Temperature-Driven Formation and Consolidation of Magnetite-Type Nano-Assemblies in a PEI-Assisted Solvothermal System

Magnetite-type iron oxide nano-assemblies are widely investigated because of their strong magnetic response, chemical stability, and tunable surface chemistry. However, the lower-temperature threshold for crystalline spinel formation in polyethylenimine-assisted solvothermal systems remains insufficiently defined. In this work, an FeCl3·6H2O–ethylene glycol–sodium acetate–branched polyethylenimine system was systematically investigated between 150 and 200 °C to determine the onset of magnetite-type spinel formation and its associated physicochemical evolution. Rietveld refinement identified the crystalline fraction of the product synthesized at 150 °C as 60.1 wt.% hematite and 39.9 wt.% goethite, whereas the materials obtained at 160–200 °C were satisfactorily described using a cubic Fd3¯m magnetite-type spinel model. This structural transformation was accompanied by an abrupt increase in saturation magnetization from 0.806 emu g–1 at 150 °C to 62.86 emu g−1 at 160 °C, reaching 79.99 emu g−1 at 200 °C. Temperature-dependent zero-field-cooled and field-cooled magnetization (ZFC–FC) measurements revealed an anomaly within 99–104 K, with a representative derivative maximum near 101 K, assigned to a Verwey-type transition and supporting the presence of magnetite-containing domains. Brunauer–Emmett–Teller surface-area (BET) analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) revealed a concurrent consolidation process involving goethite dehydroxylation, progressive removal or transformation of organic species, reduction in accessible surface area, and the emergence of spherical nano-assemblies from 160 °C onward. Additional PEI-free control syntheses at 160, 180, and 200 °C showed that a well-developed cubic spinel diffraction pattern was not obtained at 160 °C in the absence of PEI, whereas clear spinel reflections emerged at 180 and 200 °C. These results identify 160 °C as the lowest tested temperature for reproducible magnetite-type spinel formation under the investigated PEI-assisted solvothermal conditions and demonstrate that phase transformation, ferrimagnetic ordering, and nano-assembly consolidation are closely coupled.