Homogeneous AgI nanoplates 300 nm in diameter and 50 nm in thickness can be prepared by a solution-based route using poly (diallyldimethylammonium chloride) as capping agent. The formation of unusual 7H/9R polytype modifications is the origin of the huge room temperature superionic conductivity and the large hysteresis in the phase transition with respect to the superionic high-temperature phase alpha-AgI. All these properties indicate a substantial Ag disorder in the 7H/9R polytype AgI unlike the normal beta-AgI in wurtzite structure. The Ag disorder in 7H/9R polytype structure can be formally understood in terms of an equilibrium redistribution of charge carriers at heterojunctions in ionically conducting zinc blende (gamma-AgI)/wurtzite(beta-AgI) heterostructures in the subnano regime. Upon martensitic transformation of alpha-AgI with its body-centered-cubic iodine lattice, the 7H/9R stacking sequence rather than the 2H sequence of beta-AgI occurs in the presence of Ag+ attracting surface adsorbents of Cl-. The usability of these materials as electrolytes in rechargeable all- solid- state Ag batteries working at 100 S C was demonstrated by using Ag as anode and TiS2 as cathode. (c) 2007 The Electrochemical Society.