The M2Q11 unit (M = Nb, Ta; Q = S, Se): A versatile building block to generate group 5 polychalcogenides

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) A and c = 3.719(1) A. 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 (alpha)-MoO3 phase was observed at 350 °C. It was also shown that NC2 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.