Thermally induced phase transformation in multi-phase iron oxide nanoparticles on vacuum annealing

The evolution of magnetic phases in multi-phase iron oxide nanoparticles, synthesized via the transferred arc plasma induced gas phase condensation method, was investigated by X-ray diffraction, vibrating sample magnetometry and Fe-57 Mossbauer spectroscopy. The particles are proposed to be consisting of three different iron oxide phases: alpha-Fe2O3, gamma-Fe2O3 and Fe3O4. These nanoparticles were exposed to high temperature (similar to 935 K) under vacuum (10(-3) mbar He pressure), and the thermally induced phase transformations were investigated. The Rietveld refinement of the X-ray diffraction data corroborates the least-squares fitting of the transmission Mossbauer spectra in confirming the presence of Fe3O4, gamma-Fe2O3 and alpha-Fe2O3 phases before the thermal treatment, while only Fe3O4 and alpha-Fe2O3 phases exist after thermal treatment. On thermal annealing in vacuum, conversion from gamma-Fe2O3 to Fe3O4 and alpha-Fe2O3 was observed. Interestingly, we have observed a phase transformation occurring in the temperature range similar to 498 K-538 K, which is strikingly lower than the phase transformation temperature of gamma-Fe2O3 to alpha-Fe2O3 (573-623 K) in air. Combining the results of Rietveld refinement of X-ray diffraction patterns and Mossbauer spectroscopy, we have attributed this phase transformation to the phase conversion of a metastable defected and strained d-Fe3O4 phase, present in the as-prepared sample, to the alpha-Fe2O3 phase. Stabilization of the phases by controlling the phase transformations during the use of different iron-oxide nanoparticles is the key factor to select them for a particular application. Our investigation provides insight into the effect of temperature and chemical nature of the environment, which are the primary factors governing the phase stability, suitability and longevity of the iron oxide nanomaterials prepared by the gas-phase condensation method for various applications. (C) 2017 Elsevier B.V. All rights reserved.