  When a family of non symmetrical heterocycled compounds is investigated, a
variety of mesophases can be observed with rather different features. Here we
report the behaviour of seven different members among a family of such
materials, that consists of mesomorphic oxadiazole compounds. In two of these
compounds, the optical microscope investigation shows very interesting
behaviours. In their smectic phases, fan-shaped and toric textures, sometimes
with periodic instability, are observed. Moreover, the nematic phase displays a
texture transition. Texture transitions have been previously observed only
inside the nematic phase of some compounds belonging to the families of the
oxybenzoic and cyclohexane acids. In these two oxadiazole compounds we can
observe what we define as a "toric nematic phase", heating the samples from the
smectic phase. The toric nematic texture disappears as the sample is further
heated, changing into a smooth texture.
