  We show that a generalized Landau theory for the smectic A and C phases
exhibits a biaxiality induced AC tricritical point. Proximity to this
tricritical point depends on the degree of orientational order in the system;
for sufficiently large orientational order the AC transition is 3D XY-like,
while for sufficiently small orientational order, it is either tricritical or
1st order. We investigate each of the three types of AC transitions near
tricriticality and show that for each type of transition, small orientational
order implies de Vries behavior in the layer spacing, an unusually small layer
contraction. This result is consistent with, and can be understood in terms of,
the "diffuse cone" model of de Vries. Additionally, we show that birefringence
grows upon entry to the C phase. For a continuous transition, this growth is
more rapid the closer the transition is to tricriticality. Our model also
predicts the possibility of a nonmontonic temperature dependence of
birefringence.
