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

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 ($\langle$ 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°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.