PACS 61.30.-v-Liquid crystals PACS 61.30.Gd-Orientational order of liquid crystals; electric and magnetic field effects on order PACS 78.30.-j-Infrared and Raman spectra Abstract-Raman Scattering was used to investigate biaxiality in the nematic phase formed by the bent-core material, C5-Ph-ODBP-Ph-OC12. Linearly polarised light was normally incident on a homogeneously aligned sample, and the depolarisation ratio was measured over a 360 • rotation of the incident polarisation for the Raman-active phenyl stretching mode. By modeling the bent-core structure and fitting to the depolarisation data, both the uniaxial (P200 and P400) and biaxial (P220 , P420 and P440) order parameters, are deduced. We show unequivocally the presence of a uniaxial to biaxial nematic phase transition approximately 30 • C above the underlying smectic phase. Further, we report the temperature evolution of the biaxial and uniaxial order parameters, which increase in magnitude continuously with reducing temperature, reaching values of 0.1, −0.15 and −0.18 for P220 , P420 and P440 , respectively.

The depolarisation ratio for the Raman-active phenyl stretching mode has been measured over the whole of the mesophase range, and the orientational order parameters deduced, in the uniaxial nematic liquid crystal octylcyanobiphenyl (8CB). Linearly polarised light was incident normally on a homogeneously aligned sample and a chi(2) minimisation routine performed on the 360 degrees depolarisation ratio profile. The order parameters (P(200)) and (P(400)), together with the differential polarisability ratio, r , are used as fitting parameters and measured as a function of temperature. Interestingly, we show that the value for r , conventionally measured in the isotropic phase and assumed to remain constant, has a clear temperature dependence, ranging from -0.032 + or - 0.008 in the isotropic phase through to -0.245 + or - 0.015 at the nematic-to-smectic A phase transition. The measured order parameters (P(200)) and (P(400)) varied from 0.35-0.55 + or - 0.02 and 0.180-0.245 + or - 0.02, respectively, across the 8 degrees C wide nematic phase range. The values of both (P(200)) and (P(400)) are in excellent agreement with theory, but it is noteworthy that (P(400)) shows a much better quantitative match than has been reported in previous work. Crucially the temperature dependence of r is shown to be a contributing factor in the low (P(400)) values that have been conventionally reported from Raman scattering measurements. The potential for fitting the entire angular depolarisation ratio distribution in liquid crystalline systems that are described by more order parameters, specifically biaxial materials, is discussed.

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Liquid crystals are intriguing electrically responsive soft matter systems. We report previously unexplored field-induced changes in the structures of some frustrated liquid crystal phases and describe them theoretically. Specifically, we have discovered using resonant x-ray scattering that the four-layer intermediate smectic phase can undergo either a transition to the ferrielectric (three-layer) phase or to the ferroelectric phase, depending on temperature. Our studies of intermediate phases using electric fields offer a way to test theories that describe ferroelectricity in self-assembling fluids.

The fast multistate switching associated with ferroelectric and antiferroelectric smectic liquid crystals makes them important materials for electro-optic devices, and asymmetric electro-optic responses are of particular interest for some analog switching applications. We report intrinsic asymmetric switching in a ferrielectric liquid crystal device observed using time-resolved small angle x-ray scattering and electro-optic measurements. The experiments reveal a marked variation in the response time depending on field polarity not present in the ferroelectric or antiferroelectric phase. We suggest that this is a consequence of the ferrielectric structure itself and interactions with the device surfaces.