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

Highlights: • The iron-oxide nanoparticles synthesized by gas-phase thermal plasma route are multi-phased. • A metastable phase, (defected) d-Fe₃O₄, was observed in the nanoparticles. • This metastable phase d-Fe₃O₄ transforms to α-Fe₂O₃ phase at low temperature (498 K–538 K). • The phase transformation of γ-Fe₂O₃ to mostly Fe₃O₄ (reduction) is observed after thermal treatment in vacuum. - Abstract: 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 ⁵⁷Fe Mössbauer spectroscopy. The particles are proposed to be consisting of three different iron oxide phases: α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄. These nanoparticles were exposed to high temperature (∼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 Mössbauer spectra in confirming the presence of Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃ phases before the thermal treatment, while only Fe₃O₄ and α-Fe₂O₃ phases exist after thermal treatment. On thermal annealing in vacuum, conversion from γ-Fe₂O₃ to Fe₃O₄ and α-Fe₂O₃ was observed. Interestingly, we have observed a phase transformation occurring in the temperature range ∼498 K–538 K, which is strikingly lower than the phase transformation temperature of γ-Fe₂O₃ to α-Fe₂O₃ (573–623 K) in air. Combining the results of Rietveld refinement of X-ray diffraction patterns and Mössbauer 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 α-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.