In situ XAFS study of phase transitions and hydrogen intercalation in WO₃-MoO₃ system

The WO₃-MoO₃ system has several reconstructive and displacive phase transitions which can be induced by temperature and/or by change of stoichiometry or valence state of metal ions under hydrogen intercalation. We have studied in situ the local electronic and structural changes arising in WO₃-MoO₃ system with hydrogen intercalation and temperature. In situ XAFS measurements at the Mo K and W L₃ edges in H_yMo_xW_1-xO₃ compounds, performed at different hydrogen intercalation level and temperatures (from RT to 500°C), allowed us to determine the rearrangement in the first and second coordination shells of Mo and W cations during phase transitions. It was found that hydrogen insertion leads, depending on W/Mo ratio, to the appearance of localized (mixed valence) or delocalized electronic states with different Mo and W surrounding.