Steam-Assisted Ammonolysis of MoO2 as a Synthetic Pathway to Oxygenated δ-MoN

A common route for the synthesis of molybdenum nitrides is through the temperature-programmed reaction of molybdenum oxides with NH₃, or ammonolysis. In this work, the role of precursor phase, gas phase chemistry (impact of H₂O), and temperature profile on the reaction outcome (700 °C) was examined, which resulted in varying amounts of MoO₂, H₂MoO₅, and the nitride phases-cubic γ (nominally Mo₂N) and hexagonal δ (nominally MoN). The phase fraction of the δ phase increased with precursor in the sequence MoO₂ > MoO₃ > H₂MoO₅. Steam in the reaction gas also favored the production of δ over γ, but with too much steam, MoO₂ was obtained in the product. Synthesis conditions for obtaining nearly phase-pure δ were identified: MoO₂ as the precursor, 2% H₂O in the gas stream, and a moderate heating rate (3 °C/min). In situ X-ray diffraction provided insights into the reaction pathway. Extensive physico-chemical analysis of the δ phase, including synchrotron X-ray and neutron diffraction, electron microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and prompt gamma activation analysis, revealed its stoichiometry to be MoO_0.108(8)N_0.892(8)H_0.012(5), indicating non-trivial oxygen incorporation. The presence of N/O ordering and an impurity phase Mo₅N₆ were also revealed, detectable only by neutron diffraction. Notably, a computationally predicted MoON phase (doi: 10.1103/PhysRevLett.123.236402), of interest due to its potential to display a metal-insulator transition, did not appear under any reaction condition examined.