Rare earth (RE) materials have widespread applications and constitute an indispensable part of modern lives. Tailored electrical, optical, and magnetic properties can be achieved by various RE elements in isomorphic structures, providing materials with the desired properties for specific applications. However, the processing of RE materials encounters difficulties in separation and recycling of RE elements due to the similar physical and chemical properties. For better utilization of REs, including mining, recovery, and incorporation into devices, it is essential to have comprehensive knowledge about thermodynamic properties of RE compounds.The present work comprises not only new experimental thermodynamic studies on selected RE compounds but also a fundamental understanding of the energetics of RE compounds. By means of high temperature oxide melt solution calorimetry, formation enthalpies of RE oxycompounds (REOOH, REOF, and REOCl) and ternary sodium fluorides (NaREF4) were measured to provide reliable data for energetic assessments of REs in diverse structures. NaF–NdF3 nanocrystals and Ca1−xRExF2+x solid solutions were also investigated to shed light on the energetics of RE fluoride nanocrystals and solid solutions containing defect clusters. Combining other published thermochemical data, lattice energies of numerous RE compounds were evaluated to reveal the correlations between the energetics and structures of RE materials.
RE oxycompounds are important components of RE precipitates and play a key role in the leaching of RE elements. By different synthesis methods, high-purity single-phase REOOH, REOF, and REOCl with various REs were prepared and characterized by powder X-ray diffraction (PXRD). The formation enthalpies from binary components, determined by high temperature oxide melt solution calorimetry, are all negative but become less exothermic from light to heavy RE elements. They confirm the thermodynamic stability of these RE compounds relative to binary oxides and/or halides, The thermal decomposition of REOOH and a structural phase transition in YbOF were studied by differential scanning calorimetry (DSC).
Mixed cation (alkali or alkaline earth) RE fluorides have drawn extensive research interest due to their unique luminescent properties and potential applications in phosphors and laser hosts, but limited thermodynamic studies hinder the rational design and synthesis of these materials. The formation enthalpies of NaREF4 follow an opposite trend compared to most ternary RE oxide compounds, becoming increasingly negative for heavier REs. Energetic studies of NaF–NdF3 nanocrystals coupled with chemical analysis and structural characterizations indicate that the α → β phase conversion in aqueous solutions is associated with compositional variations and distinct from the β → α transition driven by temperature for stoichiometric NaNdF4. Formation enthalpies of Ca1−xRExF2+x (RE = La, Pr, and Nd) solid solutions were measured and the relationships between the energetics and structures were discussed.
Based on experimental thermodynamic studies, an energetic evaluation of RE crystalline compounds was conducted to identify correlations between structures and thermodynamics. Lattice energies were calculated by Born–Haber cycles, which highly depend on ionic strength and increase in magnitude (becoming more exothermic) from light to heavy RE elements. The slopes of lines relating lattice energies to RE ionic radii are related to RE coordination numbers, contributing to the opposite trends in formation enthalpies for RE oxides and fluorides. A correlation between the magnitude of lattice energies and the structures occurring in RE compounds across the RE series was identified.
