The extraordinary high ionic conductivity and the phase transition with large thermal hysteresis of AgI:Al2O3 nanocomposites can be ascribed to the presence of unusual polytype 7H and 9R modification of AgI as mesoscopic heterostructures. In addition to the 7H AgI phase in the composites previously identified by a Rietveld analysis of X-ray diffraction, a new 9R AgI polytype with a characteristic three-fold contrast was directly evidenced by high resolution transmission electron microscopy. The epitaxial intergrowth of various polytypes was also microscopically evidenced which is in accordance with the Rietveld analysis.