Ammonium tungsten bronzes

Under a slightly reducing atmosphere between 220 and 325 °C amorphous, and above 325 °C, crystallized, ammonium tungsten bronzes form (NH4) tW03. Under stronger reducing conditions, conversion to lower tungsten oxides takes place. 

All of these complexes, including those with the lanthanide cations, are prepared directly from W04 and the appropriate An" (Ln ), and have only a narrow pH-range of stability (ca5.5-6.5) in aqueous solution. However in the solid state the ammonium salt of [Th(W50is)2] yields the cubic tungsten bronze ThxWOs (x 0.1) when heated in an inert atmosphere to 725°C. It has been proposed that bronzes prepared in such fashion from appropriate POM salts could be used as inert waste forms for actinides and radioactive lanthanides [40]. 

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