Intrinsic Paramagnetic Defects Probe the Superionic Phase Transition in Mechanochemically Synthesized AgI Nanocrystals

Abstract

Electron paramagnetic resonance (EPR) of two intrinsic paramagnetic centers generated by soft mechanochemistry of Ag and I to yield zinc blende gamma-AgI nanoparticles (approximately 38 nm) has been used for the first time to probe the gamma-alpha (body centered cubic) superionic phase transitions in AgI at (423 +/- 1) K. These results are agreeable with the differential scanning calorimetric studies. A transmission electron microscope picture shows the average crystallite size in the range of approximately 30-40 nm. A hole-type Ag-related paramagnetic center (Ag2+) with an average g = 2.21025 value is remarkably sensitive to the first-order phase transition exhibiting sharp drops at the phase transition temperature (T(t)) and complete reversibility. The T(t) is characterized by a sharp, abrupt rise in the inverse paramagnetic susceptibility 1/chi by 1 order (7.4 x 10(10) to 3.17 x 10(11) in kg m(-3)) which reflects changes in the bonding of the material. Furthermore, a sharp signal at g = 2.0019 (deltaH(PP) = 10 G) due to an electron-excess center (Ag0) as a result of Ag metal nanoclusters also formed during the mechanochemical reaction (MCR) yields an abrupt and drastic decrease in the intensity observed at T(t) = 423 K. From high-temperature (323 to 433 K) I-V characteristics, the evolution of nonohmic behavior is observed on the order of 10(-9)-10(-6) A with increasing temperature until below T(t) which becomes ohmic thereafter. The reason could be the creation of an electronic defect such as Ag0 metal nanoclusters formed during the near-equilibrium mechanochemical reaction, with the increased excess free energy favoring the formation of gamma-AgI nanoparticles.