Thermal decomposition of hydrogen molybdenum bronze, H0.25MoO3, in a nitrogen atmosphere: defects and phase transformations

Abstract

The dehydrogenation and subsequent phase transformations of hydrogen molybdenum bronze, H_0.25MoO₃, 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(MoO_2.875) 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 γ-Mo₄O₁₁ was found to be ca. 20 K lower than the former literature value.