  We use the discrete phenomenological model to study theoretically the phase
diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical
purity and temperature. Recent experiments have shown that in some systems the
number of phases is reduced if the optical purity is extremely high. In some
materials the SmC$_{A}^{\star}$ phase is the only stable tilted smectic phase
in the pure sample. In the scope of the presented model this high sensitivity
of the phase sequence in the AFLCs to optical purity is attributed to the
piezoelectric coupling which is reduced if optical purity is reduced. We limit
our study to three topologically equal phases - SmC$^{*}$, SmC$_{\alpha}^{*}$
and SmC$_{A}^{*}$ and show that the reduction of optical purity forces the
system from the antiferroelectric to the ferroelectric phase with a possible
SmC$_{\alpha}^{\star}$ between them. The effect of the flexoelectric and
quadrupolar coupling is considered as well. If the phase diagram includes only
two phases, SmC$^{\star}$ and SmC$%_{A}^{\star}$, the flexoelectric coupling is
very small. The materials which exhibit the SmC$_{\alpha}^{\star}$ in a certain
range of optical purity and temperature, can be expected to have a significant
flexoelectric coupling that is comparable with the piezoelectric coupling. And
finally, when temperature is lowered the phase sequence SmA $\to $
SmC$%_{\alpha}^{\star}$ $\to $ SmC$^{\star}$ $\to $ SmC$%_{A}^{\star}$ is
possible only in materials in which quadrupolar coupling is very strong.
