AgI nanoparticles were prepared by solution-based routes using water-soluble anionic or cationic polyelectrolytes as capping agents. Depending on the polyelectrolytes, AgI nanoparticles with well-defined morphology, size, and phase compositions were obtained: the use of poly (sodium 4-styrenesulfonate) (PSS) resulted in AgI nano-rods of beta-AgI in wurtzite structure (2H); with poly(acrylic acid sodium salt) (PAS) truncated-tetrahedron shaped gamma-Agl nanoparticles (nanotetrahedra) in zinc-blende structure (3C) were obtained; by employing poly (diallyldimethylammonium chloride) (PDADMAC) plate-like AgI nanoparticles (nano-plates) consisting of unusual polytype phases of AgI (7H and 9R) were formed. Macroscopically unstable gamma-Agl and 7H and 9R phases could be stabilized in the form of nanocrystalline powders. They transform reversibly into the high temperature alpha-AgI phase and exhibit unusually high ionic conductivity and substantially smaller transformation enthalpy values compared to the macroscopic beta-AgI. (c) 2006 Elsevier B.V. All rights reserved.