A smectic C*-smectic A tricritical point in a mixture of two compounds exhibiting high spontaneous polarization

X-ray diffraction, D.S.C. and miscibility studies have been performed on inary mixtures whose constituent compounds exhibit large values of spontaneous polarization. These studies have led to the observation of a tricritical point for the smectic C*-smectic A transition. The origin of this tricritical behaviour is discussed in relation to the range of the smectic A phase and the large spontaneous polarization.