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

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.