On the role of grain boundaries in nanocrystalline γ-Fe2O3 under high pressure.

In situ impedance spectroscopy of nanophase and bulk γ-Fe2O3 were performed using a fabricated microcircuit on a diamond anvil cell. The results provide evidence for the existence of grain and grain boundary effects that are separated in the frequency region. The analysis establishes that the grain boundary conductance of nanocrystalline γ-Fe2O3 is higher than that of the bulk material. Both the grain and grain boundary resistances of γ-Fe2O3 smoothly decrease with pressure. A remarkable phenomenon of discontinuity appears at 7.4 GPa for the nanocrystals. This critical value is interpreted by the space charge model and changes in charge densities between the grain boundaries. The impedance analysis further reveals that the relaxation frequency gradually increased with the effect of pressure and the γ-Fe2O3 samples' impedance imaginary part exhibits a typical capacitive characteristic. Within the given pressure range, the relaxation frequency of γ-Fe2O3 follows the Arrhenius behavior. This result was attributed to a dominant interfacial effect. [ABSTRACT FROM AUTHOR]