Orientational order of a ferroelectric liquid crystal with small layer contraction

We present spectroscopic and optical studies of a non-layer-shrinkage ferroelectric liquid crystal DSiKN65. The orientational order parameters S, measured with respect to the smectic layer normal using IR spectroscopy on a sample aligned homeotropically, does not exhibit any significant variation between the smectic-A^ and smectic-C^ phases. In contrast the birefringence of a planar homogenous sample abruptly increases at the smectic-A^ to smectic-C^ transition. This suggests a general increase in the orientational order, which can be described by the orientational order parameters S^' defined with respect to the director. Simultaneous increase of S^' and the director tilt Θ may explain the low shrinkage of smectic layers, which is consistent with recent theoretical models describing the smectic-A^ to smectic-C^ transition for such materials.