X-ray and Raman scattering study of orientational order in nematic and heliconical nematic liquid crystals

The temperature dependence of the orientational order parameters (P2(cosβ)) and (P4(cosβ)) in the nematic (N) and twist-bend nematic (Ntb) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The (P2(cosβ)) obtained from both techniques are the same, while (P4(cosβ)), determined by Raman scattering is, as expected, systematically larger than its x-ray value. Both order parameters increase in the N phase with decreasing temperature, drop across the N-Ntb transition, and continue to decrease. In the Ntb phase, the x-ray value of (P4(cosβ)) eventually becomes negative, providing a direct and independent confirmation of a conical molecular orientational distribution. The heliconical tilt angle α, determined from orientational distribution functions in the Ntb phase, increases to ∼24 at ∼15 K below the transition. In the Ntb phase, α(T) -(T∗-T)λ, with λ=0.19±0.03. The transition supercools by 1.7 K, consistent with its weakly first-order nature. The value of λ is close to 0.25 indicating close proximity to a tricritical point.