  Using a generalized Landau theory involving orientational, layering, tilt,
and biaxial order parameters we analyze the smectic-A* and smectic-C* (Sm-A* --
Sm-C*) transition, showing that a combination of small orientational order and
large layering order leads to Sm-A* -- Sm-C* transitions that are either
continuous and close to tricriticality or first order. The model predicts that
in such systems the increase in birefringence upon entry to the Sm-C* phase
will be especially rapid. It also predicts that the change in layer spacing at
the Sm-A* -- Sm-C* transition will be proportional to the orientational order.
These are two hallmarks of Sm-A* -- Sm-C* transitions in de Vries materials. We
analyze the electroclinic effect in the Sm-A* phase and show that as a result
of the zero-field Sm-A* -- Sm-C* transition being either continuous and close
to tricriticality or first order (i.e for systems with a combination of weak
orientational order and strong layering order) the electroclinic response of
the tilt will be unusually strong. Additionally, we investigate the associated
electrically induced change in birefringence and layer spacing, demonstrating
de Vries behavior for each, i.e. an unusually large increase in birefringence
and an unusually small layer contraction. Both the induced change in
birefringence and layer spacing are shown to scale quadratically with the
induced tilt angle.
