Thermochemical reference data for rare earth elements (Sc, Y, La–Nd, Sm–Lu) dissolved in aqueous 5 N hydrochloric acid at 298 K and in molten sodium molybdate at 1,073 K

Abstract Rare earth elements (REE) and their compounds are widely used for the production of components of consumer technology. Current geopolitical considerations have brought them to the forefront of international discussion and highlight the need for development of strategies for more efficient mining, processing, use, and recycling. However, advanced technology relies on robust thermodynamic data to understand the formation of REE-bearing compounds and limitations of their use, and such data are not well constrained and sometimes contradictory. In this study we use solution calorimetry to accurately determine the energetics of two of the most important groups of rare earth materials: the rare earth metals and their common oxides. Rare earth metals were dissolved in aqueous 5 N HCl at 298 K and yielded solution enthalpies ranging from −645 kJ mol −1 to −725 kJ mol −1 . The common oxides, including most sesquioxides (REE 2 O 3 ) and other stable oxide phases where appropriate, were dissolved in the melt of 3 Na 2 O·4 MoO 3 at 1,073 K and yielded drop solution enthalpies ranging from −220 kJ mol −1 to −50 kJ mol −1 , with the exception of CeO 2, which had a positive drop solution enthalpy of about 90 kJ mol −1 . The formation enthalpies of REE oxides from the literature were combined with our data to calculate the respective drop solution enthalpies of the metals in sodium molybdate melt and the dissolution enthalpies of the oxides in aqueous 5 N HCl. These data are currently the most reliable reference state data for the majority of rare earth metals and oxides, including results for previously unstudied materials, and can be considered state of the art for studies and software requiring thermodynamic data going forward.