Preparation of hexagonal MoO₃ by "Chimie Douce" reaction with NO₂

The metastable hexagonal form of MoO₃ has been successfully prepared starting from the ammonium molybdate, using a "Chimie Douce" reaction with NO₂ gas. The lattice parameters of the as-prepared material were determined by XRD analysis to be : a= 10.633(2) Å and c = 3.719(1) Å. The composition of this material obtained by physical and chemical analysis is MoO₃, 0.09 H₂O, showing that a small amount of water molecules remained while there were no more ammonium ions in this structure. The structural transformation from hexagonal to stable orthorhombic α-MoO₃ phase was observed at 350°C. It was also shown that NO₂ reaction is a very useful method to remove topotactically ammonium ions from a parent structure at low temperature. This "Chimie Douce" process appears to be very promising for obtaining new metastable oxides with large tunnels or interlayer distances.