Nanophases in mechanochemically synthesized AgI–CuI system: structure, phase stability and phase transitions Original Research Article

Nanoscale crystallites of Ag-rich (Ag1−xCuxI, x=0.05, 0.10, 0.15 and 0.25), Cu-rich (Cu1-yAgyI, y=0.05, 0.10, 0.15 and 0.25) and intermediate Ag1-xCuxI (x=0.50) solid solutions and end members AgI, CuI with sizes in the range of 46–13 nm were synthesized by attrition at ambient temperature in a soft mechanochemical reaction (MCR) of Ag, Cu and I. Monophasic γ-AgI (zincblende, ) with disordered Ag+ sublattice and the crystallite size of about ∼31 nm was realized in the case of Ag0.75Cu0.25I (x=0.25) composition. Lattice parameter decreases linearly from 649 to 604 pm with increasing Cu concentration in the AgI–CuI system validating Vegardʹs law. Smallest size (∼13 nm) agglomerated nanocrystals were realized in the Cu-rich composition Cu0.75Ag0.25I (), while unagglomerated uniform-sized (∼17 nm) and spherical shape nanocrystallites of Ag0.50Cu0.50I () with maximum strain were synthesized for sensor applications using MCR. Differential scanning calorimetry study shows the systematic changes in the phase transition temperature with Cu substitution. Ag-rich composition posses less enthalpy (ΔH (x or Cu=0.05, 0.10, 0.15, 0.25)=6.0, 6.11, 6.6, 6.3 in kJ/mol) and entropy (ΔS (y or Ag=0.05, 0.10, 0.15, 0.25)=14.15, 14.1, 15.03, 13.6 in J/mol K) when compared to undoped AgI () implying greater thermal stability of γ-phase due to Cu-strengthened Ag–I bond. Enhanced entropy () in Cu0.75Ag0.25I (Cu-rich) solid solutions relative to CuI () indicates Ag-induced cation disorder. Fifteen percent Ag-doped CuI (Cu0.85Ag0.15I) nanocrystals apparently behave like microscopic p–n junctions with currents in the range of 10−6–10−8 A characterized by a non-linear I–V curve.