A Hydrothermal Phase Diagram for the Low-Temperature Synthesis of Nonstoichiometric Nickel Ferrite Nanoparticles

Corrosion products in pressurized water reactors are challenging to study in situ, yet understanding their properties is key to improving reactor performance and radiation reduction. In this study, a hydrothermal synthesis technique was used to produce nickel ferrite (NiFe₂O₄) particles from goethite (α-FeOOH) and nickel nitrate hexahydrate [Ni(NO₃)₂ 6H₂O] in the presence of sodium hydroxide (NaOH). X-ray diffraction was used for phase identification, with scanning electron microscopy used for particle shape and size analysis. By varying the [Ni]:[Fe] ratio of the precursors and synthesis temperature between 100°C to 250°C, a phase diagram was developed to determine the stability field in both composition and temperature for obtaining a single-phase, nonstoichiometric nickel ferrite product. The compositional boundaries of the single-phase region of the diagram are a function of temperature, consistent with the increased solubility and reaction rates at temperatures above 125°C. The single-phase nickel ferrite encompasses [Ni]:[Fe] ratios in a very narrow range at 150°C, only 0.35 to 0.375, but widens as a function of temperature and reaches its greatest breadth at 250°C. At this temperature, a single-phase product is obtained for a range of starting compositions from 0.30 to 0.425. Outside of this window, additional nanoparticles are obtained whose identity and composition vary with both temperature and starting mixture. On the lower nickel content side of the single-phase region, the mixture contains either unreacted goethite (for temperatures below 200°C) or hematite (α-Fe₂O₃) at 200°C or higher. On the Ni-rich side of the single-phase region, theophrastite [β-Ni (OH)₂] was obtained along with the nickel ferrite, at all temperatures studied. The single-phase window was widest at 250°C, resulting in nickel ferrites with a Ni mole fraction between 0.23 and 0.31.