Interference pattern, biaxial nematics

The conoscopic interference patterns are also quite useful to identify uniaxial and biaxial nematics. For a uniaxial specimen, the conoscopic image consists of a dark cross... 

Figure 8. Interference patterns of the biaxial nematic phase of compound 9 (/j=12) at 137°C. (a) The trace of the optic axial plane lies parallel to the polarizer or analyzer (extinction position) (b) and (c) represent the trace of the optic axial plane enclosing an angle of +45° or -45° with the polarizer position (from [24]).

The main features of the two experimental interference patterns in Figure 5-23 - two pairs of diffuse peaks with an azimuthal angular separation of 80° in the absence and one pair of reflections in the presence of the electric field - were found in the calculated patterns as well. This unambiguously proves the biaxial nature of the investigated nematic phase. Latest polarization microscopy and X-ray experiments conducted on azo-substituted achiral bent-core mesogens revealed the existence of a uniaxial and a biaxial nematic phase as well as three smectic phases [36]. 

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