EPL (Europhysics Letters)

EPL, 82 (2008) 56001
DOI: 10.1209/0295-5075/82/56001

Thermotropic biaxial nematic order parameters and phase transitions deduced by Raman scattering

C. D. Southern¹, P. D. Brimicombe¹, S. D. Siemianowski¹, S. Jaradat¹, N. Roberts¹, V. Görtz², J. W. Goodby² and H. F. Gleeson¹

¹ School of Physics and Astronomy, The University of Manchester - Manchester, M13 9PL, UK, EU
² Department of Chemistry, The University of York, Heslington - York, YO10 5DD, UK, EU

helen.gleeson@manchester.ac.uk

received 27 December 2007; accepted in final form 17 April 2008; published June 2008
published online 26 May 2008

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 $^{\circ}$ 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 ( $\langle$ P₂₀₀ $\rangle$ and $\langle$ P₄₀₀ $\rangle$ ) and biaxial ( $\langle$ P₂₂₀ $\rangle$ , $\langle$ P₄₂₀ $\rangle$ and $\langle$ P₄₄₀ $\rangle$ ) order parameters, are deduced. We show unequivocally the presence of a uniaxial to biaxial nematic phase transition approximately 30 $^{\circ}$ 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 $\langle$ P₂₂₀ $\rangle$ , $\langle$ P₄₂₀ $\rangle$ and $\langle$ P₄₄₀ $\rangle$ , respectively.

PACS
61.30.-v - Liquid crystals.
61.30.Gd - Orientational order of liquid crystals; electric and magnetic field effects on order.
78.30.-j - Infrared and Raman spectra.

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