Deciphering the commensurately modulated monoclinic phase of Rb2ZnCl4 at low temperatures

The ferroelectric phase III of Rb2ZnCl4 is stable below Tc = 192 K. It is known to be a threefold superstructure of the centrosymmetric high-temperature structure, with space group P21cn. Below TL = 70 K, phase IV exists as a sixfold superstructure. We report the crystal structure of phase IV with monoclinic symmetry Cc (b unique), while a structure model with symmetry Pn (c unique) leads to an almost equally good, yet significantly worse fit to the diffraction data. Employing the superspace approach to these commensurately modulated structures results in modulation waves that follow the two-dimensional irreducible representation T1 of P21cn, albeit with different order parameter directions defining Cc and Pn symmetries, consistent with the literature. Standard tools of crystal-chemical analysis indicate that the sixfold superstructure is more stable than the threefold superstructure of phase III. However, crystal-chemical arguments cannot distinguish between the correct superstructure model with space group Cc (b unique) and the incorrect superstructure model with symmetry Pn (c unique) for phase IV. New crystal chemical tools are required, in order to attain a meaningful understanding of superstructure formation.