The influence of suspended nano-particles on the frederiks threshold of the nematic host

In recent years several papers reported an enhanced dielectric anisotropy when ferroelectric particles were suspended in a liquid crystal. These results seem to be sensitive to the liquid crystal used and the preparation method of the ferroelectric particles. In this paper we compare different preparation methods of suspended barium titanate in two different liquid crystal hosts. As a control experiment we have followed similar preparation steps with non- ferroelectric silica particles. In all cases we found a broadening of the optical Frederiks transition but no change in the dielectric anisotropy. Raman spectroscopy has been used to investigate the loss of tetragonal structure in the barium titanate as a function of milling time and particle size. As reported in the earlier literature barium titanate does not exhibit a well-defined tetragonal crystal structure below a certain particle size and loses its ferroelectric features. This provides a simple rationalisation for the absence of any dielectric enhancement.