Nanophases in mechanochemically synthesized AgI-CuI system: structure, phase stability and phase transitions |
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Authors: | DBharathi Mohan |
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Institution: | School of Physics, University of Hyderabad, Hyderabad 500 046, India |
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Abstract: | 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 Vegard's 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 (![View the MathML source View the MathML source](http://ars.els-cdn.com/content/image/1-s2.0-S0022369704001283-si4.gif) ) 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. |
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Keywords: | D Phase transitions |
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