Correlation between microstructure, cation distribution and magnetism in Ni1−xZnxFe2O4 nanocrystallites

ydrothermal synthesis of magnetic ferrite spinel nanoparticles, with composition Ni1−xZnxFe2O4 resulted in sizes below 27 nm in the entire composition range (x = 0.0–1.0). The nanoparticles were analyzed with regard to their microstructure, crystal structure and magnetic properties. Combined Rietveld refinements of powder X-ray diffraction (PXRD) and neutron powder diffraction data (NPD), were used to test and compare different models for the cation distribution between the tetrahedral and octahedral sites in the spinel structure. The structural modeling reveals that the Ni2+ ions have a strong preference for the octahedral coordination irrespective of composition, which is consistent with the inverse spinel structure of bulk NiFe2O4. However, the Zn2+ ions are found to partly occupy octahedral sites, despite their preference for tetrahedral coordination in the bulk. Annealing of the nanoparticles in air for 4 h at 600 °C causes a transition towards a more bulk-like cation configuration. The powder diffraction data were complemented by transmission electron microscopy (TEM), scanning transmission electron microscopy with a high-angle annular dark field detector (STEM-HAADF), energy-dispersive X-ray spectroscopy (EDS) and vibrating sample magnetometry (VSM). Quantitative analysis of EDS spectra shows that the desired stoichiometries were obtained. The highest saturation magnetization, of 66 Am2 kg−1, is achieved for Ni0.6Zn0.4Fe2O4. Furthermore, the saturation magnetization obtained from VSM was compared to the results from atomic magnetic refinements based on the neutron powder diffraction data. The present study confirms that the cation distributions observed in nanocrystalline ZnFe2O4 and NiFe2O4 are maintained in the mixed spinel Ni1−xZnxFe2O4.he authors would like to thank financial support from the European Commission through the AMPHIBIAN (H2020-NMBP-2016-720853) project and the Innovation Fund Denmark (Green Chemistry for Advanced Materials, GCAM-4107-00008B). Support from the Danish National Research Foundation (Center for Materials Crystallography, DNRF-93), and the Danish Center for Synchrotron and Neutron Science (DanScatt) is gratefully acknowledged. We would like to thank Antonio Cervellino, Hermann Emerich, and Denis Cheptiakov for assistance during beamtime at MS@SLS, Paul Scherrer Institute (PSI), Switzerland, the Swiss-Norwegian beamlines (SNBL), BM31@ESRF, France, and HRPT@SINQ, Paul Scherrer Institute (PSI), Switzerland, respectively. Affiliation with the Center for Integrated Materials Research (iMAT) at Aarhus University is gratefully acknowledged. JH acknowledges affiliation with the Sino-Danish Center for Education and Research (SDC).Peer reviewe

Mechanisms for Iron Oxide Formation under Hydrothermal Conditions: An in Situ Total Scattering Study

The formation and growth of maghemite (γ-Fe2O3) nanoparticles from ammonium iron(III) citrate solutions (C6O7H6·xFe3+·yNH4) in hydrothermal synthesis conditions have been studied by in situ total scattering. The local structure of the precursor in solution is similar to that of the crystalline coordination polymer [Fe(H2cit(H2O)]n, where corner-sharing [FeO6] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge-sharing [FeO6] units form (with extent of the structural order <5 Å). Tetrahedrally coordinated iron subsequently appears, and as the synthesis continues, the clusters slowly assemble into crystalline maghemite, giving rise to clear Bragg peaks after 90 s at 320 °C. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the clusters along the corner-sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles smaller than 6 nm are highly disordered. The final crystallite size (<10 nm) is dependent on both synthesis temperature and precursor concentration