Three dielectric constants and orientation order parameters in nematic mesophases*

Temperature dependence of the three components $\varepsilon _{1}$, $\varepsilon _{2}$, and $\varepsilon _{3}$ of dielectric constant and orientation order parameters in the nematic phase of mesogens with rod, banana, and zero-order dendritic shape were measured using the in-plane and vertical switching geometries, and micro-Raman technique. Results on the well-known uniaxial (N$_{u})$ nematogens, E7 and 5CB, revealed two components $\varepsilon _{1~}$=~$\varepsilon _{\vert \vert }$ and $\varepsilon _{2~}$=~$\varepsilon _{3~}$=~$\varepsilon _{\bot }$, as expected. The three dielectric constants were different for two azo substituted (A131 and A103) and an oxadiazole based (ODBP-Ph-C12) bent core mesogens, and a Ge core tetrapode. In some cases, two of the components became the same indicating a loss of biaxiality at temperatures coinciding with the previously reported N$_{u}$ to biaxial nematic transition. This interpretation is substantiated by micro-Raman measurements of the uniaxial and biaxial nematic order parameters.