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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 andlarge layering order leads to Sm-A* -- Sm-C* transitions that are eithercontinuous and close to tricriticality or first order. The model predicts thatin such systems the increase in birefringence upon entry to the Sm-C* phasewill be especially rapid. It also predicts that the change in layer spacing atthe 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. Weanalyze the electroclinic effect in the Sm-A* phase and show that as a resultof the zero-field Sm-A* -- Sm-C* transition being either continuous and closeto tricriticality or first order (i.e for systems with a combination of weakorientational order and strong layering order) the electroclinic response ofthe tilt will be unusually strong. Additionally, we investigate the associatedelectrically induced change in birefringence and layer spacing, demonstratingde Vries behavior for each, i.e. an unusually large increase in birefringenceand an unusually small layer contraction. Both the induced change inbirefringence and layer spacing are shown to scale quadratically with theinduced tilt angle.

de Vries behavior of the electroclinic effect in the smectic-A* phase near a biaxiality-induced smectic-A* -- smectic-C* tricritical point