⁵⁷Fe Mössbauer study of high-yield CuFe₂O₄ nanoparticles produced by the levitation-jet aerosol technique with post-synthesis annealing

Pseudo-spherical cubic-shaped nanoparticles of spinel ferrite CuFe₂O₄ have been prepared using a two-stage process. At first, an evaporation of levitating copper–iron drop into mix of helium–air gas flow took place, which resulting in copper ferrite-based powders. Then, such powders were additionally oxidized trough the heterogeneous auto-wave combustion in open air. We studied the effect of performing the synthesis in either an air or a helium environment on the phase composition of nanoparticles, and how the use of a post-synthesis annealing step modifies this, using room temperature ⁵⁷Fe Mössbauer spectroscopy. By applying this technique, we are able to distinguish between the normal (non-magnetic) and inverse cubic (magnetic) phases of CuFe₂O₄, which is usually inaccessible using X-ray diffraction, as well as quantifying their relative amounts within each sample. Furthermore, we have been able to quantify trace amounts of the tetragonal CuFe₂O₄, phases that are typically obscured by line-broadening effects within our X-ray diffraction data, which indicates that annealing using a propane flame can also cause a cubic to tetragonal distortion in the crystal structure of the spinel lattice. Finally, by combining our Mössbauer parameters, which are sensitive to the Fe-containing phases only, with X-ray diffraction data, which are sensitive to all phases, we report on the full phase composition for the first time for nanoparticles produced via this synthesis route.