TEMPERATURE DEPENDENCES OF THE REFRACTIVE INDICES OF UNIAXIAL CALAMITIC LIQUID CRYSTALS WITH HIGH BIREFRINGENCE

The temperature dependences of the refractive indices n_j(t) and the values <n(t)> = (n_|| + 2n_⊥)/3,n(t) =ε^-1/2,ε= (ε_|| + 2ε_⊥)/3, ε_j = n_j² for the light waves polarized along (j = ||) and across (j = ⊥) the director in uniaxial calamitic liquid crystals (LCs) were studied within the microscopic approach with taking into account the anisotropy of the local-field tensor f_j(t) =1 + L_j(t)[ε_j(t)–1] and the Lorentz tensor L_j(t). The known negative derivatives <n(t)>' = d<n(t)>/dt,n' and the point t_o of minimum at the dependence n_⊥(t) for calamite LCs were shown to correspond to small and medium values of the birefringence Δn = n_|| – n_⊥. For LCs with high values of δn and components L_⊥ exceeding the threshold value L_⊥e two new effects were predicted: positive derivatives <n>', and the presence of the point t_e of maximum at the dependence n_||(t). For a number of nematics with high values of Δn the components L_j(t) were determined, the presence of the predicted effects and the conditions for their manifestation were confirmed, the values _te were obtained and their connection with the magnitude Δn were established. The features of the transition from LCs with point to t_o LCs with point c under gradual increase in Δn were cleared up.