Using Atom Dynamics to Map the Defect Structure Around an Impurity in Nano-Hematite

The bulk behavior of materials is often controlled by minor impurities that create nonperiodic localized defect structures due to ionic size, symmetry, and charge balance mismatches. Here, we used transmission electron microscopy (TEM) of atom-resolved dynamics to directly map the topology of Fe vacancy clusters surrounding structurally incorporated U⁶⁺ in nanohematite (α-Fe₂O₃). Ab initio molecular dynamic simulations provided additional independent constraints on coupled U, Fe, and vacancy mobility in the solid. A clearer understanding of how such an apparently incompatible element can be accommodated by hematite emerged. The results were readily interpretable without the need for sophisticated data reconstruction methods, model structures, or ultrathin samples, and with the proliferation of aberration-corrected TEM facilities, the approach is accessible. Given sufficient z-contrast, the ability to observe impurity-vacancy structures by means of atom hopping can be used to directly probe the association of impurities and such defects in other materials, with promising applications across a broad range of disciplines.

Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe₂O₃) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L₃-edge EXAFS for U⁶⁺-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U⁶⁺ that substitutes for structural Fe³⁺, which requires two partially protonated Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions.

Self-organizing layers from complex molecular anions

The formation of traditional ionic materials occurs principally via joint accumulation of both anions and cations. Herein, we describe a previously unreported phenomenon by which macroscopic liquid-like thin layers with tunable self-organization properties form through accumulation of stable complex ions of one polarity on surfaces. Using a series of highly stable molecular anions we demonstrate a strong influence of the internal charge distribution of the molecular ions, which is usually shielded by counterions, on the properties of the layers. Detailed characterization reveals that the intrinsically unstable layers of anions on surfaces are stabilized by simultaneous accumulation of neutral molecules from the background environment. Different phases, self-organization mechanisms and optical properties are observed depending on the molecular properties of the deposited anions, the underlying surface and the coadsorbed neutral molecules. This demonstrates rational control of the macroscopic properties (morphology and size of the formed structures) of the newly discovered anion-based layers.

Trace Uranium Partitioning in a Multiphase Nano-FeOOH System

The characterization of trace elements in minerals using extended X-ray absorption fine structure (EXAFS) spectroscopy constitutes a first step toward understanding how impurities and contaminants interact with the host phase and the environment. However, limitations to EXAFS interpretation complicate the analysis of trace concentrations of impurities that are distributed across multiple phases in a heterogeneous system. Ab initio molecular dynamics (AIMD)-informed EXAFS analysis was employed to investigate the immobilization of trace uranium associated with nanophase iron (oxyhydr)oxides, a model system for the geochemical sequestration of radiotoxic actinides. The reductive transformation of ferrihydrite [Fe(OH)₃] to nanoparticulate iron oxyhydroxide minerals in the presence of uranyl (UO₂)²⁺_(aq) resulted in the preferential incorporation of U into goethite (α-FeOOH) over lepidocrocite (γ-FeOOH), even though reaction conditions favored the formation of excess lepidocrocite. This unexpected result is supported by atomically resolved transmission electron microscopy. We demonstrate how AIMD-informed EXAFS analysis lifts the strict statistical limitations and uncertainty of traditional shell-by-shell EXAFS fitting, enabling the detailed characterization of the local bonding environment, charge compensation mechanisms, and oxidation states of polyvalent impurities in complex multiphase mineral systems.

Built-In Potential in Fe2O3-Cr2O3 Superlattices for Improved Photoexcited Carrier Separation

Hematite (α-Fe2 O3) is engineered to improve photoexcited electron-hole pair separation by synthesizing Fe2O3-Cr2O3 superlattices (SLs) with precise atomic control. The different surface terminations exhibited by Fe2O3 and Cr2O3 determine the hetero-junction interface structure and result in controllable, noncommutative band offset values. This controllable band alignment is harnessed to generate a built-in potential as large as 0.8 eV in Fe2 O3-Cr2O3 SLs.

Superconductivity and disorder in PrOs(4)Sb(12)

The specific heat, and dc and ac magnetic susceptibility are reported for a large single crystal of PrOs(4)Sb(12) and, after grinding, its powder. The physical properties of the crystal are typical of the majority of reported PrOs(4)Sb(12) samples. The room temperature effective paramagnetic moment of the crystal was consistent with the Pr(3+) ionic configuration and full (or nearly full) occupancy of the Pr sublattice. The crystal showed two distinct anomalies in the specific heat and an overall discontinuity in C/T of approximately 1000 mJ K(-2) mol(-1). The upper transition (at T(c1)) was characteristically rounded. The anomaly at T(c2) was very sharp, consistent with a good quality for the crystal. We observed a shoulder in χ(') and two peaks in χ('') below T(c1). However, there were no signatures in χ(') of the lower temperature transition. Grinding to powder size smaller than 50 µm completely suppresses the upper superconducting transition in both the specific heat and magnetic susceptibility. It also strongly reduces ΔC/T(c) at T(c2). Stress annealing brings back some of this lost ΔC/T(c) but does not restore the upper temperature transition. Possible explanations of the existence of two superconducting specific heat anomalies for single crystals are discussed.