Influence of double-electron transitions on the EXAFS L edges of rare-earth systems

Aqueous Structure of Lanthanide-EDTA Coordination Complexes Determined by a Combined DFT/EXAFS Approach

Sustainable production of rare earth elements (REEs) is critical for technologies needed for climate change mitigation, including wind turbines and electric vehicles. However, separation technologies currently used in REE production have large environmental footprints, necessitating more sustainable strategies. Aqueous, affinity-based separations are examples of such strategies. To make these technologies feasible, it is imperative to connect aqueous ligand structure to ligand selectivity for individual REEs. As a step toward this goal, we analyzed the extended X-ray absorption fine structure (EXAFS) of four lanthanides (La, Ce, Pr, and Nd) complexed by a common REE chelator, ethylenediaminetetraacetic acid (EDTA) to determine the aqueous-phase structure. Reference structures from density functional theory (DFT) were used to help fit the EXAFS spectra. We found that all four Ln-EDTA coordination complexes formed 9-coordinate structures with 6 coordinating atoms from EDTA (4 carboxyl oxygen atoms and 2 nitrogen atoms) and 3 oxygen atoms from water molecules. All EXAFS fits were of high quality (R-factor < 0.02) and showed decreasing average first-shell coordination distance across the series (2.62-2.57 Å from La-Nd), in agreement with DFT (2.65-2.56 Å from La-Nd). The insights determined herein will be useful in the development of ligands for sustainable rare earth elements (REE) separation technologies.

Hydration of Heavy Alkaline-Earth Cations Studied by Molecular Dynamics Simulations and X-ray Absorption Spectroscopy

The physicochemical properties of the three heaviest alkaline-earth cations, Sr²⁺, Ba²⁺, and Ra²⁺ in water have been studied by means of classical molecular dynamics (MD) simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The polarizable and flexible model of water MCDHO2 was adopted. The theoretical-experimental comparison of structural, dynamical, energetic, and spectroscopical properties of Sr²⁺ and Ba²⁺ aqueous solutions is satisfactory, which supports the methodology developed. This good behavior allows a reasonable reliability for the predicted Ra²⁺ physicochemical data not experimentally determined yet. Simulated extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge spectroscopy spectra have been computed from the snapshots of the MD simulations and compared with the experimental information available for Sr²⁺ and Ba²⁺. For the Ra²⁺ case, the Ra L₃-edge EXAFS spectrum is proposed. Structural and dynamical properties of the aqua ions for the three cations have been obtained and analyzed. Along the [M(H₂O)_n]^m+ series, the M-O distance for the first-hydration shell is 2.57, 2.81, and 2.93 Å for Sr²⁺, Ba²⁺, and Ra²⁺, respectively. The hydration number also increases when one is going down along the group: 8.1, 9.4, and 9.8 for Sr²⁺, Ba²⁺, and Ra²⁺, respectively. Whereas [Sr(H₂O)₈]²⁺ is a typical aqua ion with a well-defined structure, the Ba²⁺ and Ra²⁺ hydration provides a picture exhibiting an average between the ennea- and the deca-hydration. These results show a similar chemical behavior of Ba²⁺ and Ra²⁺ aqueous solutions and support experimental studies on the removal of Ra-226 of aquifers by different techniques, where Ra²⁺ is replaced by Ba²⁺. A comparison of the heavy alkaline ions, Rb⁺ and Cs⁺, with the heavy alkaline-earth ions is made.

Structures and Free Energies of Cerium Ions in Acidic Electrolytes

The Ce³⁺/Ce⁴⁺ redox potential changes with the electrolyte, which could be due to unequal anion complexation free energies between Ce³⁺ and Ce⁴⁺ or a change in the solvent electrostatic screening. Ce complexation with anions and solvent screening also affect the solubility of Ce and charge transfer kinetics for electrochemical reactions involving waste remediation and energy storage. We report the structures and free energies of cerium complexes in seven acidic electrolytes based on Extended X-ray Absorption Fine Structure, UV-vis, and Density Functional Theory calculations. Ce³⁺ coordinates with nine water molecules as [Ce(H₂O)₉]³⁺ in all studied electrolytes. However, Ce⁴⁺ complexes with anions in all electrolytes except HClO₄. Thus, our results suggest that Ce⁴⁺-anion complexation leads to the large shifts in standard redox potential. Long range screening effects are smaller than the anion complexation energies but could be responsible for changes in the Ce solubility with acid.

The hydration structure of the heavy-alkalines Rb+ and Cs+ through molecular dynamics and X-ray absorption spectroscopy: surface clusters and eccentricity

Hydration shells around Rb⁺ and Cs⁺ are not symmetric; the cation and the 1st-shell water mass center are separated by ∼0.4 Å, and this is supported by agreement between the theoretical and experimental EXAFS spectrum.

Local structural disorder in REFeAsO oxypnictides by RE L(3) edge XANES

The REFeAsO (RE = La, Pr, Nd and Sm) system has been studied by RE L(3) x-ray absorption near edge structure (XANES) spectroscopy to explore the contribution of the REO spacers between the electronically active FeAs slabs in these materials. The XANES spectra have been simulated by full multiple scattering calculations to describe the different experimental features and their evolution with the RE size. The near edge feature just above the L(3) white line is found to be sensitive to the ordering/disordering of oxygen atoms in the REO layers. In addition, shape resonance peaks due to As and O scattering change systematically, indicating local structural changes in the FeAs slabs and the REO spacers due to RE size. The results suggest that interlayer coupling and oxygen order/disorder in the REO spacers may have an important role in the superconductivity and itinerant magnetism of the oxypnictides.