Studies on fluorination of Y2O3 by NH4HF2

Fluorination of Y2O3 by NH4HF2 to obtain pure YF3 is studied in this work by observing the mass changes at room temperature and by employing TGA and DTA at higher temperatures, and observing the subsequent phase changes by X-ray diffraction. Fluorination begins at room temperature with the formation of (NH4)3Y2F9 and NH4F, and continues with the later further fluorinating any leftover Y2O3 to ammonium yttrium fluorides with the evolution of NH3. Similarly, on heating a mixture of Y2O3 and NH4HF2, the reaction proceeds sequentially through the formation of (NH4)3Y2F9, NH4Y2F7 and finally YF3. Any Y2O3 still remaining reacts with YF3 to form yttrium oxyfluorides, which continue as impurity. Substantial evaporation of NH4HF2 is possible even before it could participate in the reaction. This disturbs the stoichiometry of the charge causing oxygen to remain in final YF3. The compound reported in literature as YF3·1.5NH3 appears to be NH4Y2F7.

► NH4HF2 and Y2O3 react right from room temperature to form ammonium metallofluorides.
► The other product could be NH4F or NH3 depending on the amount of NH4HF2 used.
► Substantial evaporation of NH4HF2 is possible before reaction with Y2O3 on heating.
► Rate of fluorination reactions noticeably depends on removal of product vapours.
► Melting and boiling points of NH4F are around 110 and 165 °C respectively and the boiling point of NH4HF2 is around 145 °C.
► Y2O3 does not appreciably react with NH4Y2F7.
► No indication of the existence of ammonium yttrio-oxyfluorides (NH4YxOyFz) was found.
► YF3·1.5NH3 is actually NH4Y2F7.