Influence of cooling conditions on cubicity and iron distribution in novel copper ferrites

Copper ferrites were prepared by a combustion method to obtain a porous framework. This morphology, which maximizes the surface area of the material, is important for applications in catalysis, where copper ferrites can be used for example in CO oxidations. Nominal concentration was Cu0.87Fe2.13O₄ . To get single-phase samples, combustion was followed by a calcination process at 950∘C for 1 h. Afterwards, samples were cooled to room temperature at different cooling rates to get information about the structural transition from high-temperature cubic to low-temperature tetragonal phase. Experiments were performed by XRD, dc magnetic measurements and ⁵⁷ Fe Mössbauer spectroscopy. From structural parameters, a cubicity parameter X, describing how the unit cell resembles a cube, was determined from XRD measurements. From Mössbauer spectra the inversion parameter λ , describing the distribution of iron on the different crystallographic sites, was calculated. With increasing cooling rate X increases, not reaching full cubicity, even by cooling in liquid nitrogen. Further samples become magnetically softer, λ goes in direction of random distribution of Cu over both crystallographic sites. A rather large magnetic ordering temperature of 877± 16 K for the quickest cooled sample is obtained. The comparison with samples prepared by standard coprecipitation method show that the hyperfine parameters for the samples prepared by the new combustion method are very similar.