Crystal/magnetic structure and cation inversion in hydrothermally synthesized MnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 nanoparticles: a neutron powder diffraction study†

Abstract

The crystal and magnetic structures of MnFe₂O₄, CoFe₂O₄, NiFe₂O₄ and ZnFe₂O₄ nanocrystallites are reported based on joint structural modelling of powder X-ray diffraction and neutron powder diffraction data. The nanoparticle samples were prepared using equivalent precursor preparation routes (co-precipitation of transition metal hydroxides using NH₄OH) and identical hydrothermal synthesis conditions (steel autoclave, 200 °C, 1 hour), allowing the isolated effect of the divalent cation to be evaluated. The study uncovers how variations in cation site preferences, spinel inversion degree, and crystallite size, which are challenging to discern using conventional characterization techniques, distinctly influence the magnetic structures. Diffraction peak profile analysis and scanning transmission electron microscopy images show how MnFe₂O₄ forms the largest crystallites (17.13(2) nm), followed by NiFe₂O₄ (10.31(1) nm) and CoFe₂O₄ (7.92(1) nm), while ZnFe₂O₄ forms ultrafine nanoparticles of only 3.70(1) nm. The transition metal ions have different affinities for the tetrahedral and octahedral crystallographic sites as evident from the obtained spinel inversion degrees, x, [M²⁺_1−xFe³⁺_x]^tet[M²⁺_xFe³⁺_2−x]^octO₄. The MnFe₂O₄ and CoFe₂O₄ nanocrystallites exhibit mixed/semi-inverse spinel structures with x = 0.87(3) and 0.954(6), respectively, while NiFe₂O₄ is fully inverse (x = 1.00) and ZnFe₂O₄ is closer to a normal spinel (x = 0.138(4)). The combination of neutron diffraction and magnetic measurements illustrates how cation identity impacts site occupancy, crystallite size, and magnetization, providing new insights into the design of ferrite-based nanomaterials for magnetic applications.