Temperature Dependence of Birefringence and Refractive Indices

For 4-n-propyloxybenzylidene-4-n-heptyl-aniline the temperature dependence of the principal refractive indices and and the refractive index in the isotropic i liquid at different wavelengths (509, 546, 589, 644 nm) is shown in Fig. 3 [10]. It can be seen that the birefringence is positive and decreases with increasing temperature. The temperature dependence is pronounced in the neighbourhood of Tn-i- It should be emphasized that at the transition to the isotropic liquid the birefringence does not disappear continuously but discontinuously, indicating a phase transition of first order. 

The principal refractive indices fig and n have different temperature dependences. Whereas rig strongly decreases with increasing temperature, for there is a weak increase with temperature. The refractive index of the isotropic phase Mj decreases linearly with rising temperature. As shown in Fig. 4, there are cases where the temperature dependence of the birefringence is less pronounced and the n curve goes through a flat minimum [63]. 

The birefringence of nematics containing calamitic metal has been measured [16, 62]. For palladium complexes of 4,4 -bis(hepty-loxy)azoxybenzene it was found that com-plexation increases and but decreases the birefringence with respect to the free ligand [16]. The inverse case has also been observed [62, 64, 65]. 

The experimental results can be easily understood on the basis of equations that correlate the refractive indices with the molecular polarizability anisotropy and the orientational order. For a merely qualitative 

Whereas the temperature dependence of Hi is determined by the temperature dependence of p, the temperature change in and 

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