Low-Temperature Nitridation of FeO by Reaction with NaNH.

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 NH gas. One such method is the reaction between metal oxides and NaNH 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 FeO by reaction with NaNH at 170-190 °C, via a pseudomorphic reaction. The more reduced FeO precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with FeO. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and Fe Mössbauer spectroscopy, comprised 49-59 mol % ε-FeN, accompanied by 29-39 mol % FeON 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 NaNH, 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 FeON oxynitride phase identified in this study opens the possibility for the synthesis of phases in high yield, by reaction of a metal oxide substrate with NaNH, with either careful control of HO concentration in the system or postsynthetic hydrolysis and crystallization.