Rare earth elements are critical to materials used in technologies ranging from magnets to batteries to LED lighting. They are considered 'critical' materials in the sense that such technologies cannot work without them and, at the same time, their supply is limited and/or subject to geopolitical manipulation. The source of rare earths, as of all elements, is the Earth, and a variety of rock types are mined for these elements, which must then be concentrated, separated, and purified. These rocks (ores) contain a variety of host minerals, which typically contain rare earths in low concentrations in complex minral structures together with other elements such as uranium, thorium and niobium. Though the thermodynamic properties of these multicomponent mineral solid solutions define their formation and occurrence in the Earth and constrain processes for the mining, processing, and purification of the rare earth elements, these properties are poorly known at present. This project entails a systematic study of the thermodynamics of formation of a selected set of synthetic rare earth materials in order to understand the effects of different rare earths and other elements, as well as of the structures, on stability. The results will provide fundamental data for more complete, efficient, and economical extraction and separation of rare earths from mineral sources, thus fulfilling the Sustainable Chemistry, Engineering, and Materials (SusChEM) objectives. At the same time the data will be useful in modeling the transport of rare earths in the geologic environment, the origin of rare earth deposits, and the use of rare earths as geochemical indicators in petrologic processes.
High temperature oxide melt solution calorimetry, a specialized technique tailored to determing heats of formation of refractory oxide systems, will be used to measure heats of formation of selected systems. The samples will be synthesized and characterized extensively prior to calorimetry. The overall goal is to quantify the thermochemistry of charge-balanced ionic substitutions of rare earths in minerals. Phases in the RE2O3- H2O- CO2- HF system, including the major rare earth ore bastnasite, will be studied. The enthalpies of formation and mixing of RE solid solutions in perovskite and pyrochlore systems will be characterized.