Open structure tungstates: Synthesis, reactivity and ionic mobility

Sodium tungstates with the hexagonal tungsten bronze and the pyrochlore structure have been synthesized at around 150°C using hydrothermal methods. The phase formed is a function of the pH of the reaction medium. Their structures have been determined using Rietveld analysis on X-ray and neutron powder data. The pyrochlore phase readily undergoes ion-exchange with a wide range of monovalent cations giving the compounds, M_xW₂O_{6+ x 2}·yH₂O; the value of y is strongly dependent on the identity of the cation, M. Thermogravimetric analysis was used to determine the water content and the thermal stability of the cation exchanged compounds. WO₃ with the pyrochlore structure could be formed from the hydronium and ammonium complexes. Lithium can be readily intercalated both chemically and electrochemically into both phases. Surprisingly more lithium is incorporated in most cases in the hexagonal than in the pyrochlore phase. Silver is ion-exchanged most rapidly into the pyrochlore structure, and this compound also reacts with the most lithium. The ionic mobility has been determined by complex impedance methods, with the hydronium ion showing the greatest mobility.