A new insight into the synthesis mechanism of ScF3 by the NH4HF2 solid-phase fluorination method

As the important precursor to produce Sc, ScF3 is mainly produced by the NH4HF2 solid-phase fluorination method. However, the fluorination mechanism has not yet been sufficiently described, which severely hinders the development of the ScF3 synthesis technology. This work comprehensively investigated the reaction mechanisms of NH4HF2 and Sc2O3 with different molar ratios of raw materials (at both room temperature and high temperature) and the generation conditions of ScOF. The results show that at room temperature (NH4)(3)ScF6 were generated firstly, then it reacted with Sc2O3 to produce (NH4)(5)Sc3F14 after full consumption of NH4HF2. A new ScF3 synthesis mechanism, including three stages and a first discovered intermediate compound NH4Sc3F10, was put forward. The crystal structure of NH4Sc3F10 was analyzed by Rietveld analysis. Based on the analysis of ScOF formation conditions, some strategies on regulation of raw material molar ratio, heating rate, heating temperature, and holding time were proposed to avoid ScOF formation. The results will be beneficial to the solid-phase fluorination technology development for high purity ScF3 production.