Steam-Assisted Ammonolysis of MoO 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 NH, or ammonolysis. In this work, the role of precursor phase, gas phase chemistry (impact of HO), and temperature profile on the reaction outcome (700 °C) was examined, which resulted in varying amounts of MoO, HMoO, and the nitride phases-cubic γ (nominally MoN) and hexagonal δ (nominally MoN). The phase fraction of the δ phase increased with precursor in the sequence MoO > MoO > HMoO. Steam in the reaction gas also favored the production of δ over γ, but with too much steam, MoO was obtained in the product. Synthesis conditions for obtaining nearly phase-pure δ were identified: MoO as the precursor, 2% HO 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 MoONH, indicating non-trivial oxygen incorporation. The presence of N/O ordering and an impurity phase MoN 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.