Structural stability of α/β-Mo2C during thermochemical processing

The structural stability and its influence on the microstructure of thermochemically synthesized ultrafine Mo2C was investigated. The precursors for the thermochemical process were chemically synthesized and consisted of crystalline orthorhombic MoO3 and different concentrations of amorphous C. Crystalline β-Mo2C (hexagonal space group: P63/mmc) was obtained on heat treatment of a C-rich precursor at 1000 °C after 1 h of isothermal holding under hydrogen atmosphere. The as-synthesized β-Mo2C showed spherical grain morphology with less than about 100 nm size and highly agglomerated in plate form. The carbon analysis revealed about 6.24 wt.% of carbon in the β-Mo2C, which is slightly higher than the equilibrium C-level in Mo2C (5.89 wt.%), while the precise lattice parameters, as determined by Rietveld structural refinement, was close to the theoretical values of stoichiometric β-Mo2C. The precursor containing lower concentration of C, on the other hand, led to crystalline orthorhombic α-Mo2C with space group: Pnab (non-standard setting of Pbcn) at 1000 °C under identical heat treatment conditions. The as-synthesized α-Mo2C powders were highly agglomerated with a tendency to acquire spherical morphology with increase in C-level. The preferential formation of hexagonal β-Mo2C over orthorhombic α-Mo2C at higher C in the precursor was attributed to the faster kinetics of carbon diffusion due to its higher activity, which possibly led to the disordered distribution of carbon in the octahedral sites of hexagonal Mo sublattice.