Hydrazine method of synthesis of gamma-Fe2O3 useful in ferrites preparation. Part III - Study of hydrogen iron oxide phase in gamma-Fe2O3

Abstract

Direct current electrical conductivity (sigma) measurements as a function of temperature have been carried out on gamma-Fe2O3 prepared from precursors, iron (II) carboxylatohydrazinates, gamma-FeOOH and hydrazinated gamma-FeOOH. The conductivity variation obeys an Arrhenius equation, sigma(1) = sigma(o)e(-Delta E/kT) and the plots of log sigma versus 1/T of the as prepared gamma-Fe2O3, which are in general linear, during the very first heating up to 350 degrees C and cooling to room temperature (RT) do not overlap. This indicates a hysteresis behavior of conductivity, thereby suggesting involvement of two different conductivity mechanisms. When the heat treated sample was equilibrated in a known partial pressure of moisture at similar to 200 degrees C and then conductivity measured from RT, the log plots during heating and cooling did not overlap and a hysteresis behavior similar to the as prepared gamma-Fe2O3 is observed again in the conductivity. Water is considered to be crucial during the synthesis of gamma-Fe2O3 through magnetite, Fe3O4. Protons, H+, are thought to be introduced in the spinel Fe3O4 making it defective and the oxidation product of this is gamma-Fe2O3 which retains few protons in its spinel structure. From the structural similarity of such proton incorporated gamma-Fe2O3 and lithium ferrite, LiFe5O8, (Fe3+)(8)[Fe-12(3+) Li-4(1+)]O-32, a formula HFe5O8, (Fe3+)(8)[Fe123+H41+]O-32 is suggested. A hydrogen iron oxide of formula H1-xFe5+x3O8, where x similar to 0.1 is probably formed as a maximum limit. Protons are removed during the very first heating of the as prepared sample in the present studies and hence the conductivity of proton free gamma-Fe2O3 is different and therefore a hysteresis behavior is observed. Moisture equilibration reintroduces the protons. The lithiated samples in the present studies were found to substitute for protons in gamma-Fe2O3 and no hysteresis behavior is observed in such samples even after moisture equilibration.