Low-Temperature Nitridation of Fe₃O₄ by Reaction with NaNH₂

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH₃ gas. One such method is the reaction between metal oxides and NaNH₂ at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe₃O₄ by reaction with NaNH₂ at 170-190 °C, via a pseudomorphic reaction. The more reduced Fe₃O₄ precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe₂O₃. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and ⁵⁷Fe Mössbauer spectroscopy, comprised 49-59 mol % ε-Fe₂₊N, accompanied by 29-39 mol % FeO_1-N and 8-14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH₂, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO_1-N oxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitride phases in high yield, by reaction of a metal oxide substrate with NaNH₂, with either careful control of H₂O concentration in the system or postsynthetic hydrolysis and crystallization.