Conversion of Rare-Earth Oxides to Halides: How to Prevent Formation of Oxyhalides?

There is controversy regarding the formation of rare-earth oxyhalides during halogenation of rare-earth oxides. The formation of oxyhalide appears to be dependent on the conditions of experiment and the halogenating agent used. To clarify the issue, the stability field diagram for the system Nd-O-F is developed at 1200 K, showing unequivocally the stability of NdOF when both the sesquioxide and trifluoride are at unit activity. Methods to avoid the precipitation of solid NdOF and the feasibility of the process in scale-up are discussed.