the N* phase is transformed into a specific state known as
"heliconical cholesteric". In this work, we have studied the well-known
Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChD)
using optical microscopy, differential scanning calorimetry (DSC), and
measurements of temperature-dependent optical transmission using optical
microscopy. Effects of 5CB and different chiral dopants (of steroid or
non-steroid nature) upon thermal stability of Ntb phase were analyzed, and
selective reflection in the visible range could be observed in the N* phase
with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of
the cholesteric helix was observed at lower temperatures when approaching the
N*tb phase, which appears to be similar to the helix unwinding close to the
smectic-A transition.
