THERMAL-DECOMPOSITION OF HYDROGEN MOLYBDENUM BRONZE, H0.25MOO3, IN A NITROGEN ATMOSPHERE - DEFECTS AND PHASE-TRANSFORMATIONS

The dehydrogenation and subsequent phase transformations of hydrogen molybdenum bronze, H0.25MoO3, have been investigated in a nitrogen atmosphere. Two dehydrogenation processes were identified, which depend on the density of defects. For low defect density, the removal of lattice oxygen accompanied by dehydrogenation results in disordered shear defects in the parent Mo-O framework, while for a high defect density the removal of lattice oxygen leads to an MoO2 phase as the Mo-O framework can not maintain the structure based on MoO3. The phase transformations after the dehydrogenations are, according to the literature, different from those at equilibrium of MoO3-MoO2 or MoO3-Mo(MoO2.875) mixtures. However, they were found to be similar to that of MoO3-MoO2 under non-equilibrium conditions. It was observed that in a stream of nitrogen (i.e. in an open system) Mo18O52 was formed from, a mixture of MoO3 and MoO2 at temperatures > 1023 K, although it had been known to transform to Mo9O26 at these temperatures. The formation temperature range of gamma-Mo4O11 was found to be ca. 20 K lower than the former literature value.