Structural, magnetic and size transformations induced by isothermal treatment of ferrous oxalate dihydrate in static air conditions

摘要

Thermal decomposition of the FeC_2O_4.2H_2O powder has been studied isothermally in static air conditions at minimum decomposition temperature of 180 ℃ using ~(57)Fe Moessbauer spectroscopy, XRD, HRTEM, AFM and BET surface area measurements. Dehydration and liberation of carbon oxides from powdered sample is accompanied by direct oxidation of Fe(II) to nanocrystalline Fe_2O_3 without any indications of the stabilization of the magnetite (Fe_3O_4) phase. Decomposition process is completed after two hours and as-prepared nanopowder (3-5 nm), with a large surface area of about 250 m~2/g, is comprised of ultrafine superparamagnetic particles of γ-Fe_2O_3 (maghemite) and α-Fe_2O_3 (hematite) as proved by low temperature Moessbauer spectroscopy, XRD and HRTEM analysis. The simultaneous formation of both polymorphs is probably related with the non-equivalent diffusion conditions on the surface and in the bulk of oxalate particles. With increasing time, the particle size induced phase transformation of maghemite to hematite has been observed by AFM and XRD. Moessbauer spectra demonstrate that maghemite particles are formed only in superparamagnetic state (doublet), while magnetic components (sextet) in room temperature spectra can be assigned exclusively to hematite particles with size distribution produced by gradual crystallization of primary superparamagnetic nanoparticles of the same structure as well as by the phase transition of maghemite phase.