Issue
EPL
Volume 82, Number 5, June 2008
Article Number 56001
Number of page(s) 6
Section Condensed Matter: Structural, Mechanical and Thermal Properties
DOI http://dx.doi.org/10.1209/0295-5075/82/56001
Published online 26 May 2008
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.

© EPLA 2008