Crystal shapes of cubic mesophases in pure and mixed carboxylic acids observed by optical microscopy

Two different cubic phases in pure (Ia (3) over bar d cubic space group) and mixed (Im (3) over bar m cubic space group) thermotropic carboxylic acids are observed by optical microscopy. In unpolarized light: the presence of the cubic phases is identified unambiguously by the observation of facetted single crystals with highly symmetrical shapes. The observed crystal habits are growth shapes, rather than equilibrium shapes. The facetting is described, both at the interfaces with air and with the isotropic liquid phase. The facets correspond to various reticular planes that are discussed in relation with crystallographic data.