Hydrofluoride synthesis of niobium and tantalum compounds

Hydrofluoride synthesis is based on the simultaneous fluorination by ammonium hydrofluoride of niobium or tantalum oxides with other metals compounds (oxides, halides, carbonates etc.) [105]. Table 13 presents some properties of ammonium hydrofluoride, NH4HF2 [51, 71]. Ammonium hydrofluoride is similar to anhydrous HF in its reactivity, but possesses some indisputable advantages. The cost of ammonium hydrofluoride is relatively low, it can be dried and handled easily, recycled from gaseous components, and its processing requires no special equipment. 

The interaction between metal oxides and ammonium hydrofluoride at 100-250°C yields complex fluorometalates of ammonium and water. The water formed evaporates from the mixture, but can also lead to hydrolysis of the fluorination product in the process. In the very common form the fluorination of metal oxides by ammonium hydrofluoride can be described as follows  

Ammonium complex fluorometalates are generally susceptible to hydrolysis by water formed in the course of the fluorination process (see 25). Such interactions, resulting in the formation of oxyfluoride metalates can be described as follows  

Furthermore, both complex fluoromataletes and oxyfluorometalates of ammonium are prone to thermal decomposition. The decomposition is 

Complex oxyfluorometalates usually decompose by the same mechanism, maintaining the same Me/O ratio  

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