Stacking Disorder by Design: Factors Governing the Polytypism of Silver Iodide upon Precipitation and Formation from the Superionic Phase

Silver iodide (AgI) is used for a wide range of applications from photocatalysis and antimicrobial coatings to photography and ice nucleation. By fitting powder X-ray diffraction patterns with DIFFaX, we show that AgI displays a strong tendency to form stacking-disordered materials. Its polytypism is determined by the silver cation to iodide molar ratio during precipitation. Under iodide-rich conditions, fully hexagonal <i>b</i>-AgI is obtained whereas a maximal percentage of cubic stacking of 80% is obtained at a 1:2 molar ratio in the silver cation-rich regime. These findings are explained on the basis of a concentration-dependent competition between kinetically and thermodynamically-favored adsorption processes. Furthermore, the previously reported memory effects observed upon transforming hexagonal and cubic AgI to the high-temperature superionic phase and back are now followed quantitatively. We propose that the memory effects originate from excess ions at the surfaces of AgI crystals that stabilize the pyroelectricity of AgI associated with hexagonal stacking. The ability to ‘design’ the polytypism of AgI by tuning the precipitation conditions provides a first example where the stacking disorder of a material can be controlled in a continuous fashion. Future studies will clarify if this design principle can be applied to other materials as well.