A series of dialkyldimethylammonium bromides bearing a 4,4-oxynitrostilbene calamitic core at the end of one of the alkyl chains was synthesized. Its thermotropic liquid crystal behaviour was examined using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. An ordered smectic phase was identified in which the ionic ammonium bromide groups are arranged according to a square two-dimensional crystal lattice, and the oxynitrostilbene cores are tilted away from the layer normal by an angle of 54 degrees. This novel smectic structure is discussed in terms of structural compatibility of the smectic sublayers: the sublayers of the ionic groups and calamitic cores adapt their internal structure in order to become adequate for coherent superposition in a smectic structure.